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Fair Play rules make ice hockey safer

Fair Play (PDF) is an initiative developed and implemented by Mayo Clinic Sports Medicine Research to reduce the incidence of concussions and make hockey a safer sport.

Research improves all aspects of sports mechanics

Mayo Clinic Sports Medicine Research is using research discoveries to improve strength, power, agility and speed and prevent common injuries such as ACL tears in athletes.

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Mayo Clinic Sports Medicine Research is using biomechanics research to show how pitching and swing mechanics can put an athlete at risk for injury.

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Mayo Clinic Sports Medicine Research: Preventing and treating sports injuries to improve and enhance athletic performance.

Mayo Clinic Sports Medicine Research performs leading-edge research to define the mechanism of injury and utilizes these findings to implement educational programs and therapeutic interventions. This work helps prevent injuries, enhances athletic performance and increases injury prevention during play.

Sports Medicine Research investigates all aspects of sports injury evaluation, treatment and prevention to provide optimal care to those involved in sports- or fitness-related activities. Recent work has addressed neuromuscular interventions to prevent anterior cruciate ligament (ACL) injuries, the use of subsymptom exercise to return athletes who've had concussions to a pre-head-trauma state and the modeling of lower extremity athletic joint injuries.

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Mayo Clinic Sports Medicine Research focuses on the following areas for optimal care and results for athletes:

• Mechanisms that may identify the anatomical and structural causes of injuries
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• Treatment strategies (surgical and nonsurgical) to optimize outcomes after injury

Comprehensive care comes from bringing research directly into practice. A complete team of surgeons, physicians, researchers, specialists and therapists work together to return people to physical activity as soon as possible.

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Research is focused into specialized areas to diagnose, treat and prevent diseases or conditions that affect athletic performance for the professional and recreational athlete.

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Clinical research areas in ACL, concussion, hip, knee and shoulder apply scientific discoveries to diagnose and treat athletes for the best outcomes.

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ACSM Supports Funding for National Concussion Surveillance System and Physical Activity Alliance Releases New Report 

Last month, ACSM joined more than 30 other organizations in signing a letter requesting at least $2 million to fund the National Concussion Surveillance System. The funding would allow the Centers for Disease Control and Prevention (CDC) to develop an initial system that will collect necessary data to determine the incidence of sports-related concussions. This letter will go to House and Senate leadership and both the House and Senate Chairs and Ranking Members of the Appropriations Labor-HHS-Education Subcommittee. The Physical Activity Alliance (PAA), of which ACSM is a founding member, just released a new report: “Advancing Key Actions to Enhance Physical Activity Surveillance in the United States.” The report summarizes actions taken to advance surveillance in four areas: children and youth, workplace, health care and community support for physical activity. It also lays out steps to advance tracking physical activity engagement as a key public health priority and underscores the benefits of systematic data collection to drive policy and programming that supports equitable physical activity opportunities across the U.S. Read the report.

Team Physicians Update Guidance on Musculoskeletal Injuries

A group of delegates from ACSM, the American Academy of Family Physicians, American Academy of Orthopaedic Surgeons, American Medical Society for Sports Medicine, American Orthopaedic Society for Sports Medicine and the American Osteopathic Academy of Sports Medicine recently met to write updated guidelines on the care and treatment of select musculoskeletal injuries as part of the annual Team Physician Consensus Conference (TPCC). The TPCC Executive Committee, consisting of ACSM Fellows Stan Herring, M.D.; Ben Kibler, M.D.; and Margot Putukian, M.D., along with delegates from the partner organizations, reviewed the current scientific literature and clinical evidence to write the new evidence-based statement for clinicians. The resulting document, which will be published in 2022, will provide guidance and serve as a teaching tool for physicians, residents and fellows working in the sports medicine field. Learn More About TPCC

The Female Athlete Triad

MYTH: It's "normal" for female athletes to lose their period, or stop menstruating. REALITY: Amenorrhea due to working out is unhealthy. Mary Jane De Souza, Ph.D., FACSM, and Nancy Williams, Sc.D., FACSM, both researchers and professors at Pennsylvania State University, have spent many years studying the Female Athlete Triad.

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(BJSM) is the leading, peer-reviewed journal in sports medicine, with additional multimedia resources.

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British Journal of Sports Medicine (BJSM) is a  Plan S compliant Transformative Journal .

British Journal of Sports Medicine (BJSM) is a multimedia portal for authoritative original research, systematic reviews, consensus statements and debate in sport and exercise medicine (SEM). We define sport and exercise medicine broadly. BJSM’s web, print, video and audio material serves the international sport and exercise medicine community which includes 25 clinical societies who have over 13,000 members. You can access BJSM on Facebook , Twitter , Instagram and Youtube as well as via our podcasts , Stitcher and blog.

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A collection of key resources, providing an overview of the current medical issues and research landscape in #GolfMedicine

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For over two decades, the Concussion in Sport Group has held meetings and developed five international statements on concussion in sport. This 6th statement summarises the processes and outcomes of the 6th International Conference on Concussion in Sport held in Amsterdam on 27–30 October 2022 and should be read in conjunction with the (1) methodology paper that outlines the consensus process in detail and (2) 10 systematic reviews that informed the conference outcomes. Over 3½ years, author groups conducted systematic reviews of predetermined priority topics relevant to concussion in sport. The format of the conference, expert panel meetings and workshops to revise or develop new clinical assessment tools, as described in the methodology paper, evolved from previous consensus meetings with several new components.

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Safe return following testing positive.

by Kyle P. Harris | Dec 6, 2023

Elite-level athletes with a history of COVID-19 had cardiac measurements similar to unexposed elite athletes. Hence, return to sports participation is safe for athletes following COVID-19.

Eliminating Body Checking has Been Positive; However, Female Ice Hockey Players are Still at Risk!

by Jane McDevitt | Nov 29, 2023

Youth ice hockey policy eliminating body checking decreases the risk of concussion; however, concussion rates are still high among female ice hockey players.

The Biological Basis of Sex Differences in Athletic Performance: Consensus Statement for the American College of Sports Medicine

by Jeffrey B. Driban | Nov 6, 2023

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The American Sports Medicine Institute (ASMI) is a national and international leader in sports medicine research related to clinical and surgical outcomes, biomechanics, and rehabilitation. The foci of ongoing studies at ASMI includes both clinical and biomechanical research, and our team includes researchers with expertise in motion capture biomechanics, cadaver joint biomechanics, outcomes research, clinical trials, biostatistics, and human anatomy.

ASMI’s mission is to improve the understanding, prevention, and treatment of sports-related injuries through research and education. In our strive for this mission, ASMI studies are regularly published in high-impact sports medicine journals, presented at scientific meetings, publicized in the media, and utilized in bodies creating sports safety policies. 

Frequently Asked Questions

• What is sports medicine? Sports medicine is a field of medicine concerned with the prevention and treatment of injuries and disorders that are related to participation in sports.
• Does your research only include athletes? The mission of ASMI is to improve the understanding, prevention, and treatment of sports-related injuries through research and education. This includes everyone from youth to professional athletes, as well as recreational adult athletes and people working to be healthy and fit. Our research team also explores treatment outcomes of non-athletes, particularly those with orthopaedic injuries or surgeries.
• What is the difference between biomechanical research and clinical research? ASMI’s biomechanics studies the motions and forces to minimize injury risk and maximize performance in sports. Other biomechanical studies at ASMI investigate the strength of human joints after surgery compared to their natural, undamaged state. Clinical research at ASMI involves both observational studies and clinical trials. Observational studies generally include examination of what is already occurring (no intervention), whereas clinical trials are carefully designed experiments that involve a specific treatment or intervention. Specifically, our clinical team works to evaluate surgical outcomes, rehabilitation efficacy, as well as injury mechanisms and prevention to improve treatment of sports-related and orthopaedic injuries. Importantly, our biomechanical and clinical research teams work closely together to fulfill the mission of ASMI.
• How can I get involved in research at ASMI? Being involved in ASMI research means contributing to knowledge that directly improves the prevention and treatment of sports-related injuries. We work closely with our local physician and surgeon partners and other collaborators around the country and world to achieve these impactful research goals. Opportunities to participate in research at ASMI’s center in Birmingham are available for students and scholars of all levels. Learn more about how to get involved here .

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Our Biomechanical Research department works extensively to further our goal of preventing injury in sports.

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Submaximal Fitness Tests in Team Sports: A Theoretical Framework for Evaluating Physiological State

• Review Article
• Open access
• Published: 11 July 2022
• Volume 52 , pages 2605–2626, ( 2022 )

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• Tzlil Shushan   ORCID: orcid.org/0000-0002-0544-1986 1 ,
• Shaun J. McLaren   ORCID: orcid.org/0000-0003-0480-3209 2 , 3 ,
• Martin Buchheit 4 , 5 , 6 , 7 ,
• Tannath J. Scott   ORCID: orcid.org/0000-0003-4336-2370 8 , 9 ,
• Steve Barrett   ORCID: orcid.org/0000-0002-6751-9937 10 &
• Ric Lovell   ORCID: orcid.org/0000-0001-5859-0267 1  

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Team-sports staff often administer non-exhaustive exercise assessments with a view to evaluating physiological state, to inform decision making on athlete management (e.g., future training or recovery). Submaximal fitness tests have become prominent in team-sports settings for observing responses to a standardized physical stimulus, likely because of their time-efficient nature, relative ease of administration, and physiological rationale. It is evident, however, that many variations of submaximal fitness test characteristics, response measures, and monitoring purposes exist. The aim of this scoping review is to provide a theoretical framework of submaximal fitness tests and a detailed summary of their use as proxy indicators of training effects in team sports. Using a review of the literature stemming from a systematic search strategy, we identified five distinct submaximal fitness test protocols characterized in their combinations of exercise regimen (continuous or intermittent) and the progression of exercise intensity (fixed, incremental, or variable). Heart rate-derived indices were the most studied outcome measures in submaximal fitness tests and included exercise (exercise heart rate) and recovery (heart rate recovery and vagal-related heart rate variability) responses. Despite the disparity between studies, these measures appear more relevant to detect positive chronic endurance-oriented training effects, whereas their role in detecting negative transient effects associated with variations in autonomic nervous system function is not yet clear. Subjective outcome measures such as ratings of perceived exertion were less common in team sports, but their potential utility when collected alongside objective measures (e.g., exercise heart rate) has been advocated. Mechanical outcome measures either included global positioning system-derived locomotor outputs such as distance covered, primarily during standardized training drills (e.g., small-sided games) to monitor exercise performance, or responses derived from inertial measurement units to make inferences about lower limb neuromuscular function. Whilst there is an emerging interest regarding the utility of these mechanical measures, their measurement properties and underpinning mechanisms are yet to be fully established. Here, we provide a deeper synthesis of the available literature, culminating with evidence-based practical recommendations and directions for future research.

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1 Background

Monitoring the training process and its outcomes within team-based sports requires a systematic approach that: (1) is grounded on a rigorous conceptual framework; (2) can be implemented pragmatically on a frequent basis; (3) uses proxy outcome measures possessing sufficient measurement properties; and (4) is sensitive to identify acute (e.g., post-match) and chronic (e.g., post-training program) training effects [ 1 , 2 , 3 , 4 ]. Such an approach can be used to inform decision making around athlete and training management, including future programming, adjustments to training delivery, or the implementation of recovery interventions [ 1 ].

Assessing aerobic-oriented training effects has traditionally been made via distinct maximal-effort exhaustive tests. For example, improvements denoted in maximal intermittent field tests (e.g., 30–15 Intermittent Fitness Test) can infer improved aerobic capacity (amongst other systems), whereas decreased values may be interpreted as a negative response or de-training [ 5 , 6 ]. However, given the nature of the in-season phase common to professional teams, which frequently experiences fixture congested periods, it is considered less feasible to expose athletes to serial exhaustive assessments [ 7 , 8 ]. With regard to neuromuscular function, a variety of test protocols are administered to quantify chronic training effects on athletic qualities such as strength and power, but more frequently as indicators of acute and transient responses (e.g., post-match recovery kinetics) [ 7 , 9 ]. Similarly, the practicability of such assessments in the team-sports environment is challenged by different factors such as the number of athletes, the time available for discreet testing protocols, and the numerous contextual and individual elements that may undermine inferences derived from the data (e.g., motivation, physical qualities, season stage) [ 1 , 7 , 8 ].

Submaximal fitness tests (SMFT) have been proposed to deliver a feasible alternative to evaluate an athlete’s physiological state, presumably because of their time-efficient nature, low physical/physiological burden, and relative ease of administration [ 10 ]. In essence, SMFT provide a pragmatic and systematic approach of observing response(s) to a standardized physical stimulus [ 11 , 12 ]. Such assessments have been investigated since the late 1940s [ 13 ] and were mainly adopted among clinicians diagnosing health conditions or physical fitness in non-athletic populations, whereby exposure to maximal or exhaustive activities was thought to be ill-advisable because of the health risk it posed to patients [ 13 , 14 ]. Over the years, a number of walking [ 15 , 16 , 17 , 18 , 19 ], cycling [ 14 , 20 , 21 , 22 ], and running [ 23 , 24 , 25 ] SMFT have been administered among clinical and healthy populations. These tests involve single or multiple continuous steady-state protocols, with some prescribing an absolute standardized intensity, while others include relative intensity ranges, or self-paced protocols (refer to File 1 in the Electronic Supplementary Material [ESM]).

1.1 Elite Sports

The implementation of SMFT in elite sports has been traditionally used for quantifying relevant physiological transitions between exercise intensity domains (e.g., anaerobic threshold [i.e., the threshold indicates an equal rate of lactate production and disposal] and the onset of blood lactate accumulation [4-mmol·L −1 lactate threshold]) [ 26 , 27 ], often administered to inform training prescription or determine exercise economy. However, these tests generally necessitate a laboratory environment, are resource intensive and obtrusive, and therefore considered less feasible in the day-to-day field context, particularly with large cohorts of athletes [ 7 ].

Individual endurance sport practitioners were the first to develop and implement SMFT as part of their training monitoring processes [ 28 , 29 ]. Throughout the years, a broad range of cycling and running SMFT have been adopted across a variety of endurance sport athletes such as cyclists [ 28 , 29 , 30 , 31 , 32 ], runners [ 33 , 34 , 35 ], and triathletes [ 29 , 36 , 37 ]. Extensively used SMFT in endurance sports include exercise tasks prescribed by fixed internal intensities (% of an individual heart rate [HR] maximum) [ 32 , 34 ], while the outcome measures are considered both external (e.g., speed [ 34 ], cadence, power [ 32 ] collected throughout the test) and internal responses (HR recovery [HRR] [ 38 , 39 ], ratings of perceived exertion [RPE] [ 32 ] collected immediately post-exercise). Alternatively, researchers have adopted tests using standardized external intensities (usually via absolute running speed values) [ 33 , 37 ]. Initially, the primary purpose of these SMFT was to predict performance (e.g., time trial) or physiological capacities (i.e., maximal oxygen uptake) [ 32 ]; however, more recently, they have been used to identify impaired performance (e.g., functional overreaching) [ 40 , 41 ].

Because of the simplicity of implementing SMFT, their non-exhaustive nature, and their potential to provide information regarding both positive and negative training effects, SMFT have become common in team sports. Indeed, the adoption of SMFT in team-sports research [ 8 , 10 , 42 ] and practice [ 43 , 44 ] has increased substantially over the last decade. However, given the broad range of SMFT adopted, including various protocols (continuous vs intermittent) [ 45 , 46 ], activity modes (running vs cycling) [ 47 , 48 ], intensities (fixed vs incremental, absolute vs individualized) [ 12 , 48 ], outcome measures (cardiorespiratory/metabolic, subjective, or mechanical) [ 12 , 49 , 50 ], and purposes (monitoring positive vs negative effects) [ 47 , 51 ], a synthesis appears warranted. Accordingly, the aims of this scoping review were to (1) develop an operational definition of SMFT and protocol taxonomy, (2) identify previously used SMFT in the team-sports research and discuss their conceptual and methodical aspects, (3) provide an audit of outcome measures, collection methods, and analytical processes, as well as evaluate the theoretical rationale underpinning their inclusion, (4) provide a narrative synthesis of the available research on SMFT as indicators of training effects in team sports, and (5) conclude with practical recommendations and future directions.

Systematic searches of the electronic databases MEDLINE, Scopus, and Web of Science were used to identify relevant studies. From 2170 records identified in the original searches, we accepted 87 team-sport studies meeting our inclusion criteria. A detailed description of the searching strategy, screening process, and the inclusion–exclusion criteria are provided in File 2 in the ESM.

3.1 Submaximal Fitness Test Definition

A table presenting the characteristics of the included studies is provided in File 3 in the ESM. Based on the available literature, we defined SMFT as a short exercise bout, undertaken at a standardized intensity that is intended to be non-exhausting, and performed with the purpose of inferring an athlete’s physiological state through the monitoring of relevant outcome measures. In this regard:

Exercise is typically a cyclic activity involving large muscle groups. In team-sports settings, this is often administered as running activities, however, cycling has also been featured.

Standardized intensity refers to a threshold(s) that is standardized based on an internal response or external intensity parameter, and can be either fixed for all athletes (i.e., absolute) or individualized to a capacity anchor (relative; e.g., fraction of HR maximum or maximal aerobic speed).

Non-exhausting generally excludes frequent or prolonged ‘all-out’ maximal intensities, intensities that would cause voluntary cessation, or intensities that elicit an excessive training stimulus beyond that originally intended. From a practical standpoint, in team sports, the test should not have negative carry-over effects for the subsequent training session (for instance, if administered during the warm-up), or elsewhere it is implemented as an integrated standardized training component within the session plan (see for example, Sect.  3.2 ; intermittent-variable category).

Physiological state can be defined as a particular condition or function of an individual’s physiological system, or a combination of systems—primarily, cardiovascular, respiratory, nervous, and muscular—at a specific point in time. In the context of SMFT, it may be used to infer an athlete’s current (physical) performance capacity or training effects (i.e., training responses).

Training effects [ 4 ] indicate the direction (i.e., positive, negative) and the time course of the effect. Considering the challenges of dichotomizing time course criteria, we opted to use commonly referenced durations [ 4 , 42 ] that align with the context and design of the included studies in our review, classified as acute (i.e., immediate [ 49 ] and up to a 1-week duration [ 47 , 52 ]), short term (typically 1–3 weeks; e.g., congested or intensified periods [ 53 , 54 ], training camps [ 54 , 55 ], exposure to extreme environments [ 56 , 57 ], season break [ 58 ]), and chronic (usually established over several weeks or months of training; e.g., pre-season [ 51 , 59 ], training intervention [ 60 , 61 ]). The first two are commonly referred as transient effects, while the latter typically indicates more ‘persistent’ or ‘enduring’ changes [ 4 ].

Outcome measures include cardiorespiratory/metabolic, subjective, mechanical, or a combination, and are used as proxy (surrogate) measures that reflect (either directly or indirectly) the physiological systems they intend to assess. These are collected continuously within exercise and then aggregated into a summary metric (e.g., mean HR, accumulated ground impacts estimated via accelerometery-derived data), or measured immediately post-exercise (e.g., HRR, blood lactate, RPE).

3.2 Protocol Taxonomy

Information on SMFT protocols was extracted and categorized in reference to two main levels of classification: (1) exercise regimen (continuous or intermittent) and (2) manipulation of exercise intensity (fixed, incremental, or variable). Regarding exercise regimen characteristics, continuous activity represents a constant load exercise bout (typically for at least several minutes), without frequent alterations in velocity or rest periods [ 14 , 62 ]. Alternatively, intermittent is defined as an activity that is interrupted and restarts after a particular time span, characterized by alternated loads and rest intervals [ 63 , 64 ]. Considering all possible combinations of these categories, we subsequently identified five distinct SMFT categories from the available literature (Fig.  1 ), with each category further sub-divided based on the activity mode (running or cycling), movement pattern (linear, change of direction, or multi-directional), and exercise environment (closed, semi-open, or open):

Continuous-fixed category represents a fixed-intensity exercise bout that remains constant for the entire SMFT and intends to elicit a steady state (e.g., 4 min running at 12 km·h −1 ) [ 65 , 66 ].

Continuous-incremental category is characterized by a progressive increase in intensity within (single) or between (multiple) exercise bout(s), whereas each bout lasts for several minutes (e.g., 4 min running with progressive increases in speed, 3 sets × 3-min bouts at 10, 11, and 12 km·h −1 , interceded by 1-min rest periods) [ 11 , 67 ].

Intermittent-fixed category involves reoccurring activities performed at a constant intensity and rest intervals (e.g., four running bouts × 50–60 m at 18–22.5 km·h −1 , separated by 30 s of recovery) [ 49 , 68 ].

Intermittent-incremental category predominantly involves fixed rest periods, while intensity is increased between exercise bouts (e.g., 30-s shuttle runs at 10–14 km·h −1 , alternated by a 15-s rest period and with a-0.5 km·h −1 increment after each bout) [ 59 , 69 ].

Intermittent-variable category represents specific and non-specific standardized drills, and therefore locomotive demands fluctuate during the exercise (i.e., multi-directional movements). This category can be further categorized into drill-based and game-based exercises. Drill-based exercises refer to exercises that do not include competition features (e.g., passing drills) [ 70 ], whereas game-based exercises are characterized with competition features (small-sided games [SSG]) [ 71 , 72 ].

Submaximal fitness tests (SMFT) protocol taxonomy. Each protocol category consists of two levels: (1) exercise (EXE) intensity intermittency (continuous or intermittent) and (2) manipulation of exercise intensity (fixed, incremental, or variable), together yielding five distinct SMFT protocol categories ( shaded areas ). Intermittent-variable can be further categorized into drill-based and game-based formats. Each category can be further manipulated based on the movement (MOVE) pattern (linear, change of direction [CoD], and multi-direction), activity mode (running or cycling), and exercise environment (closed, semi-open, or open)

3.2.1 Application of the Taxonomy to the SMFT Team-Sports Literature

From the 87 included team-sport studies, we identified 100 independently described SMFT. As illustrated in Fig.  2 , the majority of studies in the literature adopted continuous-fixed SMFT (37%), followed by intermittent-incremental (34%), intermittent-variable (15%), continuous-incremental (8%), and intermittent-fixed (6%). Table 1 provides a summary of these SMFT as described in these studies. Continuous-fixed protocols were administered in both running and cycling exercise modes and include linear and change of direction (running protocols) movement patterns performed at absolute or relative standardized intensities. Continuous-incremental protocols comprised incremental exercises that were terminated when a specific internal (e.g., HR) or external (e.g., speed) intensity was achieved. Intermittent-fixed SMFT solely involved short-duration (8–12 s), high-intensity standardized bursts (~ 50–60 m). Intermittent-incremental SMFT incorporated shorter versions of the most common intermittent shuttle fitness tests, such as the Yo-Yo Intermittent Recovery Tests (Yo-YoIR1&2) [ 46 ] and 30–15 Intermittent Fitness Test [ 5 ]. Finally, intermittent-variable SMFT were mostly administered as game-based practices, including non-specific (handball, touchdown games) and specific (SSG of the sport) exercises, while drill-based practices included a variety of passing exercises (Table 1 ).

Frequency of submaximal fitness test (SMFT) categories and their highlighted individual tests as identified in the team-sports literature. A detailed description of these tests is highlighted in Table 1 . 30-15IFT 30–15 Intermittent Fitness Test, HIR high-intensity runs, Int. intensity, ISRT interval shuttle run test, MSFT multi-stage fitness test, PCW physical capacity work, Yo-YoIE2 yo-yo intermittent endurance level 2, Yo-YoIR1 yo-yo intermittent recovery test level 1, Yo-YoIR2 yo-yo intermittent recovery test level 2

4 Outcome Measures

We identified 202 total outcome measures used in previous team-sports research. As shown in Fig.  3 , cardiorespiratory/metabolic were the most used outcome measures (66%), followed by mechanical (28%) and subjective (6%). The following sections present the outcome measures corresponding to each response type, discuss their putative underlying mechanisms, and synthesize the current available evidence examining their changes within the SMFT framework.

Frequency of submaximal fitness test (SMFT) outcome measures as identified in the team-sports literature. Heart rate (HR)-derived indices are the most common cardiorespiratory/metabolic outcome measures and include variables representing exercise intensity (HRex) and recovery (HRR and HRV). Level 1–2 mechanical outcome measures represent locomotor activity variables collected during SMFT that are standardized by an internal stimulus or intermittent-variable exercise such as small-sided games (SSG). Level 3 mechanical outcome measures are response measures derived from inertial measurement units (micro-electrical mechanical systems [MEMS]) for monitoring neuromuscular status. Subjective outcome measures represent tolerance to effort and have been monitored solely via ratings of perceived exertion (RPE). AL accelerometry-load, AL V (AU) the vertical vector magnitude component of tri-axial AL, AL V (%) percentage contribution of the vertical vector magnitude component to tri-axial AL, ANS autonomic nervous system, CT contact time, Force-load (fL) sum of estimated ground-reaction forces during all foot impacts, GPS global positioning system, Ln rMSSD natural log of rMSSD, Ln SD1 log-transformed standard deviation of successive R spikes measured from Poincaré plots, rMSSD root mean square of the sum of all differences between successive normal heartbeats, Velocity-load (vL) sum of distance covered weighted by the speed of displacement (in SMFT refers to the actual mean velocity), + positive, − negative

4.1 Cardiorespiratory/Metabolic Outcome Measures

In team-sports settings, which can include a large number of individuals who may possess different aerobic capacities [ 8 , 12 ], it is difficult to implement SMFT that are standardized by internal intensity variables. Accordingly, the majority of the tests were applied by standardizing the external intensity and measuring the corresponding internal responses [ 10 , 73 ]. A variety of cardiorespiratory/metabolic outcome measures have been used in the literature, with the most common being HR-derived indices (Fig.  3 ). These include variables collected during (exercise HR [HRex]), immediately after (HRR) the SMFT, and soon after (HR variability [HRV, vagal-related HRV]) the SMFT. Other measures include blood markers (e.g., blood lactate) [ 67 , 74 , 75 , 76 , 77 , 78 , 79 , 80 ] and oxygen consumption-related parameters (e.g., oxygen uptake) [ 75 , 76 , 81 , 82 ]. As blood and oxygen uptake outcome measures are time consuming, expensive, and obtrusive, their viability to provide standardized and repeatable response measures is considered limited, particularly in team sports [ 11 , 83 ]. Accordingly, we focused on HR-derived indices.

4.1.1 Exercise HR

Exercise HR is collected during the SMFT and is analyzed in absolute (beats per minute; HRex[a]) [ 47 , 59 , 66 , 69 , 74 , 75 , 76 , 81 , 82 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 ], relative to maximal HR (HRex[%]) [ 6 , 11 , 12 , 45 , 46 , 48 , 51 , 54 , 56 , 57 , 59 , 60 , 61 , 66 , 67 , 70 , 79 , 83 , 84 , 89 , 91 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 ], or HR reserve (HRex[reserve]) [ 53 ] values. A variety of methods are used to derive HRex, with the majority calculating the mean HR during the last 10–60 s of the test [ 11 , 12 , 45 , 47 , 48 , 51 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 66 , 69 , 71 , 74 , 75 , 82 , 83 , 85 , 86 , 87 , 89 , 92 , 93 , 96 , 97 , 103 , 105 , 106 , 107 , 108 , 111 , 113 , 114 , 115 , 117 , 119 , 120 , 121 ]. Other approaches calculate the mean HR during the overall test (particularly during intermittent-variable protocols such as SSG) [ 70 , 71 , 79 , 94 , 98 , 101 , 104 , 116 ], specific fixed timepoints [ 6 , 46 , 59 , 60 , 84 , 91 , 95 , 99 , 105 , 109 , 110 , 112 , 122 ], or peak values observed [ 61 , 90 ].

4.1.2 HR Recovery

Heart rate recovery can be defined as the rate at which HR declines after exercise cessation [ 10 , 39 ] and may be collected with athletes lying supine [ 123 ], sitting upright [ 12 , 45 ], standing [ 89 , 121 ], or walking [ 95 ]. Similarly, HRR can be assessed as the absolute (HRR[ d ]) [ 45 , 47 , 48 , 53 , 54 , 57 , 59 , 61 , 66 , 88 , 100 , 103 , 107 , 114 , 119 , 121 , 123 , 124 ] or relative (HRR[% d ]) [ 12 , 47 , 53 , 60 , 66 , 102 , 104 , 107 , 113 , 114 , 121 ] difference between HRex and HR value observed between 10 and 180 s post-test. Alternative approaches include the actual HR value observed at the end of the designated recovery period in absolute (HRR[a]) [ 66 , 74 , 89 , 95 , 99 , 121 , 125 ] or relative (HRR[%]) [ 66 , 89 , 109 , 110 , 113 ] values, or the overall mean HR during a variety of fixed time intervals [ 90 , 97 , 107 ]. Other approaches calculate time-based variables such as the time required to decrease from between fixed HR values (HRR[ s ]; e.g., time between 80 and 70% HR maximum) [ 79 ], or the time constant of HRR derived from mono-exponential modeling [ 61 , 126 ].

4.1.3 HR Variability

Vagal-related HRV is defined as the variability in the time intervals (usually in milliseconds) between adjacent heartbeats and reflects the regulation of cardiac autonomic nervous system (ANS) balance [ 10 , 127 ]. Heart rate variability indices are commonly collected in a seated or supine position and resting state in a laboratory or quiet room [ 10 , 42 ]; however, as the aim in SMFT is to monitor the response during and after a given submaximal workload, only HRV parameters observed immediately or soon after the test are relevant for SMFT in team sports. These measures are usually analyzed within a window of 3–5 min post-test cessation, and predominantly calculated as time domain-related variables such as the square root of the mean of the sum of the squares of all differences between successive normal heartbeats (rMSSD) [ 61 , 107 ] or its natural log (Ln rMSSD) [ 45 , 47 , 53 , 59 , 100 , 106 , 114 ], natural log of standard deviation of successive ‘R spikes’ (the peak of the QRS complex, reflective of ventricular depolarization, recorded from an electrocardiogram wave) measured from Poincaré plots (Ln SD1) [ 57 , 106 ], and the standard deviation of mean interval differences between normal heartbeats (SDNN) [ 61 , 107 ].

4.1.4 Putative Mechanisms

Heart rate-derived indices are commonly used to inform chronic aerobic-oriented training effects, attributed to the linear relationship between HRex and oxygen uptake during an intended steady-state activity [ 10 , 42 ]. A reduction in HRex to a standardized submaximal stimulus may represent improved exercise economy, which may translate to the development of aerobic fitness [ 51 , 120 ], whereas an increment of HRex is considered to reflect a negative response (de-training of the cardiorespiratory system) [ 87 ], likely due to central adaptations (i.e., left-ventricular function) [ 128 ]. In addition, an increment of HRR or vagal-related HRV measures is considered a positive effect [ 59 , 60 , 107 ], reflecting the reactivation of the parasympathetic system and hemodynamic adjustments post-exercise [ 10 ]. That said, in addition to being more time consuming, these measures may be influenced by preceding exercise, with higher intensities eliciting increased blood acidosis that simulate the metaboreflex, and therefore may reduce HR decay post-exercise and alter HRR and vagal-related HRV results [ 10 , 53 ].

The use of HR-derived indices to infer negative transient training effects is inconclusive in the SMFT research. The theoretical basis for their inclusion is due to the potential influence of various training-induced physiological processes that originate in central (i.e., ‘central command’) and peripheral (e.g., afferent feedback from skeletal muscles) body regions to alter cardiac ANS function (i.e., the balance between the sympathetic and parasympathetic systems), and subsequently HR activity [ 10 , 38 , 42 ]. It has been hypothesized that training-induced fatigue or an incomplete recovery might result in a greater muscle activation at a given intensity [ 129 ], promoting increased oxygen demands [ 129 ] and yielding accelerated cardiac sympathetic activity that consequently increase HRex, and reduce HRR and HRV [ 114 , 129 ]. In contrast, previous research has proposed that increased training stress (leading at least to an overreaching state) may cause opposite responses — increased parasympathetic activity or blood plasma volume, consequently lowering HRex and increasing HRR and HRV [ 37 , 38 , 130 ].

4.1.5 Inferring Physiological State

Generally, across a standard micro-cycle, HR measures tend to stay relatively stable [ 45 , 47 ]. For example, studies among youth and senior athletes have observed no changes in the day-to-day variability of SMFT HRex [ 45 , 47 , 75 ] and HRR [ 45 , 47 ] derived from multiple SMFT administered throughout the week, despite substantial variations in training loads. Whilst vagal-related HRV indices are considered to provide a better insight into the cardiac ANS [ 10 , 53 ], daily variations in training loads were associated with stable [ 45 , 47 ] and lower [ 53 ] responses. The disparity across studies may be due to the differences in training loads and HRV variables [ 53 ], an athlete’s fitness levels [ 45 ], or simply reflective of measurement ‘noise’ (either measurement errors or biological variations) [ 10 , 61 , 107 ], thus challenging between-study comparisons.

The current research suggests that the use of HR-derived indices appears more relevant after acute (3–7 days) and short-term (~ 2 weeks) altered training stress, albeit it may cause misleading interpretations. In team sports, 3–4 consecutive days of accumulated training loads have been associated with both increased [ 114 ] and decreased [ 54 ] SMFT HRex. Lower SMFT HRex values have been also observed after 2 weeks of intensified training [ 84 ], but also following a substantial decline in training loads due to season breaks [ 58 ]. Whilst the information in team-sports settings is limited, studies among various cohorts of individual endurance athletes (e.g., cyclists, triathletes) provide encouraging evidence that short-term intensified periods (1–3 weeks) of training-induced fatigue (incurring functional overreaching) lowered SMFT HRex [ 36 , 38 , 130 ] and increased SMFT HRR [ 36 , 37 , 39 ]. These responses likely reflect a complex interplay between acute cardiac ANS function (usually referred to as larger parasympathetic activity) and increased plasma volume [ 37 , 54 , 104 ], leading to an enhanced stroke volume for a similar cardiac output [ 10 , 131 ]. In summary, the use of HR-derived indices (especially HRex and HRR) to infer transient training effects associated with cardiac ANS dysfunction is currently questionable (at least, not straightforward), with no consensus around the underlying mechanisms and conflicting results in the literature.

Overall, SMFT HRex has small to very large inverse relationships with performance indicators (i.e., maximal oxygen uptake, intermittent endurance capacity) when measured concurrently (lower HRex is associated with higher test results) [ 46 , 48 , 105 ], suggesting its validity as an indicator of an individual’s current endurance capacity. Indeed, it has been highlighted that a chronic exposure to internal (e.g., HR-based training impulse, session RPE) [ 86 , 90 ] and external (e.g., total distance covered, force load) [ 71 , 120 ] loads is associated with reduced SMFT HRex. However, the studies examining training effects within athletes have reported contrasting findings. For example, studies in soccer players [ 46 , 91 ] have observed moderate to very large relationships between SMFT HRex and intermittent endurance performance at different timepoints across the season, albeit with no interaction between the magnitude of these effects. Likewise, improved [ 91 ] or maintained [ 87 ] intermittent running ability did not necessarily coincide with reduced or stable SMFT HRex, respectively. In contrast, studies conducted on a variety of team sports and age groups have observed significant relationships between changes in similar markers from pre-season to in-season [ 51 , 59 ], and across a full season [ 107 ]. Improved SMFT HRex were also largely correlated with the changes in running speed at 4 mmol·L −1 blood lactate [ 11 , 83 ] or a reduced oxygen uptake at fixed submaximal intensities (i.e., exercise economy) [ 81 ].

Research observations are more consistent where SMFT HRex has been administered to evaluate the adaptation time course to changes in extreme environments (heat and altitude) during training camps or competitions [ 55 , 56 , 57 , 93 , 104 , 106 , 132 ]. Collectively, HRex displayed substantial deteriorations upon arrival and up to days of exposure [ 56 , 57 , 93 , 104 ], and generally return to baseline values within 6–10 [ 55 , 106 ] or 14 days [ 56 ], with quicker adaptations among highly trained individuals or across repeated exposures within individuals [ 133 , 134 ]. Taken together, whilst it appears that SMFT HRex has the potential to serve as a valid and sensitive marker of positive training-induced effects, it remains questionable whether it can be used as a surrogate measure of within-athlete changes in maximal aerobic capacity. There is, however, considerable evidence suggesting its use during exposure to changes in environments for monitoring the athlete’s acclimatization.

The magnitude of correlations between cardiac parasympathetic-related variables (HRR and vagal-related HRV) and performance indicators is less clear and ranged from no correlation to a very large relationship [ 48 , 66 , 100 ]. The disparity could be the consequence of varied protocol intensities [ 10 , 35 , 42 , 45 ], collection time (e.g., 60 s and 180 s post-exercise) [ 48 ], analysis approaches [ 66 ], and fitness criterion measures [ 45 , 79 ]. Importantly, inferences regarding their long-term validity as proxy measures of chronic training effects can be somewhat impacted by SMFT intensity (i.e., HRex) [ 10 ]. The rate of the sympathetic withdrawal and parasympathetic reactivation post-exercise is altered when the recovery period starts from different intensities. As an example, different absolute HRR values (HRR[ d ]) may be expected 60 s post-SMFT if the exercise intensity varies (e.g., 90% vs 75% HR maximum). Hence, in theory, a significant reduction in HRex might influence the concurrent interpretation of post-exercise outcomes. Although this issue has been addressed by analyzing HRR in relative values (HRR[% d ]) [ 60 , 114 ], the time necessary to decrease from two fixed HR values (HRR[ s ]) [ 79 ], or employing individualized intensity protocols [ 12 ], these are not yet fully understood. In support of this, regardless of the HRR analysis used, authors examining training-induced changes in both HRex and HRR did not always find congruent trends [ 60 , 61 , 102 ]. Other studies observed a lack of association between changes in HRR and endurance performance, despite significant relationships with HRex [ 59 , 107 ]. However, increased SMFT vagal-related HRV has been shown to be more appropriate, with studies reporting its validity for monitoring endurance-oriented training effects [ 59 , 61 , 107 ].

4.1.6 Considerations

Of critical importance when using HR-derived indices as proxies of training effects is the range of confounding factors, such as environmental (e.g., temperature), habitual (e.g., sleep, diet), circadian (time during the day), and psychological (e.g., emotions, stress). These could all contribute to the error of measurement of SMFT HR measures — HRex (coefficient of variation (CV): 1.0–3.5%) [ 12 , 45 , 48 ], HRR (CV: 2.8–13.8%) [ 45 , 48 , 66 ], and vagal-related HRV (CV: 6.6–19.0%) [ 45 , 61 , 106 ] — and should be considered (or standardized where pragmatic) when interpreting changes in HR-derived indices responses to SMFT [ 10 , 42 ].

Further research is warranted to explore the use of all HR-derived measures to infer acute and short-term effects, in particular, verifying the interaction between temporary changes in cardiac ANS function and plasma volume responses. In this respect, it should be highlighted that HR responses may still be less appropriate to denote peripheral neuromuscular fatigue, which are considered more important to monitor delayed recovery and injury risk mitigation in team sports [ 49 , 135 ]. In the longer term, HRex is probably the easiest to collect and most reliable HR measure, and its utility in observing positive changes in aerobic capacity has stronger empirical support. Accordingly, the utility of adding post-exercise (HRR, vagal-related HRV) SMFT HR responses to infer chronic effects may be redundant. In order to enhance interpretations, future research should first determine meaningful changes in SMFT HRex (i.e., smallest worthwhile change) in reference to variations in physiological states.

Finally, when monitoring the responses to intermittent-variable SMFT (e.g., SSG), an athlete’s HRex may be influenced by their locomotor activity. These are likely to differ between tests and should be accounted for when interpreting data. Therefore, we recommend quantifying intermittent-variable SMFT locomotor activity for consideration. Given the reasonable association between internal and external measures during field-based sessions [ 136 ], it is also possible to standardize HRex to a given (fixed) external intensity parameter. Whilst some have attempted to achieve this by dividing the former by the latter, creating a ratio [ 137 ], there are complex statistical properties and assumptions associated with such indexes, presenting as a major validity concern [ 138 ]. To appropriately examine HRex while holding external intensity parameters constant, we recommend linear regression techniques, which do not violate statistical assumptions and achieve the desired outcome ratios [ 139 ].

4.2 Subjective Outcome Measures

Subjective measures are recognized by their ability to serve as gestalt measures that can be used across different exercise typologies, given their feasibility and low cost [ 140 ]. These are commonly applied to quantify an athlete’s perception of intensity and training effects. The former is derived solely from RPE, while the latter are commonly referred to as athlete-reported outcome measures [ 141 ] of latent response constructs such as readiness, wellness, and stress. [ 4 , 142 ]. Accordingly, RPE are the only subjective outcome measures that can be applied to SMFT, a notion supported by their exclusivity (albeit the limited number of studies) in the team-sport SMFT literature (Fig.  3 ). Among the available studies, different scales such as the Category-Ratio 10 (CR10 deciMax) [ 50 , 56 , 57 , 67 , 70 , 98 , 108 ], 100 (CR100® centiMax) [ 91 ], and 6–20 (Borg’s 6–20) [ 75 , 76 ] have been adopted, using a variety of collection protocols (during the last 180 s, immediately, and up to 5 minutes post-exercise).

4.2.1 Putative Mechanisms

Perception of effort is defined as the ‘conscious sensation of how hard or strenuous a physical task is’ [ 142 ], and mainly depends on how easy or hard it is to breathe and drive the working muscles during exercise [ 143 ]. As part of SMFT, the athlete provides a retrospective appraisal of perceived effort to a standardized stimulus that can be prescribed by either an objective internal [ 32 ] or external [ 91 ] means. Because RPE is strongly associated with cardiorespiratory, metabolic, and neuromuscular measures of exercise intensity [ 144 , 145 ], and influenced by the mental state [ 142 ], changes in RPE may reflect positive [ 32 , 91 ] or negative [ 37 , 41 ] alterations in the psycho-physiological state. However, given their gestalt nature, it is perhaps difficult to align RPE as a proxy to a single physiological system during SMFT. For example, RPE has been empirically associated with HRex during a continuous exercise [ 144 ] and might therefore be used as a cardiorespiratory proxy measure. However, spinal or supraspinal motoneuron inhibition, which is a neuromuscular phenomenon, can increase central motor command and subsequently RPE [ 146 ]. This is not to say that RPE cannot be used to infer a physiological state during SMFT, but rather the mechanisms may be less precise. In addition, given that RPE has also been used to regulate exercise intensity [ 147 ], RPE can be used as an anchoring intensity variable (i.e., running or cycling at fixed RPE), whereby external outcomes such as velocity or power output are used as response measures [ 148 ]. However, to our knowledge, there is no published team-sports research evaluating the theoretical basis of such SMFT and their actual utility.

4.2.2 Inferring Physiological State

Whilst some evidence supports the use of SMFT RPE [ 58 , 91 , 98 ], it is difficult to support or refute their utility to determine training effects, given the limited data available in team sports. In endurance athletes, RPE have been shown to detect negative transient effects associated with functional overreaching and disturbances in endurance performance [ 30 , 37 , 41 ], while their sensitivity to chronic positive effects is less certain [ 34 , 40 ]. In the team-sports context, one study [ 91 ] showed that reduced SMFT RPE (albeit maintained HRex[a]) was accompanied with enhanced intermittent running performance (Yo-YoIR1) and soccer match high-intensity running [ 91 ]. Another study [ 50 ] in professional soccer players did not observe any significant relationships between RPE collected immediately after an individualized SMFT and athlete-reported outcome measures across 6 in-season standard weeks. Despite the conflicting results, researchers have suggested the potential usefulness of SMFT RPE when measured concomitantly with other objective outcome measures (e.g., HRex) as part of a multivariate monitoring approach [ 37 , 42 , 130 ].

4.2.3 Considerations

It is noteworthy to highlight some of the challenges associated with collecting and interpreting RPE in team sports, where interpretation is challenged by the presence of the coach and peers biasing ratings, the application of unvalidated collection tools, lack of or inappropriate athlete familiarization/education, and their gestalt nature [ 142 , 146 , 149 ]. Moreover, RPE may be confounded by other sensations associated with exercise, such as mood, discomfort, pain, and enjoyment [ 142 ]. In view of these challenges, researchers may consider other perceptual measures such as ratings of fatigue [ 150 ], or techniques such as numerically blinded [ 149 ] and differential [ 151 ] RPE, alongside the implementation of rigorous familiarization processes to facilitate authentic and sensitive perceptual ratings associated with SMFT.

With this in mind, a benefit of SMFT in team sports is to provide an assessment that can be seamlessly integrated into the training session such as the warm-up or standardized drills. The need to collect RPE from all athletes individually (~ 20–40) under controlled conditions is likely to be disruptive and impractical, which is perhaps why there are few studies using this practice. It is probably reasonable to assume that unless SMFT are completed as a discreet activity, with smaller groups, or RPE collection procedures are made more accessible (e.g., mobile devices, human resources), these outcome measures may be less pragmatic or sustainable in team sports.

4.3 Mechanical Outcome Measures

Within the current monitoring schemes in sport, there is a broad classification of external load parameters. These indices represent kinematic outputs [ 152 ] performed by an athlete throughout an exercise bout/session [ 153 ] and have been classified into three distinct levels, which we will use here to audit their implementation in the context of SMFT [ 8 , 154 ].

4.3.1 Level 1–2 Measures

Level 1 variables are typically the locomotor performance outputs including distance covered, time spent, or the count of efforts in different velocity zones, whilst Level 2 variables reflect changes in velocity such as accelerations and decelerations (i.e., change of directions) [ 8 ]. Such kinematic parameters are routinely collected using global positioning systems (GPS) or other tracking technologies (i.e., semi-automated pixel tracking, local positioning systems). In the team-sports SMFT framework, the use of level 1–2 mechanical outcome measures can occur in two scenarios: (1) monitoring the speed achieved to a submaximal exercise that is standardized by internal intensity responses [ 50 , 155 ] and (2) monitoring the changes in these variables during intermittent-variable standardized drills [ 54 , 58 , 70 , 71 , 72 , 79 , 93 , 98 , 106 , 116 , 132 , 156 ], as they are standardized by a variety of other parameters such as duration, sets, recovery, and unique constraints (e.g., number of players, rules modifications). Whilst the former is considered less practical in team sports for many pragmatic reasons, the latter are implemented as a part of the training plan, encompassing sport-specific actions and are perhaps the most feasible to apply routinely [ 8 , 131 ]. Conceptually, higher values (e.g., accumulated distance covered), coupled with stable or lowered internal responses are indicative of positive effects (i.e., improved exercise performance) [ 8 , 58 , 98 ]. In fact, studies examining level 1–2 variables during drill (passing) and game (SSG) exercises have highlighted their pragmatic advantages to deliver information related to an athlete’s performance [ 58 , 71 , 72 , 98 ]. However, it should be noted that intermittent-variable SMFT are influenced by a variety of individual and contextual factors such as technical level, motivation, and tactics [ 8 , 72 ], and have a higher degree of variation (test–retest reliability) compared with other SMFT modalities [ 70 , 72 , 156 , 157 ], and therefore, should not necessarily be interpreted in the aforementioned simplistic manner [ 8 ].

4.3.2 Level 3 Measures

Level 3 external load variables are derived from inertial-measurement units such as tri-axial accelerometers, magnetometers, and gyroscopes [ 8 ] (collectively referred to as micro-electrical mechanical systems [MEMS] [ 158 , 159 ]). Unlike level 1–2 variables, these outcome measures can be used for the majority of SMFT applied in the team-sport context and have been proposed to provide an insight into an athlete’s neuromuscular system, given their potential link with lower limb vertical stiffness [ 135 , 154 , 160 , 161 , 162 , 163 ]. Vertical stiffness is considered to affect several athletic parameters, including elastic energy storage and utilization (i.e., stretch shortening cycle) [ 164 ] and has traditionally been measured through a variety of jump assessments (counter-movement, hopping, and drop jumps) using variables such as jump height, contact time, and flight time [ 52 , 165 , 166 ]. However, because of the limited viability of these assessments in field conditions and their lack of specificity (jumping activities may be less sensitive to detect changes in running strategies) [ 52 , 161 ], researchers and practitioners have started to collect proxy variables required to estimate vertical stiffness derived from MEMS during SMFT [ 49 , 52 , 55 , 65 , 68 , 72 , 167 , 168 , 169 ].

To date, studies have adopted accelerometer-derived vector magnitudes (collectively termed in this review as accelerometery load [AL]) [ 49 , 52 , 72 ] and individual vector components (vertical AL [AL V ], antero-posterior AL [AL AP ], and medio-lateral AL [AL ML ]) using MEMS-embedded accelerometers [ 49 , 52 , 68 , 167 , 168 ], predominantly collected during intermittent-fixed protocols comprising high-intensity running bursts [ 49 , 52 , 55 , 68 , 168 ]. Generally, reduced AL in the vertical plane (AL V [arbitrary units; AU]), or the percentage contribution of the AL V to the overall tri-axial vector magnitude (AL V [%]) during SMFT have been postulated as an indicator of reduced leg vertical stiffness and subsequently inferred a degree of lower limb neuromuscular fatigue [ 167 , 170 ]. Theoretically, reduced vertical stiffness may lead to reduced efficiency for the same speed through altered kinetic and kinematic parameters, such as reduced vertical ground-reaction forces and increased ground-contact time [ 161 , 170 ], which likely lead to decreased stride length [ 52 , 72 ] and elevated energy cost [ 162 , 170 ].

4.3.3 Inferring Physiological State

Studies investigating acute effects on mechanical outcome measures during SMFT are scarce, and those available are quite disparate in terms of protocols, variables, and their analytical processes [ 49 , 52 , 65 ]. Nonetheless, there is an emerging agreement from these studies that suggest AL measures can provide sensitive indicators of an athlete’s neuromuscular fatigue and efficiency. In a group of professional soccer players who performed an intermittent-fixed SMFT (4 ×  ~ 60-m runs, alternated by an ~ 30 s recovery) before and immediately after a training session, Buchheit et al. [ 49 ] found that various AL variables respond differently to different training modes (strength, speed, endurance-oriented conditioned sessions). Estimated vertical stiffness slightly increased after all training modes, whereas propulsion efficiency (the ratio between velocity loads and force load; refer to Fig.  3 for variables) was session dependent (largely increased after strength, small and moderate decreases after endurance and speed, respectively), suggesting its sensitivity to detect changes in running strategies (hypothetically, horizontal force application capability) [ 8 , 49 ]. A study [ 52 ] using a similar SMFT protocol and outcome measures among university rugby union players reported large relationships between the vector magnitude and vertical accelerometer components derived from the constant phase of the run, versus leg stiffness measured more directly via submaximal hopping test performed on a force platform [ 52 ]. Whilst only trivial effects were observed in leg stiffness over the week, the changes in SMFT AL data were large [ 52 ].

Similar trends were also found in a study investigating the alterations in triaxial AL data collected before, 48 h, and 96 h after an Australian football match [ 68 ]. A main finding reported in this latter study was that AL V (AU) and AL ML (AU) derived from the constant phase of the SMFT were still impaired 96 h post-match among players who were classified as ‘fatigued’ (> 8% reduced counter-movement jump at a 48 h post-match) [ 68 ]. Finally, a within-individual longitudinal study among soccer players [ 72 ] showed reductions in AL m·min −1 and AL slow·min −1 collected during a standardized SSG (5v5 + 5) 1 day before a match were concomitant with a reduction in neuromuscular function (flight-time:contact-time ratio measured from counter-movement jump) and an altered match running profile—increased AL ML (%) and decreased AL V (%) contribution to AL—indicative of potential neuromuscular fatigue [ 72 , 167 ]. Collectively, it appears that specific mechanical level 3 metrics may be useful for identifying acute variations in performance (neuromuscular fatigue and efficiency) associated with changes in lower limb function.

Inferring longer term training effects from mechanical variables ascertained during SMFT has received limited attention, and insights have been drawn exclusively from tracking locomotor outputs during intermittent-variable SMFT in the form of SSG. For example, moderate to very large positive relationships have been reported between higher level 1–2 outputs during SSG and intermittent running capacity [ 98 ]. In addition, during intensified training camps (1–2 weeks), within-individual increases in running parameters (e.g., total and high-speed running distance) measured during intermittent-variable SMFT were also concordant with improved intermittent running capacity [ 54 , 106 , 132 ]. Of note, the utility of all mechanical outcome measures (levels 1, 2, and 3) derived during SMFT in detecting chronic training effects in neuromuscular function such as improved running efficiency (enhanced muscle–tendon unit recoil) is unknown.

4.3.4 Considerations

Most studies adopting mechanical outcome measures to denote acute neuromuscular effects have administered SMFT characterized by intermittent high-intensity bursts. These outcome measures are often sampled from the constant running velocity phases of the SMFT and using the vertical accelerometer component [ 52 , 68 , 167 ], perhaps owing to an enhanced association with vertical stiffness [ 52 , 68 ]. Typically, such techniques have demonstrated an inferior degree of reliability (CV: 6.7–17.5%, standardized TE: small to moderate [ 49 , 52 , 68 ]) versus maximal and non-running-based assessments of neuromuscular function (jump and force indices; CV: 2.9–6.1%, standardized TE: small [ 49 , 65 , 171 ]). One study [ 65 ] using a continuous-fixed SMFT and lower running intensity (mean velocity of 12 km·h −1 compared to 18–22.5 km·h −1 ) reported lower measurement noise (CV: 2.1–8.0%, standardized TE rated as small). Additionally, the changes found in AL V (%) [decreased] and AL ML (AU) [increased] 24 h after a strenuous soccer training session were greater than smallest worthwhile change (signal-to-noise ratio >  ± 1) [ 167 ]. Although these findings indicate enhanced reliability and sensitivity for SMFT involving lower running speeds in a more continuous manner, this is based on only one study and the utility of different SMFT protocols has not yet been compared in the available literature, and barriers to implementation should also be considered. Similarly, the questionable reliability of locomotor outputs recorded during intermittent-variable SMFT in the form of SSG (total distance CV: 2.3–11.7%, high-speed thresholds CV: 8.1–83.0% [ 70 , 72 , 94 , 98 , 156 , 157 ], small to moderate in magnitude [ 70 , 156 , 157 ], perhaps limits their utility to denote moderate-to-large effects only (i.e., larger CV may decrease the signal-to-noise ratio).

Although studies have suggested that changes in AL variables may reflect effects on lower limb stiffness [ 162 , 172 , 173 ], few have directly assessed stiffness [ 52 , 174 ]. Moreover, these studies have typically used MEMS mounted between the scapulae, which may be influenced by upper-body kinematics during running or dampening of ground-contact vibrations [ 172 , 175 ]. Whilst unit placement may have limited the impact under standardized conditions, intermittent-variable SMFT may be more susceptible to positioning noise as changes in orientation of the MEMS devices are not considered in the quantification of accelerometer metrics. In addition to positioning, users should be cautious of other extraneous factors such as movement artifacts within the device harness, running surface (e.g., ground stiffness), and footwear [ 158 ].

Future work should address the overall convergent validity of MEMS-derived data to obtain an accurate estimation of running strategy characteristics such as vertical stiffness. It is also necessary to investigate the theoretical framework for the sensitivity of these measures and their potential mechanisms (i.e., with respect to human tissue and gait mechanics). Furthermore, more research is required to examine whether protocol characteristics (e.g., exercise regimen, running intensity) and unit placement (e.g., center of mass, foot-mounted MEMS unit) can enhance measurement properties, and therefore facilitate inferences regarding lower limb stiffness and ultimately neuromuscular fatigue or efficiency.

5 Summary and Conclusions

Our review provides an overview of the literature regarding SMFT in team sports, including the development of the SMFT definition, protocol categorization, and a systematic audit of protocols and outcome measures. We also provide a narrative synthesis of the applications of SMFT within the training continuum of sport teams and future research directions (outlined in Table 2 ). In summary, SMFT have the potential to serve as time-efficient, non-exhaustive, and feasible standardized tests that can be administered to a group of athletes simultaneously as a part of the warm-up and using specific drill(s) during the training session. Multivariate outcome measures such cardiorespiratory/metabolic (e.g., HR-derived indices), subjective (e.g., RPE), and mechanical (GPS and MEMS-derived data) can be collected simultaneously, and in theory, provide a multifactorial evaluation for athlete monitoring in team sports. Collectively, the literature suggests that several outcome measures collected during and immediately post-SMFT can inform on an athlete’s physiological state. Heart rate-derived indices seem more appropriate to denote positive chronic training effects on endurance performance, whereas their role in detecting negative transient effects associated with variations in ANS function is questionable. Despite the lack of knowledge about the underlying mechanisms and the inconsistent findings between studies, their sensitivity appears to improve after days or over short periods that are characterized by substantial alterations in training stress, seemingly caused by an interaction between cardiac ANS status and plasma volume responses. Subjective outcome measures are less common in team-based sports and only global RPE have been adopted thus far. Although their validity and practicability have yet to be established, researchers have proposed their utility when measured concomitantly with other objective measures (e.g., HRex) as part of a multivariate monitoring system. Mechanical outcome measures are relatively novel and have been mostly investigated using intermittent-variable and intermittent-fixed protocols, whereby the former primarily involves GPS-derived kinematic variables (levels 1–2) to monitor exercise performance, while the latter includes response measures derived from inertial measurement units (level 3) to monitor lower limb neuromuscular function. Whilst monitoring locomotor outputs during standardized training drills is more feasible and has shown to provide valuable data on an athlete’s performance, practitioners should consider the large influence of various individual and contextual factors (e.g., technical/tactical level, motivation) that may undermine their interpretations. Accelerometery load parameters can provide sensitive indicators of acute changes in lower limb function and therefore neuromuscular fatigue and efficiency, albeit the overall validity of these outcome measures and the physiological mechanisms underpinning their changes have not yet been fully evaluated. Moreover, there is an absence of information on the use of all mechanical metrics (levels 1–3) to monitor chronic training effects. Finally, future research should also examine the methodical elements (e.g., protocol characteristics, collection, and analytical processes) related to SMFT to derive the most appropriate protocol to capture reliable, valid, and sensitive outcome measures that provide useful inferences regarding an athlete’s physiological state.

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The authors thank the authors who provided additional data for the studies included in our review. We also thank the reviewers for their constructive comments and contributions.

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Shushan, T., McLaren, S.J., Buchheit, M. et al. Submaximal Fitness Tests in Team Sports: A Theoretical Framework for Evaluating Physiological State. Sports Med 52 , 2605–2626 (2022). https://doi.org/10.1007/s40279-022-01712-0

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The Training Characteristics of World-Class Distance Runners: An Integration of Scientific Literature and Results-Proven Practice

• Thomas Haugen   ORCID: orcid.org/0000-0001-5929-0389 1 ,
• Øyvind Sandbakk 2 , 3 ,
• Stephen Seiler 4 &
• Espen Tønnessen 1  

Sports Medicine - Open volume  8 , Article number:  46 ( 2022 ) Cite this article

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In this review we integrate the scientific literature and results-proven practice and outline a novel framework for understanding the training and development of elite long-distance performance. Herein, we describe how fundamental training characteristics and well-known training principles are applied. World-leading track runners (i.e., 5000 and 10,000 m) and marathon specialists participate in 9 ± 3 and 6 ± 2 (mean ± SD) annual competitions, respectively. The weekly running distance in the mid-preparation period is in the range 160–220 km for marathoners and 130–190 km for track runners. These differences are mainly explained by more running kilometers on each session for marathon runners. Both groups perform 11–14 sessions per week, and ≥ 80% of the total running volume is performed at low intensity throughout the training year. The training intensity distribution vary across mesocycles and differ between marathon and track runners, but common for both groups is that volume of race-pace running increases as the main competition approaches. The tapering process starts 7–10 days prior to the main competition. While the African runners live and train at high altitude (2000–2500 m above sea level) most of the year, most lowland athletes apply relatively long altitude camps during the preparation period. Overall, this review offers unique insights into the training characteristics of world-class distance runners by integrating scientific literature and results-proven practice, providing a point of departure for future studies related to the training and development in the Olympic long-distance events.

This review bridges the gap between science and results-proven practice regarding how training principles and training methods should be applied for the Olympic long-distance events and identified clear distinctions in training organization between track runners and marathon specialists

The weekly running distance is in the range 160–220 km for marathoners and 130–190 km for track runners, with both groups performing 11–14 sessions per week, and ≥ 80% of the total running volume at low intensity

Training intensity distribution varies across mesocycles and differs between marathon and track runners, but common for both groups is that volume of race-pace running increases as the main competition approaches

Training for long-distance running (LDR) aims to improve the “big three” performance-determining variables: maximum oxygen uptake (VO 2 max; the highest rate at which the body can take up and utilize oxygen during severe exercise), fractional utilization (the ability to sustain a high percentage of VO 2 max when running), and running economy (VO 2 at a given submaximal running velocity). Together, these variables integrate the sustained ability to produce adenosine triphosphate (ATP) aerobically and convert muscular work to power/speed [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. International runners demonstrate different combinations of these determinants, as an “acceptable value” in one variable can be compensated for with extremely high values in the other variables. In addition, a “fourth variable,” neuromuscular power/anaerobic capacity, plays an important role in the decisive end phase of tactical track races [ 12 ]. Further, classic laboratory testing may not capture a “fifth variable,” fatigue resistance associated with specific adaptations that delay muscular deterioration and fatigue and enable maintaining race pace over the final 7–10 km of an elite marathon [ 13 , 14 ]. Different time courses in the development of these performance determinants are very likely. This is exemplified by a case study of former marathon world record holder Paula Radcliffe who improved running economy by ~ 15% between 1991 and 2003, while \(\dot{\text{V}}\) O 2 max remained essentially stable at ~ 70 ml kg −1  min −1 [ 5 ].

Most world-class long-distance runners engage in systematic training for 8–10 years prior to reaching a high international standard [ 15 ]. Different pathways to excellence have been described, as both early and late specialization, and different backgrounds from other sports, can provide a platform for later elite LDR performance [ 15 , 16 , 17 , 18 ]. Several scientific publications during the last two decades have described the training characteristics of world-leading distance runners [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. However, our understanding of best-practice LDR continues to evolve, and it is fair to say that positive developments in modern long-distance training methods have often been driven by experienced coaches and athletes rather than sports scientists [ 32 ]. Sport scientists have historically found themselves testing hypotheses regarding why elite athletes train as they do rather than driving innovation around the how in the training process. Tightly controlled and adequately powered laboratory studies that span the months-to-years timescales associated with maximizing all the above-mentioned physiological variables impacting LDR performance have been essentially infeasible if not impossible.

Publicly available coaching philosophies and training logs of podium contestants from international athletics championships and world marathon majors constitute a corpus of descriptive training information for the international long-distance community. It is tempting to call this corpus of information made available by international champions a description of training “best practice,” but some of our colleagues in the sports science community would reasonably argue that we can only know that this is results-proven practice, not if it is best practice. Combining and cross-checking data sources from available research evidence and results-proven practice provides a valid point of departure for outlining current training recommendations and for generating new hypotheses to be tested in future research [ 33 , 34 , 35 , 36 ]. This integrative approach also facilitates unique insights into training characteristics that previously have been scarcely investigated, altogether allowing a more holistic picture of “state-of-the-art” LDR training.

The objective of this review is therefore to integrate scientific and results-proven practice literature regarding the training and development of elite LDR performance. Within this context, we will particularly explore areas where the scientific literature offers limited information compared to results-proven training information. Moreover, the distinctions between training characteristics of the most successful marathon runners and track runners (i.e., 5000 and 10,000-m specialists) will be highlighted since they organize their training year differently. Although anchored in the standard Olympic running distances, this review is also relevant for other endurance sports.

Methodological Considerations

The scientific literature supporting this narrative review was obtained from PubMed, using varying combinations of the search terms “endurance,” “long distance,” “marathon,” “training,” “conditioning,” “running,” “elite,” “high performing,” world-class,” “runners, ” and “athletes.” In addition, we searched for non-scientific, publicly available, and English-language training information related to podium contestants from international championships (i.e., Olympic Games [OG], World Championships [WC], and continental championships) and world marathon majors. Most of the training data were obtained from websites (Runner Universe, Sweat Elite, Running Science, LetsRun, and RunnersTribe) dedicated to providing the athletics community an expansive library of information written by top athletes and coaches. Within these websites, all relevant training logs and coaching philosophies were purchased/downloaded and reviewed. Training information from doping-banned athletes or coaches were excluded. Moreover, a Google Search for podium contestants (using athlete name and “training” as search terms) and LDR books was performed. Although we cannot guarantee that relevant data have not been overlooked, the search revealed training logs/information from 59 world-leading athletes and 16 coaches of podium contestants [ 15 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 ] (Table 1 ). This information ranged from “typical training week” of various mesocycles to complete annual training logs. Interpretations of longitudinal training logs were weighted more heavily than “short-term” information. Similarly, training information from the 50 s, 60 s, and 70 s was mainly used to provide historical context.

Several limitations to our approach must be acknowledged. Firstly, the inclusion of results-proven training information can be discussed since it is not based on peer-reviewed research. However, elite athletes are systematic in their collection of training “data” and report their training accurately [ 23 , 113 ], justifying the extensive use of training logs as primary or secondary information sources in scientific training characteristics studies within LDR [e.g., 17 – 28 ]. Secondly, an initial review of both the scientific literature and results-proven practice reveals several biases, including a substantial male dominance and focus on a few successful training groups. Additionally, the lack of a common framework (e.g., intensity zones) and terminology can result in misinterpretations. Moreover, the included literature cannot be controlled for possible training prescription–execution differences or changes in training programs over the years. We are also aware that many unsuccessful athletes have applied the same “recipe” as successful runners. Hence, we particularly focus on common key features across varying athlete groups. Finally, the widespread use of doping in international athletics must also be acknowledged [ 114 , 115 ]. The outcomes of this review must therefore be interpreted with these caveats in mind. Sensitive to these limitations, we still contend that integrating scientific evidence and results-proven practice is a strong point of departure for outlining state-of-the-art training recommendations and for generation of new hypotheses to be tested in future research.

Training Periodization and Competition Scheduling

Information about the periodization pattern of LDR training over the course of a year is scarce in the scientific literature. Since Arthur Lydiard introduced his periodization system in the late 1950s [ 46 , 47 , 48 ], leading practitioners typically divide the training year (macrocycle) into distinct, ordered phases (meso- or micro-cycles) with the explicit goal of peaking for major competitions [ 15 , 21 , 26 , 27 , 28 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 63 , 67 , 73 , 76 , 92 , 94 , 99 , 100 ]. Because track and marathon specialists organize their training year and competition schedule quite differently, we will treat these groups separately in the remainder of this section.

At least three phases are typically organized within a macrocycle for track runners: a preparation period, a competition period, and a transition period. The transition period begins immediately after the conclusion of the outdoor competition season, typically consisting of 1–2 weeks with rest or recreational training/low-intensive running [ 15 , 39 , 40 , 41 , 42 , 43 , 44 , 49 , 53 , 54 , 55 , 63 , 75 , 87 , 94 ], although some athletes may take ~ 4 weeks completely off [ 73 ]. The preparation period is typically broken up into general and specific preparation. In the general preparation period, the focus is high volume to build an aerobic foundation. From the specific preparation period onward, the focus gradually shifts toward higher volume of specific race-pace intensity [ 40 , 41 , 42 , 43 , 44 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 72 , 73 , 76 , 92 , 93 , 94 , 112 ]. Such organization of training has also recently been verified as highly effective in the research literature [ 116 ] and bears some resemblance with Matveyev’s traditional periodization model based on the training of successful Soviet athletes during the 1950s and 1960s [ 117 ]. While the Matveyev model suggested a dramatic shift from volume focus to intensity focus as the competition period neared, most track runners maintain a high volume of subthreshold endurance training throughout the preparation and competition period and are careful not to overuse race-pace training or introduce it too early in their annual cycle. This is somewhat in contrast to the research literature, where under-performance caused by overtraining/under-recovery tends to be closely associated with high volumes and/or densities of training rather than reduced volume and increased intensity [ 118 ].

Some track runners apply double periodization (i.e., two peaking phases), consisting of a preparation phase, an indoor or cross-country season, a new preparation phase, and finally an outdoor track competition season (typically lasting 3–4 months, starting in May and ending in September) [ 56 , 57 , 68 ]. However, most world-class track runners apply single periodization; they may participate in cross-country or indoor competitions during their preparation phase but use these competitions as part of their training. A review of the competition schedule for the athletes listed in Table 1 (based on their most successful year in an international championship) revealed that track runners participated in 9 ± 3 (mean ± SD) annual competitions, in which 6 ± 3 where outdoor races prior to OG or WC [ 119 ]. About half of the outdoor races were so-called “under-distances” (1500–3000 m), while the remaining half consisted of 5- or 10,000-m competitions. None of the analyzed track runners competed in “over-distances” (e.g., half-marathon) in the 3–4 preceding months leading up to the OG/WC. The last competition prior to OG/WC was performed 4 ± 2 weeks ahead, and 3 ± 2 additional competitions were performed in the subsequent 2–4 weeks after their most successful championship [ 119 ].

Marathon runners periodize their training year differently. The marathon runners listed in Table 1 participated in 6 ± 2 annual competitions in their most successful year, or ~ 50% fewer races than the track runners. These competitions were distributed across 2 ± 1 marathons (separated by at least 3 months), 1 ± 1 half-marathon(s), and 3 ± 3 races over 5–15 km [ 119 ]. Their last competition prior to OG/WC or a World Marathon Major was performed 10 ± 5 weeks ahead. Marathon runners typically apply double periodization centered around spring and autumn marathons, where the 7–14 days following the marathon competitions are completely training free or very easy [ 15 , 112 ]. The 5–6 preceding months leading up to a marathon are typically divided into general and specific preparation [ 40 , 41 , 42 , 52 , 53 , 54 ]. For track runners, the focus gradually shifts throughout the preparation period from achieving high total running volume to achieving more running volume at or near race pace. Progression is either based on extending the athlete’s accumulated session duration at a goal pace [ 40 , 41 ] or establishing high intensity volume and then slowly increasing pace [ 92 ]. Some marathon runners even apply a reverse linear periodization model, with the highest running volumes registered during the preceding weeks of the tapering phase periods as the competition is approaching [ 112 , 120 ].

The underlying mechanisms for the superiority of specific periodization models in LDR remain unclear, and there is no direct evidence enabling us to compare outcomes across various periodization methodologies [ 121 ]. Although scientific comparisons of different training approaches at a macro-level are challenging to perform, future studies should aim to verify and test the concepts developed by the best practitioners over the last decades.

Training Methods

The specific training methods for LDR consist of varying forms of continuous long runs and interval training (Table 2 ). These training methods bear different labels among practitioners, mainly depending on the intention/goal of the training. For example, “easy runs” are somewhat misguidedly termed “recovery runs” or “regeneration” by some coaches [ 40 , 41 ], assuming that their value is merely to “accelerate recovery” prior to the next hard session. No scientific studies to date support this assumption, but the feeling of recovery might be caused by the low load of such short easy runs, causing very little interference with the ongoing recovery process. However, accumulation of high frequency and volume of low-intensity training (LIT) is considered an important stimulus for inducing peripheral adaptations (e.g., increased mitochondrial biogenesis and capillary density of the skeletal muscle) [ 122 ]. Accumulated volume of low intensity running seems to be a characteristic of those with better running economy [ 123 , 124 ], and continuous running is probably most beneficial in stimulating these adaptations [ 125 ]. High volumes of LIT likely promote better “neural entrainment,” decrease movement variability, and reduce energy cost of movement [ 126 ].

The historical view is that, compared to a high frequency of LIT bouts, high-intensity training (HIT) stimulates central adaptations to a larger degree (e.g., increased stroke volume of the heart) [ 127 , 128 , 129 ]. However, in well-trained athletes that are performing a high total volume of training, further increases in \(\dot{\text{V}}\) O 2 max are not consistently observed after periods of increased HIT [ 130 , 131 , 132 ]. However, there is growing evidence that HIT better stimulates peripheral adaptations in fast-twitch motor units via an adenosine monophosphate (AMP) sensitive signaling pathway [ 133 , 134 ]. In sum, HIT and LIT seem to elicit a complex suite of overlapping and complementary adaptations [ 127 , 135 , 136 , 137 ], justifying the judicious application of varying training intensities for performance development in LDR. Further, it is overly simplistic to dichotomize the LDR training process into “high volume” and “high intensity” phases or training bouts. Whether discussing LIT or HIT, resulting adaptive signaling and stress responses can only be understood when the context of accumulated duration is added. Bill Bowerman, co-founder of Nike and US coach at the 1972 Olympics in Munich where Frank Shorter won the marathon, summarized his training philosophy as follows: 2–3 weekly interval sessions, a weekly long run, and fill the rest with as much LIT as you can handle [ 15 , 38 ]. This simple training description holds true for the training organization of most successful long-distance runners during the last 5 decades (see “ Intensity distribution ” section). However, while the interval sessions are considered “key” sessions for track runners, the training organization for marathoners is most often centered around their weekly “long runs.”

Several successful long-distance runners have supplemented their sport-specific training with alternative locomotion modalities, so-called cross-training, including swimming, biking, cross-country skiing, and workouts on elliptical machines [ 15 , 39 , 57 , 94 ]. Arguments supporting the inclusion of cross-training include injury prevention and avoidance of training monotony [ 138 , 139 ]. Because running is associated with lower total training duration and higher mechanical/ballistic load compared to other locomotion modalities [ 140 ], one could speculate if cross-training should be performed to a larger extent among highly trained long-distance runners to provide the same central and peripheral training stimulus with lower muscular mechanical load. Future long-term studies should aim to investigate the possible aerobic training effects of various types of cross-training.

Less specific training forms such as strength, power and plyometric training in small doses (relative to running training dosage) are commonly applied by world-leading long-distance runners [ 15 , 44 , 56 , 57 , 58 , 60 , 65 , 70 , 93 , 94 , 97 , 104 , 111 ]. Even though these training forms do not duplicate the holistic running movement, they likely target specific neuromuscular qualities that underlie running economy. A review of the results-proven practice shows that such supplementary training is typically implemented as a combination of (1) resistance training using free weights or apparatus (squats, cleans, lunges, step ups, leg press, etc.) without causing noteworthy hypertrophy, (2) circuit training with body mass resistance, (3) core strength/stability (e.g., sit-ups and back exercises), and (4) plyometrics in the form of vertical and/or horizontal multi-jumps on grass, inclines, stairs, hills (e.g., bounding, skipping, squat jumps) or jumping over hurdles [ 15 , 44 , 56 , 57 , 58 , 60 , 65 , 70 , 93 , 94 , 97 , 104 , 111 ]. Overall, this supplementary training is poorly described in terms of resistance loading, sets and repetitions, and caution must therefore be made when drawing conclusions. However, it appears that more strength, power and plyometric training are implemented during early-to-mid preparation (about twice a week) compared to the competition period (typically zero or one weekly session) [ 15 , 44 , 56 , 57 , 58 , 60 , 65 , 70 , 93 , 94 , 97 , 104 , 111 ]. Several studies have shown that strength, power and plyometric training 2–3 times per week can improve running economy in long-distance runners [ 11 , 29 , 141 , 142 , 143 ]. Paula Radcliffe improved her vertical jump performance from 29 to 38 cm between 1996 and 2003, a period where she improved her running economy and marathon performance considerably [ 5 ].

Training Volume

Most world-leading marathon runners train 500–700 h year −1 , while most corresponding track runners are in the range 450–600 h year −1 [ 15 , 40 , 41 , 42 , 43 , 54 , 73 , 76 , 79 , 87 , 94 ]. The relatively broad ranges in training volume are also present in other endurance sports [ 132 , 144 , 145 , 146 , 147 , 148 , 149 , 150 , 151 , 152 , 153 ] and are likely explained by individual differences in mechanical training load tolerance, intensity distribution, risk willingness, training age/career stage, application of cross-training, genetics and perhaps also psychological factors. The present training volume observations are in line with other studies of top-class long- and middle-distance athletes [ 19 , 20 , 21 , 27 , 28 , 34 ], but a larger proportion of middle-distance training is devoted to strength, power, and plyometric training (particularly in 800-m runners) [ 34 ]. Successful endurance athletes in cross-country skiing, biathlon, cycling, triathlon, swimming, and rowing train considerably more (800–1200 h per year) [ 132 , 144 , 145 , 146 , 147 , 148 , 149 , 150 , 151 , 152 , 153 ]. This is likely explained by the fact that LDR is a weight-bearing exercise where rapid plyometric muscle actions put high loads on muscles and tendons during each step. Accordingly, both total training volume and the duration of low-intensity sessions are relatively low for LDR compared to the other endurance sports [ 140 ]. To obtain a relatively high training volume, world-leading athletes seem to compensate by running twice a day most of the week [ 40 , 41 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 79 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 ].

Many long-distance runners accumulate much of their running kilometers on dirt roads/forest paths instead of paved roads to reduce mechanical loading and maximize training volume. This indicates that the running movement per se is not the main contributor to limited training tolerance, but rather the leg-surface interaction and resulting forces [ 140 ]. Running surface is a specific aspect of training periodization for marathoners. Because major marathons are performed exclusively on hard, paved roads, marathon specialists will build in continuous runs of increasing duration on asphalt or similar hard surfaces as they specifically prepare for these events [ 15 , 41 ].

A discussion of training volume and the constraints created by mechanical interactions between runner and running surface would be incomplete without mentioning running shoes. Recent developments on the footwear front have received massive attention in the LDR community. The “super-shoe” was introduced to road running in 2016 and to track running in 2019, chronologically coincident with a wave of LDR records. These shoes are now subject to strict guidelines and testing [ 154 ]. The footwear features behind these performance improvements include shoe weight, material composition, heel thickness, and bending stiffness, altogether improving running economy (and thereby performance) significantly [ 155 , 156 , 157 , 158 ]. Importantly in the context of LDR training, anecdotal evidence (i.e., our discussions with national-level distance runners) also suggests less muscle soreness and increased training tolerance with the recent shoe technology, altogether facilitating slightly increased running volume. Future studies should investigate how the current rapid development in shoe technology will affect LDR training characteristics.

While most scientific studies tend to only report training volume across macro- and mesocycles [e.g., 17 , 21 , 27 , 28 ], the results-proven practice describes more detailed fluctuations throughout the training year. Because most injuries are attributed to rapid and excessive increases in training load [ 159 , 160 ], elite performers increase the total running volume gradually during the initial 8–12 weeks of the macrocycle. The initial training week is performed with ~ 40–60% of peak weekly running volume, increasing by ~ 5–15 km each week until maximal volume is reached [ 62 , 63 , 90 , 94 , 95 , 100 , 103 ]. This volume progression is mainly achieved by increasing training frequency in the initial phase, then subsequently raised further by lengthening individual training sessions. Variations in training volume progression rate seem to depend on training experience and individual predispositions. The younger the training age, and the longer the transition period, the more careful progression from early to mid-preparation within the macrocycle.

Typical weekly running volume in the mid-preparation period is ~ 160–220 km for marathon runners [ 15 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 111 , 112 ] and 130–190 km for track runners [ 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 112 ], distributed across 11–14 sessions. Peak weekly volume can reach 20–30 km higher values for both groups, but only for short periods (2–3 weeks) of time. These wide ranges must be interpreted in the context of running intensity. Some marathon runners cover “only” 130–150 km wk −1 ; however, a considerably higher proportion of their volume (25–30%) is at or near marathon race pace, compared to others who cover 220–240 km wk −1 , with only 15–20% at or near marathon pace [ 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 112 ]. Training volume in elite LDR increases ≤ 8–10% annually in their late teens and early 20s, before slightly declining and stabilizing in their mid-20s [ 17 , 18 , 49 , 53 , 54 ]. The difference in volume between marathon and track runners is mainly explained by fewer running kilometers per session for track runners, as training frequency is equal for both groups. As shown in Table 2 , some long-run sessions for marathon runners may last up to 60 min longer compared to track runners.

One could argue that the ~ 10% slower running velocity in women [ 161 ] should be compensated for with less covered distance to ensure the same running duration between sexes. A counterargument is that men and women should apply equal distances during practice because they compete in the same disciplines [ 40 , 41 ]. We observed no sex differences in distance covered among the track runners in this study. The analyzed female marathon runners covered ~ 5% (~ 10 km) less distance but trained 30–40 min wk −1 longer than males [ 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 ]. We can only speculate if the longer training duration is to compensate for the less covered distance.

Overall, total running volume has remained relatively constant among world-leading long-distance runners since the 1950–1960s [ 15 , 46 , 47 , 48 , 78 , 80 , 81 , 82 ]. Some athletes have applied considerably higher volumes (≥ 300 km wk −1 ), seemingly experiencing more challenges related to injury management and fatigue [ 15 ]. Based upon both biomechanical and physiological factors, it is tempting to speculate that lighter athletes tolerate higher running volumes over time compared to their heavier counterparts. Assuming runners spend half the step cycle time on the ground, then the vertical forces exerted upon the ground must be twice the athlete’s body weight. Hence, the higher the body weight, the higher the impact forces during the landing phase. Moreover, slim runners possess superior thermodynamical conditions, as their sweat surface area to heat producing volume ratio increases with decreasing body size [ 162 ].

Intensity Zones

While training volume in endurance sports is straightforward to quantify, training intensity quantification is more complicated. The preponderance of scientific and results-proven practice recommends that intensity scales/zones/domains in LDR should be based on physiological parameters (e.g., heart rate ranges, ventilatory/lactate thresholds), external work rates (running pace or types of training), or perceived exertion [ 17 , 18 , 21 , 22 , 25 , 27 , 28 , 30 , 40 , 41 , 42 , 54 , 112 , 135 , 163 , 164 , 165 ], but no consensus has so far been established. We would argue that this lack of consensus is consistent with an uncomfortable truth; no single intensity parameter performs satisfactorily in isolation as an intensity guide due to (1) intensity–duration interactions and uncoupling of internal and external workload, (2) individual and day-to-day variation, and (3) strain responses that can carry over from preceding workouts and transiently disrupt these relationships [ 13 , 166 , 167 ]. Consequently, combining external load, internal load, and perception regularly during training provides a triangulation of intensity characteristics that is probably complimentary and informative. Whatever intensity parameter that is chosen, describing and comparing training characteristics requires a common intensity scale. To address this, we have developed both a 3- and 7-zone intensity model (Table 3 ). These are mainly anchored around race pace and reflect the practices of world-leading track and marathon runners. In this way, we can analyze their training logs in more detail. Compared to our previously developed intensity scale for 800/1500-m specialists [ 34 ], this version was deemed more representative because (1) lactate production sessions are rarely performed in LDR, (2) long-distance runners present lower blood lactate values within each intensity zone, and (3) long-distance runners exhibit less pronounced velocity declines with increasing training/repetition duration. Admittedly, presenting two “customized” intensity scales when there is overlap among middle- and long-distance performers may be provocative, but we argue that the present scale better reflects the nature of long-distance training. Indeed, standardized intensity zone systems are imperfect tools and have been criticized for several reasons [ 34 , 135 , 168 , 169 ]. However, the potential error sources seem to be outweighed by the improved communication between coach and athlete that a common scale facilitates [ 34 , 135 ]. The intensity scale outlined here (Table 3 ) can be used as a framework for both scientist and practitioners involved in LDR.

Endurance athletes employ varying methods of intensity distribution quantification. These are anchored around blood lactate ranges, running pace references, “time-in-zone” heart rate analysis calibrated against preliminary threshold testing, or the “session goal” approach where each training session is nominally allocated to an intensity zone based on the intensity of the main workout part [ 112 , 135 , 164 , 170 ]. The method of intensity quantification can affect the calculation of the intensity distribution [ 25 , 168 ]. Based on the nature of available results-proven practice [ 15 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 ], the time/distance-in-zone approach was applied in this review to assess the intensity distribution for the analyzed running sessions.

Intensity Distribution

The description of training intensity distribution in previous studies of long-distance runners can mainly be categorized into the following three models: (1) The pyramidal model, characterized by a large volume of LIT combined with a small volume of moderate-intensity training (MIT) and an even smaller volume of HIT, (2) the polarized model, where the same large volume of LIT is combined with less MIT and more HIT, and (3) the threshold model, where a relatively larger proportion of training is performed in the threshold intensity range demarcated by lactate/ventilatory thresholds 1 (LT1/VT1) and 2 LT2/VT2 [ 17 , 18 , 21 , 25 , 26 , 28 , 112 , 135 , 163 , 164 , 170 , 171 , 172 ]. Indeed, these intensity distribution definitions have been argued to be vague and inadequate, forming a basis for misinterpretations [ 173 , 174 ]. While previous studies have tended to focus on what model is most optimal for performance based on aggregated data for the entire training year [ 17 , 18 , 21 , 25 , 26 , 28 ], the results-proven practice shows that athletes adjust intensity distribution modestly across meso- and micro-cycles (see later paragraphs in this section). It should also be noted that both MIT- and HIT-training sessions are psychologically and physiologically demanding, requiring increased recovery time between blocks or sessions compared to training at lower intensity. In this context, training at “moderate” intensity is relatively more metabolically demanding in highly trained endurance athletes because they can run at a very high percentage of their v \(\dot{\text{V}}\) O 2 max during MIT-sessions [ 6 , 175 ].

The most consistent training intensity characteristic of elite distance runners is that most of the running distance (≥ 80%) is performed at low intensity throughout the training year (corresponding to zone 1 and 2 in our 7-zone scale) [ 15 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 ], in line with previous research [ 15 , 17 , 18 , 19 , 20 , 21 , 22 , 25 , 26 , 27 , 28 , 112 , 135 , 164 , 168 , 169 , 170 , 171 , 172 ]. Most of this training is in turn executed in zone 1, and the duration of the easy runs is very stable throughout the training year. Because zone 2 is closer to marathon pace, a higher proportion of zone 2 is applied by marathon specialists, particularly during the specific preparation period [ 40 , 41 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 100 ]. Weekly long runs are one of the most important sessions for marathon runners in this period [ 40 , 41 ], typically performed as 30–40 km runs slightly below marathon pace. In contrast, an increasingly higher proportion of LIT is performed in zone 1 for track runners as the competition season approaches [ 41 , 72 , 73 , 74 , 75 , 76 ].

Training in zone 3 (in the 6-zone scale) represents 5–15% of the total running volume in elite long-distance runners [ 15 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 ]. However, this proportion can vary across meso- and micro-cycles. There is a trend among marathon runners toward performing a higher proportion of zone-3 training as the major competition approaches [ 40 , 41 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 100 ]. Track runners seem to follow an opposite organization, as the highest amount of zone-3 training is performed in the early-to-mid preparation period, before decreasing when the competition season is nearing [ 41 , 60 , 72 , 73 , 74 , 75 , 76 ]. According to Casado et al. [ 17 , 18 ], tempo runs (continuous running in zone 2–3 in our model) account for ~ 20% of the total annual running volume in world-class Kenyan long-distance runners, corresponding well with observations of Billat et al. [ 20 ] and data compiled here.

Interval training in zone 4–5 also represents 5–15% of the total running volume, but this proportion is inversely related to zone 3-training. That is, marathon runners perform most training in zones 4–5 in the early-to-mid preparation period before replacing such training with more extensive bouts of zone-3 and upper end of zone-2 training as the major competition approaches [ 40 , 41 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 100 ]. In contrast, track runners increase the proportion of zone 4–5 training at the expense of zone 3 as the competition season approaches [ 41 , 60 , 72 , 73 , 74 , 75 , 76 ].

During the pre-competition and competition period, most world-class 5000-m runners perform 1–2 weekly interval training sessions in zone 6 or in combination with zone 5 [ 56 , 68 , 72 , 73 , 74 , 75 , 76 ]. These runners may perform 10–20 km weekly in zone 5–6 between May and August, while most marathoners avoid training with such high amounts of lactic/glycolytic energy release [ 40 , 41 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 100 ].

Distance runners perform sprint training (zone 7 in our model) regularly during the annual cycle, although this accounts for less than 1% of the total running volume [ 15 , 37 , 40 , 42 , 43 , 44 , 49 , 51 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 66 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 85 , 88 , 90 , 91 , 93 , 94 , 97 , 102 , 103 , 105 , 109 , 110 , 111 ]. Sprint training is considered a supplement rather than the main goal of separate training sessions and is typically performed during the last part of the warm-up or after easy long runs. It is generally assumed that sprint training should be performed without accumulation of fatigue (often indicated by increasing levels of blood lactate). The distances are most commonly in the range 60–120 m, with sufficient recovery between each repetition. Most sprint runs are performed with low to moderate rate of acceleration (i.e., strides, progressive runs, hills sprints, or flying sprints), likely because the energy demands during maximal acceleration greatly exceed those at peak velocity [ 176 ]. However, high amounts of endurance training limit the development of muscular power [ 177 , 178 ], and it is unrealistic to expect significant sprint performance development in elite long-distance runners. Hence, sprint training is mainly performed to minimize the negative impact of aerobic conditioning on maximal sprint speed.

In summary, the annual training intensity distribution is very similar for track runners and marathon specialists, as low intensity volume dominates. However, substantial differences may be present within each mesocycle. Both groups increase the volume of race-pace running as the main competition approaches. Table 4 contrasts case study examples of typical training weeks across the annual cycle for a track runner and marathon specialist.

Tapering in elite sports refers to the marked reduction of total training load prior to important competition(s). This is a short-term balancing act, as tapering strategies are intended to decrease the cumulative effects of fatigue while maintaining fitness [ 179 , 180 ]. Because tapering strategies and outcomes are heavily dependent on the preceding training load, it is often challenging to separate tapering from periodization and training programming in general. According to previous research, a successful taper may enhance competition performance in well-trained endurance athletes by ~ 1–3% [ 179 , 180 , 181 , 182 ]. However, this claim is challenging to verify in elite LDR, as numerous confounding external variables (race tactics/pacing, weather conditions, competitors, etc.) influence performance in many important competitions where runners compete for medals and not for the best possible time [ 183 , 184 , 185 ]. It has also been shown that outstanding performances across a 3-month competition period can be achieved, without tapering for a specific competition, by merely reducing the training substantially in the last 4–5 days prior to each competition [ 73 ].

In cases where major competitions are arranged in warm and/or humid cities, and perhaps also many time zones away from the athletes’ regular location, tapering is integrated with time-, heat-, and humidity-acclimatization processes. For more details related to these topics, we refer readers to previously published reviews [ 186 , 187 , 188 ].

The general scientific guidelines for effective tapering in endurance sports include a 2- to 3-week period with 40–60% reduction in training volume adopting a progressive nonlinear format, while training intensity and frequency are maintained [ 179 , 180 , 181 , 182 ]. However, most long-distance runners do not report a substantial decrease in training volume until the last 7–10 days prior to competition [ 61 , 69 , 74 , 75 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 97 ]. Table 5 presents training volume distribution across intensity zones for 10 world-class marathon runners during the countdown to a major competition.

A review of the competition schedule for the athletes listed in Table 1 (based on their most successful year in an international championship) revealed that the last competition was performed 10 ± 5 and 4 ± 2 weeks prior to the season’s main competition for marathon runners and track runners, respectively [ 119 ]. Arrival at the championship destination typically occurs 7–10 days ahead of competition [ 39 , 54 , 57 , 94 ]. The last intensive session (e.g., 10 × 200 m at race pace with optional recoveries) is typically performed 3–5 days ahead of the main championship event [ 40 , 61 , 74 , 75 , 100 ].

Altitude Training

The LDR community became aware of the impact of altitude on endurance performance in the late 1960s and particularly in connection with the 1968 Olympics in Mexico City (2300 m above sea level). Clearly, sufficient altitude acclimatization ahead of endurance competitions at altitudes 1000 m above sea level is required to perform optimally [ 189 , 190 ]. However, many athletes additionally use longer sojourns at altitude to enhance aerobic endurance capacity and thereby performance at sea level, mainly with the goal of increasing red blood cell mass [ 191 ]. Since 1968, > 90% of all OG/WC medals from the 800 m through the marathon have been won by athletes who have lived or systematically trained at altitude [ 9 , 15 , 103 ].

The potential effect of altitude training is influenced by the hypoxic dose, which is a function of the duration of the stay and the altitude [ 192 ]. Most world-class African runners apply the "live high—train high” model, as they live and carry out LIT-, MIT-, and HIT-sessions relatively high (2000–2500 m above sea level) [ 9 ]. Athletes from lowlands typically perform relatively long altitude camps during the preparation period and one camp 2–4 weeks prior to the most important competition, with most emphasis on LIT and MIT-sessions [ 57 , 85 , 100 , 103 , 111 ]. However, the optimal time of return from altitude camps to lowland competition is disputed [ 193 ] and warrants further investigations. The ability to train effectively at altitude may be one feature that distinguishes African runners from their European, American, and Asian competitors [ 9 ]. In all cases, successful use of altitude training by the best long-distance runners is characterized by individualized, well-balanced training load and optimized recovery strategies through adequate sleep, rest and nutritional factors as described in detail elsewhere [e.g., 19 , 194 ].

It has been questioned whether altitude training has positive effects on endurance capacity and sea-level performance beyond the effects achieved with similar training performed at sea level. Here, high-quality scientific evidence is limited, and researchers interpret the current scientific data differently [ 195 , 196 ]. Altitude training research is methodologically demanding due to the difficulty of standardizing the intervention, including control groups, and controlling other psychological and physiological confounders during altitude training. Although research provides limited support for a positive effect of altitude training on sea-level performance in endurance sports, these studies remain scarce and underpowered to detect the small adaptations that may be of importance in elite LDR. This is illustrated through the large individual differences in blood responses documented in connection with altitude training [ 197 ]. Consequently, a nuanced view on altitude training is warranted.

Conclusions

This review integrates the scientific literature and results-proven practice regarding the training and development of world-class LDR performance. Herein, we have outlined a framework for specific characteristics (i.e., training methods, volume, and intensity) and identified the training organization differences between track runners and marathon specialists. Marathon and track runners participate in 6 ± 2 and 9 ± 3 (mean ± SD) annual competitions, respectively. Typical weekly running volume in the mid-preparation period is in the range 160–220 km for marathon runners and 130–190 km for track runners. These differences are mainly explained by fewer running kilometers for each session for track runners, as training frequency (11–14 sessions per week) is equal for both groups. Moreover, ≥ 80% of total running distance is performed at low intensity throughout the training year. In the general preparation period, the focus is to build an aerobic foundation by a large total running volume. From the specific preparation period onward, the volume of race-pace running increases as the main competition approaches. Hence, training intensity distribution models vary across mesocycles and differ between marathon and track runners. While the African runners live and train at high altitude (2000–2500 m above sea level), most lowland athletes apply relatively long altitude camps during the preparation period. The tapering process starts 7–10 days prior to the main competition, typically preceded by a 2–4-week altitude camp. Overall, this review offers novel insights into areas of LDR training that formerly have been scarcely studied in the scientific literature, providing a point of departure for future studies and may serve as a position statement related to the training and development in the Olympic long-distance events.

Availability of Data and Materials

All data and materials support the published claims and comply with field standards.

Code Availability

Not applicable.

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Acknowledgements

The authors want to thank Renato Canova, Michele Zanini, Sondre Nordstad Moen, and Kristian Ulriksen for their thoughtful and valuable inputs and contributions during a process of “stress testing” our interpretation of elite training practice with top practitioners.

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Haugen, T., Sandbakk, Ø., Seiler, S. et al. The Training Characteristics of World-Class Distance Runners: An Integration of Scientific Literature and Results-Proven Practice. Sports Med - Open 8 , 46 (2022). https://doi.org/10.1186/s40798-022-00438-7

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Advances in Regenerative Sports Medicine Research

1 Department of Sports Medicine, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai, China

2 Regenerative Sports Medicine and Translational Youth Science and Technology Innovation Workroom, Shanghai Jiao Tong University School of Medicine, Shanghai, China

3 Regenerative Sports Medicine Lab of the Institute of Microsurgery on Extremities, Shanghai Jiao Tong University Affiliated Sixth People’ Hospital, Shanghai, China

4 National Engineering Research Center for Biomaterials, Sichuan University, Chengdu, China

5 School of Chemistry and Chemical Engineering, Shanghai Engineering Research Center of Pharmaceutical Intelligent Equipment, Shanghai Frontiers Science Research Center for Druggability of Cardiovascular Non-Coding RNA, Institute for Frontier Medical Technology, Shanghai University of Engineering Science, Shanghai, China

Jiangyu Cai

Jinzhong zhao.

Xingge Yu , Shanghai Ninth People’s Hospital, China

Regenerative sports medicine aims to address sports and aging-related conditions in the locomotor system using techniques that induce tissue regeneration. It also involves the treatment of meniscus and ligament injuries in the knee, Achilles’ tendon ruptures, rotator cuff tears, and cartilage and bone defects in various joints, as well as the regeneration of tendon–bone and cartilage–bone interfaces. There has been considerable progress in this field in recent years, resulting in promising steps toward the development of improved treatments as well as the identification of conundrums that require further targeted research. In this review the regeneration techniques currently considered optimal for each area of regenerative sports medicine have been reviewed and the time required for feasible clinical translation has been assessed. This review also provides insights into the direction of future efforts to minimize the gap between basic research and clinical applications.

Introduction

Regenerative medicine utilizes innovative approaches to explore and develop materials that can be used to replace, repair, improve, or reproduce tissues and organs in the human body ( Brody, 2016 ). Sports medicine focuses on aspects of physical health, including the treatment and prevention of exercise-related injuries and aging-related problems that hinder the function of the locomotor system ( Figure 1 ) ( Baby, 2000 ; Foster, 2015 ; Kweon, et al., 2019 ). In the clinical practice of sports medicine, the prevention and treatment of conditions consequently depend on the structural and functional restoration of various related tissues and structures in the locomotor system. When structural integrity cannot be restored through repair, approaches that induce tissue regeneration become necessary, to prevent or delay the use of non-organic structures such as artificial joints. Thus, we explore the field of regenerative sports medicine, which is defined as a science that focuses on the restoration of the structural and functional integrity of the locomotor system, using techniques that induce the regeneration of tissue structures or organs. The approaches currently used in regenerative sports medicine include the utilization of organic and non-organic materials at various structural levels. From a clinical perspective, regenerative sports medicine deals with sports and aging-related conditions in different parts of the locomotor system, such as the menisci, ligaments, tendons, cartilages, and bones.

Sports medicine related injuries.

A meniscus tear is a common knee disorder ( Abrams, et al., 2013 ) that is irreparable in many cases, and consequently, must be addressed via structural restoration with a partial or total meniscectomy to restore its function. Previously, allograft and synthetic menisci have been used with unfavorable or inconsistent clinical results, and thus meniscus regeneration strategies are desirable; however, they require further investigation and development ( Southworth, et al., 2020 ; Li, et al., 2021 ; Veronesi, et al., 2021 ). Knee ligament injuries often result in ligament deficiency and joint instability and necessitate ligament reconstruction ( Pike, et al., 2019 ). To prevent the donor site morbidity that can occur with autografts, the high failure rate with allografts, and the non-graft-bone healing associated with non-receiver transformable synthetic ligaments, receiver transformable regenerative ligaments are considered an alternative choice ( Wang C. et al., 2021 ). Chronic Achilles’ tendon rupture often results in tendon defects that make the direct opposition of the separated tendon ends impossible and graft bridging a necessary choice; consequently, there is demand for regenerative or receiver transformable artificial Achilles’ tendons ( Arshad, et al., 2021 ). Rotator cuff tears are mainly an aging-related condition and quite often irreparable; thus, rotator cuff grafts are required to repair defects and restore the native force chain ( Novi, et al., 2018 ). Though various graft choices are available ( Sunwoo and Murrell, 2020 ), receiver transformable artificial rotator cuff patches are currently considered the ideal option. Sports-related cartilage injuries and osteoarthritis are the main conditions addressed in clinical sports medicine. Clinical approaches such as micro-fractures and autogenous osteochondral graft transplantations are the main strategies used to treat small cartilage defects ( Beck, et al., 2016 ; Guo, et al., 2018 ), while for large cartilage defects, cartilage regeneration is required. Bone defects, fractures, or osteotomies, osteoporosis, and osteonecrosis in the locomotor system, such as glenoid and humeral head bone defects with shoulder dislocations, glenoid bone absorption with severe shoulder osteoarthritis (OA), bone defects with high tibial osteotomy, and tibial plateau depression fractures with knee dislocations, may require bone structure regeneration ( Xie C. et al., 2021 ). In tendon or ligament-to-bone repair, the most important goal is to restore a normal tendon–bone connection with an important fibrocartilage layer ( Tits and Ruffoni, 2021 ). However, after soft tissue-to-bone repair the cartilage layer has been found to reappear inconsistently, which makes tendon–bone interface regeneration a critical issue ( Patel, et al., 2018 ).

In general, regenerative sports medicine has high clinical requirements. In recent years, there has been a large amount of research in this field leading to promising outcomes. In this review, we have assessed the recent progress and assessed the time required for feasible clinical translation of the new techniques and products.

Research Progress in Specific Fields of Regenerative Sports Medicine

Meniscus regeneration, meniscus physiology and the hurdles in regeneration.

The menisci are the semilunar and wedge-shaped fibrocartilaginous tissues between the articular cartilage of the femur and tibia plateau. They have complex 3D structures to absorb shock and distribute its load through collagen fibers which are specifically aligned in a circumferential pattern. The unique zonal phenotypes in the meniscus are histologically and physiologically characterized by two distinct regions: the avascular inner zone (white–white zone), which mainly consists of glycosaminoglycan and type-II collagen with a rounded chondrocyte-like cellular phenotype, and the vascular outer zone (red–red zone), which predominantly contains higher type-I collagen with an elongated ligament-like cellular phenotype. Moreover, these two regions are separated by a middle region (red–white zone), which is a mixture of the inner and outer zones. The limited vascularity of the meniscus indicates a poor healing ability, especially in the avascular white–white zone. Thus, the major hurdles in meniscus regeneration include the inability to replicate its native anisotropic zonal structure and hence, its specialized mechanical function. Moreover, avascularity due to its unique structural properties, and the tibiofemoral articular environment that hinders the healing potency both mechanically and biochemically, have made it challenging for biomedical scientists to create matched engineering constructs for meniscus regeneration.

Surgical Techniques for Meniscus Regeneration

Partial lesions or defects in the meniscus reduce its propensity to heal spontaneously because of mechanical stimuli from the tibiofemoral motions and the avascularity in the white–white zone, leading to degeneration over time. Advances in techniques and tissue engineering strategies have enabled researchers to attempt to repair or regenerate these meniscal defects. Some biological promotion techniques are recommended in clinical scenarios to augment meniscus repair, such as the introduction of bone marrow stem cells using marrow venting techniques, the exogenous addition of fibrin clot, and the stimulation of adjacent healthy meniscus and synovium ( Taylor and Rodeo, 2013 ; Dean, et al., 2017 ; Kwon, et al., 2019 ). Notably, concurrent anterior cruciate ligament (ACL) reconstruction that used bone tunnel to release the cells and growth factors from the bone marrow, has been proven to enhance meniscal repair ( Chahla, et al., 2017 ; Dean, et al., 2017 ; Westermann, et al., 2017 ; Tagliero, et al., 2018 ; DePhillipo, et al., 2019 ). Moreover, partial meniscus replacements offer promising approaches to treat patients with segmental meniscus defects. Both collagen meniscus implants (CMI) from the USA and polyurethane polymeric implants (Actifit) from Europe have been shown to improve clinical outcomes and substantially relieve pain in patients with meniscus defects in both medium- and long-term follow-ups ( van Tienen, et al., 2009 ; Bulgheroni, et al., 2015 ).

Tissue Engineering Strategies for Meniscus Regeneration

Current surgical techniques have failed to promote meniscus regeneration, while many natural or synthetic materials, such as decellularized extracellular matrices, alginate, hyaluronan, polylactides, polyglycolides, and silk have been successfully utilized as scaffolds for meniscus engineering ( Makris, et al., 2011 ). Among these scaffolds, decellularized extracellular matrices derived from the white–white and red–red regions of the meniscus have been shown to promote the differentiation of MSCs toward fibroblastic and fibrochondrocyte phenotypes ( Shimomura, et al., 2017 ). Other types include injectable hydrogels that can be used to address structural defects due to their ability to form structural adaptations ( Athanasiou, et al., 2013 ; Liu, et al., 2017 ). While scaffolds are beneficial due to their ability to incorporate growth factors and their initial mechanical stability, they also indicate recapitulation of the mechanical and biochemical architecture of the native meniscus, with matched stiffness and ingredient gradients ( Higashioka, et al., 2014 ; Steele, et al., 2014 ; Zhu, et al., 2018 ; Zitnay, et al., 2018 ). Notably, when engineering menisci for regeneration, although an obvious choice for cell source might be fibrochondrocytes, regeneration effects are best when fibrochondrocytes are cocultured with other cell subsets ( Hadidi, et al., 2016 ; Koh, et al., 2017 ; Son and Levenston, 2017 ; Sasaki, et al., 2018 ; Xie, et al., 2018 ). Zhang C. H. et al. (2018) used a pre-mechanically stimulated poly (ε-caprolactone) (PCL) scaffold, cocultured with rabbit bone marrow stem cells, for meniscus replacement and found that the pretreated scaffold was a better choice for inducing tissue regeneration ( Figure 2 ).

Orchestrated biomechanical, structural, and biochemical stimuli for engineering anisotropic meniscus. (A) Schematic diagrams for reconstruction of functional anisotropic meniscus; (B) Gross view and low-magnification immunofluorescence (IF) images of native or regenerated menisci at 24 weeks after in vivo implantation in rabbit knees. Green, COL-1; red, COL-2. Copyright 2019 American Association for the Advancement of Science.

Current Clinical Studies and the Challenges in Clinically Translating Meniscus Regeneration

Currently, a few scaffolds for meniscus engineering are undergoing clinical trials with a focus on cell-based therapies. Cell Bandage, a collagen sponge embedded with autologous bone marrow-derived MSCs, was applied in clinical practice to close the torn edges and defects of the meniscus, and is assumed to potentially promote healing. In another clinical trial in humans, a chondrogenic composed of allogeneic bone marrow-derived MSCs was administered to the knee, and reported to effectively decrease visual analog scale pain scores. While meniscus repair products for clinical applications are currently lacking, preliminary outcomes suggest that cell-based therapies are a positive and promising road ahead; however, they have also identified challenges that must be overcome. In the clinical translation of engineered meniscus products, an insufficient source of autologous cells is the primary issue. The use of non-articular cells, however, seems to be a potential strategy to alleviate the scarcity of cells for autologous meniscus therapies ( Makris, et al., 2011 ; Utomo, et al., 2016 ). Additionally, the high-quality autologous neo-tissues required to consistently regenerate the meniscus are difficult to obtain, as demonstrated by the large biological variability observed between donors ( Martin, et al., 2017 ; Vapniarsky, et al., 2018 ; Kwon, et al., 2019 ). Therefore, well-characterized allogeneic tissues and cell banks should be established to enable suitable neo-tissues to be provided stably and avoid disease transmission, and this is likely to solve the intractable problem of biological variability. Furthermore, mechanical biomimicry when engineering the meniscus should be achieved, as the native meniscus allows for frictionless tibiofemoral joint movement and load distribution, which may be related to positive long-term healing outcomes ( Elder and Athanasiou, 2009 ; MacBarb, et al., 2013 ; Huwe, et al., 2018 ). If the meniscus can be successfully generated after overcoming the aforementioned challenges, then the avascular white–white zone of the meniscus leads to difficulties in both implant protection and its integration into existing native tissues ( Arvayo, et al., 2018 ; Vapniarsky, et al., 2018 ). For surgeons and biomechanical researchers, developing appropriate techniques and protocols to enhance the vascular supply to implants should be a priority ( Vapniarsky, et al., 2018 ; Kwon, et al., 2019 ). In addition to vascularity, the engineering meniscus must also adjust to the inflammatory microenvironment, especially in an injured or diseased joint with its complex biochemical conditions. Therefore, modifications by decellularization and antigen removal when engineering the meniscus are required to minimize the immunoreaction of xenogeneic or allogeneic menisci to ensure implant survival and integration. Li et al . (2021) fabricated silk/graphene oxide-based meniscus scaffolds, which consisted of tannic acid and Sr 2+ . The scaffold exerts anti-inflammatory and reactive oxygen species elimination effects, which protect against cartilage degeneration and delay OA development after meniscus injury.

The promising progression which will ultimately lead to the application of tissue-engineered therapies for meniscus regeneration in clinical practice, is evident in current clinical trials. In the near future, tissue engineering strategies may rapidly emerge for the development of meniscus regeneration products, which could potentially provide long-term solutions for patients.

Cruciate Ligament Regeneration

Common strategies for cruciate ligament regeneration.

Knee crucial ligament injuries are common in sports medicine, and often occur during adolescence and young adulthood ( Petersonand Krabak, 2014 ). Ligament reconstruction is the main solution to prevent subsequent cartilage and meniscus damage, thus improving quality of life ( Mastrokalos, et al., 2005 ; Petersonand Krabak, 2014 ). Clinically, autografts and allografts are the two most common graft types used for surgical ligament reconstruction ( Cai J. Y. et al., 2021 ). However, donor site morbidity remains an inevitable problem associated with their use ( Yilgor, et al., 2012 ), and allografts carry additional risks of disease transmission, infection, rejection, low availability and quality, and high failure rates ( Jackson, et al., 1993 ). Xenografts [porcine bone-patellar tendon–bone (BTB)] were harvested by Galili et al. and treated with recombinant alpha-galactosidase and glutaraldehyde for ACL reconstruction ( Galili and Stone, 2021 ). The authors have completed preclinical trials with monkeys and progressed to clinical trials, and they have reported no significant differences in the functional performances of the porcine BTB group and cadaveric allograft group at the 24 months follow up, if the missing/contaminated cases were excluded ( Stone, et al., 2007 ; Van Der Merwe, et al., 2020 ). The potential advantages of the xenografts are that they could help address the quality concerns and availability problems that occur with allografts. However, like allografts, xenografts also have the disadvantages including the possibility for disease transmission, infection, and rejection. Moreover, the process of utilizing an animal originated graft with human tissue, namely graft “humanization,” is difficult and will require further investigation ( Van Der Merwe, et al., 2020 ). To address this, artificial ligaments have been developed in recent years. To date, those ligaments that are clinically available, have been made of non-degradable materials or non-receiver transformable materials, such as polyethylene terephthalate (PET) and polyethylene, which are characterized by their hydrophobic properties and inferior biocompatibility and lead to poor graft-host bone healing after implantation ( Figure 3 ) ( Ai, et al., 2017 ; Cai J. et al., 2021 ). The development of receiver transformable artificial ligaments is another scope and direction for future ligament research.

Electrodeposition of calcium phosphate onto polyethylene terephthalate artificial ligament enhances graft-bone integration after anterior cruciate ligament reconstruction. (A) Electrodeposition of calcium phosphate onto polyethylene terephthalate artificial ligament; (B) The viability and SEM morphology of MC3T3-E1 in the PET, PET/BM-CaP and PET/ED-CaP groups; (C) Micro-CT analysis of the PET, PET/BM-CaP and PET/ED-CaP groups at 12 weeks after surgery; (D) Masson and toluidine blue staining results of pathological sections in the PET, PET/BM-CaP and PET/ED-CaP groups at 6 and 12 weeks after surgery. Copyright 2021 Elsevier.

Artificial Materials for Cruciate Ligament Regeneration

Teuschl et al. ( Teuschl, et al., 2016 ) fabricated novel degradable silk fiber-based artificial ligaments and used biological materials, biodegradable polymers, and composite materials in ligament fabrication for ACL reconstruction in a sheep model. The silk ligaments could induce new tissue ingrowth and stimulate ACL regeneration in vivo . However, the balance between the degradation rate of the materials and the regeneration and remodeling rate of the tissues was not controllable. Furthermore, it is unknown whether the regenerated tissues could maintain the function of the knee at a level similar to that of the native ACL, as functional recovery and a return to sports cannot be fully evaluated in quadruped animal models.

The combined use of receiver transformable and non-transformable materials is an additional option. Mengsteab et al. ( Mengsteab, et al., 2020 ) incorporated PET fibers into the poly (L-lactic) acid (PLLA) bioengineered ACL matrix to fabricate a PET/PLLA hybrid ligament. The hybrid ligament demonstrated great peak loads and promoted the regeneration of ACL in a rabbit model.

Future Perspectives

Despite these encouraging results, further work is required to optimize the properties of newly developed grafts for crucial ligament reconstruction of the knee. We believe that decellularized scaffolds with ready-made collagen and degradable artificial ligaments are the two most promising graft types for ligament reconstruction in future clinical practice. However, prior to application, issues regarding ligament development must be addressed, as it is vital that the host-graph response be regulated and controlled. Moreover, there is a need to explore the regenerative competent microenvironment, which is the articular cavity that can induce tissue ingrowth into the graft. For clinical use, the functional assessment of the knee is more important than the regeneration and healing assessment, as the regenerated or remodeled ligament should be able to mimic the function of the native ligament.

Achilles Tendon Regeneration

Surgical techniques for achilles tendon repair.

End-to-end repair of the chronic Achilles tendon is appropriate when the gap is 2 cm or less, while the V-Y technique, turndown flaps, autograft tendon transfer, and reconstructions using allograft, xenogeneic, or synthetic biomaterials are required for larger defects with or without the preservation of the paratenon ( Kraeutler, et al., 2017 ; Maffulli, et al., 2018 ; Muller, et al., 2018 ; Chen and Hunt, 2019 ). Regardless of the multiple surgical management strategies, the ideal treatment for tendon injury is to promote Achilles tendon regeneration after gap formation ( Sun, et al., 2018 ).

Strategies for Achilles Tendon Repair

The literature on Achilles tendon regeneration is limited mostly to laboratory studies using porcine small intestinal submucosa ( Badylak, et al., 1995 ), acellular tendon matrix ( Gungormus, et al., 2015 ; Zhang C. H. et al., 2018 ), and collagen ( Sun, et al., 2018 ) or collagen gel ( Shen, et al., 2010 ) as scaffolds. Moreover, exogenous cell transplantations such as for tenocytes ( Gungormus, et al., 2015 ) and human amniotic epithelial cells ( Barboni, et al., 2018 ) have been applied but restricted by the cell source, immune rejection, ethics, and injured microenvironment ( Figure 4 ) ( Harris, et al., 2004 ; Sun, et al., 2018 ). Chemokines like SDF-1α and recombinant SDF-1α containing a collagen-binding domain (CBD) have also been reported to promote endogenous tendon regeneration by inducing extracellular matrix production and avoiding the above drawbacks of exogenous cell transplantation in clinical applications ( Shen, et al., 2010 ; Sun, et al., 2018 ). The local application of combined ascorbic acid and T 3 also showed the potential benefits for accelerated tendon healing ( Oliva, et al., 2019 ). Platelet-rich fibrin (PRF), with the delivery of cytokine and growth factor, induced more organized collagen fibers in vivo and promoted tenocyte viability and tenogenic phenotypic differentiation in vitro ( Wong, et al., 2020 ). Similarly, platelet-rich plasma (PRP) is widely used and has been proven to be effective for tendon healing in vivo ( Chiou, et al., 2015 ). However, Zhang et al. ( Zhang C. H. et al., 2018 ) found that the combined PRP was no better at repair-augmenting effects than the scaffolds alone for Achilles tendon regeneration.

Therapeutic potential of hAECs for early Achilles tendon defect repair through regeneration. (A) Circular defects of 5 mm created in the Achilles tendons. One defect was filled with fibrin glue, whereas the contralateral with 10 × 106 PKH26-stained cells suspended in fibrin glue (bottom); (B) Representative haematoxylin–eosin-, Herovici and immunofluorescent staining of CTR (control) and human amniotic epithelial cell (hAEC)-treated tendons. (C) Key functions associated with genes found to be up-regulated in hAECs and the top-scored network; (D) Key functions associated with genes found to be down-regulated in hAECs and the top-scored network. Copyright 2017 Wiley.

Promising laboratory findings reported in the literature suggest that there will be important implications for the practical application and clinical translation of tendon regeneration ( Cai C. et al., 2021 ; Cai et al., 2022 ; Wang et al., 2022 ). However, few clinical studies have been reported to date, and most are case reports and small case series ( Chen and Hunt, 2019 ) with a few translational animal models (dog, horse, etc .) ( Sun, et al., 2018 ; Badylak, et al., 1995 ; Gungormus, et al., 2015 ; Zhang C. H. et al., 2018 ; Shen, et al., 2010 ). Additionally, therapeutic perspectives are not achievable until the critical challenges relating to the scaffold, cell, or chemokine use (source, induction condition, genomic compatibility, dose, etc .) are solved. The combination of scaffold implanted with cell or chemokines, however, is encouraging for future studies and promising for human clinics.

Rotator Cuff Regeneration

Treatment of rotator cuff repairs is an ongoing challenge.

Rotator cuff defects are the main issues that occur in shoulder repair, and the rate of surgical failure is reportedly up to 94%, especially for large and massive tears after simple repairs ( McElvany, et al., 2015 ; Lewington, et al., 2017 ; Saveh-Shemshaki, et al., 2019 ). Various scaffolds have been used to replace the native tissue structures in rotator cuff repairs. Most scaffolds consist chiefly of extracellular matrix and chemical polymer, which provides a bridge for connecting tendon and bone tissues, and adsorbs the fibroblast secreted collagen matrix ( Guevara, et al., 2020 ). These scaffolds have been utilized in rotator cuff tendon tissue engineering for several decades ( Steinhaus, et al., 2016 ; Zhao, et al., 2017 ). They usually combine bioactive substances to promote rotator cuff regeneration, such as stem cells and growth factors. There are currently three types of rotator cuff tendon scaffold used: xeno-patches, allo-matrices, and synthetic films ( Steinhaus, et al., 2016 ; Coons and Alan, 2006 ; Wang D. et al., 2021 ). However, imperfect tissue regeneration is an ongoing problem.

Xeno-Patches for Rotator Cuff Repair

Xeno-patches extracted from extracellular matrices are effective bioactive scaffolds for tendon engineering and can be applied in surgical implantations to rotator cuff defects ( McGovern, et al., 2018 ). The acellular xenografts derived from the porcine dermis and small intestine were used in a large animal model for infraspinatus repair to evaluate the effects of tendon regeneration. Nicholson et al. ( Nicholson, et al., 2007 ) found that intestinal and porcine dermis patches were almost replaced by tendon-like tissues at 24-week, but a foreign body reaction was observed in the conjunction site of the tendon and xenograft. Ultimately, the cause of failure was the same for the dermal and intestinal groups. The potential immune reaction and associated chronic foreign body responses were the main concerns. This reaction may result from the residual DNA in the Xeno-tissue, even if processed by decellularization. Another problem is the hyper acute rejection caused by α-Gal. The α-Gal epitope exists in non-primate mammals ( Naso, et al., 2011 ; Platts-Mills, et al., 2021 ), and therefore, the epitope antibody is produced in humans, which specifically binds to xeno-tissue.

Allo-Matrices for Rotator Cuff Repair

Allo-matrices originated from decellularized cadaveric human tissues and were found to have the capability to bridge tendon tissue defects, with a low risk of tissue-scaffold rejection ( Fini, et al., 2012 ). A study by Adams et al. ( Adams, et al., 2006 ) explored the histological and biomechanical processes of allo-matrices, by utilizing allo-dermal matrices to bridge tendon and bone in an animal model for infraspinatus repair. The fibroblast infiltration and new collagen deposition were surrounded by dermal matrices 6 weeks after implantation, and at 24 weeks, a more mature tendon-like tissue was formed in the allo-dermal group. The biomechanical properties of the regenerated tissues were promising. However, there were only small-scale clinical trials conducted to evaluate their performance ( Zhao, et al., 2017 ). Even though no serious allo-matrix related complications were observed, and clinical outcomes appeared to be good, some potential problems still existed. Similar to the xeno-patches, there were concerns about the residual DNA. The residual DNA may cause immune inflammatory reactions and increase the proliferation of the scaring tissue ( Lewington, et al., 2017 ). There is also evidence that the mechanical properties of the allogenic matrices are decreased when compared with that of the auto-tendons. To better induce tendon–bone interface regeneration, Chen et al. (2019) added recombinant SDF-1α to the decellularized bone–fibrocartilage–tendon composite and injected synovium-derived mesenchymal stem cells (SMSCs) into the repair site. They found that the fabricated scaffold was better at recruiting the SMSCs and resulted in well regeneration of the tendon–bone interface 8 weeks after surgery. However, the option proposed in this study is too complicated for clinical application. Neither the addition of recombinant SDF-1α nor the injection of stem cells has been approved by the administration ( Figure 5 ).

Functional decellularized fibrocartilaginous matrix graft for rotator cuff enthesis regeneration: A novel technique to avoid in-vitro loading of cells. (A) Developing a cell-free graft with chemotaxis to recruit postoperative injected cells; (B) Macroscopic observation, histological analysis, and synchrotron radiation-Fourier transform infrared spectroscopy analysis of the book-type nature fibrocartilage tissues and C-SDF-1α/BDFM, sections stained with hematoxylin and eosin (H&E), DAPI, toluidine blue (TB), and picrosirius red (PR); (C , D) Histological analyses of regenerated fibrocartilage during RC healing. Copyright 2020 Elsevier.

Synthetic Polymers for Rotator Cuff Repair

Owing to the ongoing focus on immune reactions for both the xeno-patches and allo-matrices, synthetic polymers that are of great interest for tendon regeneration engineering have been identified ( Qiu, et al., 2021 ; Zhao, et al., 2021 ). Degradable polymers, including PLLA, poly-dioxanone (PDO), and poly (lactic-co-glycolic) acid (PLGA) have been used as supports to create multifunctional scaffolds. These synthetic films consisting of regularly arranged fibers exhibit stronger mechanical characteristics than the scaffolds derived from bio-tissues ( Gachon and Mesquida, 2021 ). The controllable arrangement plays a role in cell migration, with fibroblasts well aligned with the axis of the polymer fibers when compared to the un-aligned films in a random orientation. The polymer film had fewer immune responses when compared with that of the xeno-and allo-patches, indicating that it could be a potential scaffold to bridge the tendon gap. Yokoya et al. ( Yokoya, et al., 2012 ) used the PLGA sheet to repair the full-thickness defect of the rotator cuff in a rabbit model and observed that a greater proportion of type-I collagen was generated in the PLGA sheet with mesenchymal stem cells. The repaired site also had a better ultimate strength when compared with that of the controls without mesenchymal stem cells. Studies that have explored synthetic films have achieved encouraging results, but the products of the polymer film degradation were identified as a concern ( Silva, et al., 2020 ). High levels of chemical composite have a toxic effect on fibroblast proliferation and inhibit collagen deposition. These toxic effects vary with different polymers, and therefore, more research is required to ensure that the degradation product levels are safe.

These bio-scaffolds are assumed to offer ideal structural binding sites for tissue integration. Synthetic films have the advantage of mechanical properties and low rates of immune reaction. Several scaffolds have emerged in recent years; however, they have not yet been used in routine clinical surgery ( Zurina, et al., 2020 ). The ideal scaffold should be able to meet the mechanical strength of the cuff tendon and provide bioactive binding sites that promote fibroblast-mediated healing and tendon regeneration.

Cartilage and Osteochondral Regeneration

Simple cartilage regeneration.

The common types of joint bone defects include partial cartilage injury, full-thickness cartilage injury, and osteochondral defect ( Chow, et al., 2004 ; Shkhyan, et al., 2018 ; Kato, et al., 2019 ). The repair and regeneration of damaged cartilage tissue is one of the most challenging problems in the field of tissue engineering and regenerative medicine.

Until now, promoting cartilage regeneration has been one of the greatest difficulties in the field of regenerative medicine, due to the limited self-healing ability of cartilage tissues ( Wang, et al., 2020 ; Zhang, et al., 2020 ). Natural and synthetic substances are the two main biomaterials used to restore cartilage ( Chae, et al., 2021 ; Qiao, et al., 2021 ; Schuurmans, et al., 2021 ). The aim is mainly to improve cell adhesion and promote the growth and dynamic migration of regenerative tissues. Scaffolds are not only regarded as physical substrates, but in the biological environment, scaffolds are related to each other through clear chemical exchanges and physical stimulation through cells and adjacent tissues. Therefore, scaffolds are mainly used to support cell culture, infiltration, proliferation, and differentiation caused by signal factors and mechanical stimulation ( Gardner, et al., 2016 ; Lynch, et al., 2016 ). Scaffolds are dissimilated into categories such as nanomaterials, biomimetic materials, biological enhancers, and hydrogels, and they can be used to bind chondrocytes or can be placed at cartilage defect sites. Hua et al. (2021) fabricated multifunctional hybrid optical crosslinking (HPC) hydrogels by photopolymerization and photopolymerization of imine crosslinking. Loaded with chondrocytes, the scaffold was used for cartilage defect repairing through arthroscopy in a pig model. Six months after implantation, an ideal layer of cartilage was regenerated. However, the development of an optimal scaffold that can induce cartilage regeneration is ongoing. The other critical issue in isolated cartilage regeneration is how to increase the adherence of the regenerated cartilage to the bone underneath.

Scaffolds for Osteochondral Regeneration

A method to repair osteochondral defects, the terminal stage of a cartilage defects, is urgently required ( Eldridge, et al., 2020 ; Hall, et al., 2021 ; Kim, et al., 2021 ). At present, serious defects can only be treated by arthroplasty ( Pirosa, et al., 2021 ; Xie J. et al., 2021 ). However, the commonly used artificial joints based on non-degradable materials, such as metals and ceramics have some disadvantages, including their high cost, limited biocompatibility, foreign body rejection, and long-term loosening ( Labek, et al., 2011 ; Valdes, et al., 2012 ; Sakellariou, et al., 2016 ). Achieve bone–cartilage composite tissue regeneration and permanent joint physiological function reconstruction remains a challenging problem. Using biomimetic scaffolds to induce in situ osteochondral regeneration is expected to be an important option for future joint function reconstruction ( Figure 6 ) ( Radhakrishnan, et al., 2018 ; Chen et al., 2019 ; Xu, et al., 2019 ; Zhu, et al., 2019 ). The tissue engineering of osteochondral integrated scaffolds imitates a normal osteochondral structure, as well as the natural osteochondral components in composition, to achieve a double bionics structure and components, which will eventually enable the effective repair and regeneration of osteochondral defects. However, due to the complex anatomical structure and component content of normal bone and cartilage, as well as the dynamic changes in time and space that occur in the regeneration area, the repair and regeneration of the osteochondral defect area is not just a simple “filling” of new tissue. It requires subchondral bone regeneration to support hyaline cartilage, and hyaline cartilage is closely combined with bone to generate cartilage–bone interface integration and thus the concurrent regeneration of both cartilage and bone.

3D printing of a lithium-calcium-silicate crystal bioscaffold with dual bioactivities for osteochondral interface reconstruction. (A) Schematic illustration of application of Li2Ca4Si4O13 scaffolds for osteochondral reconstruction; (B) SEM images of 3D-printed Li 2 Ca 4 Si 4 O 13 scaffolds after fabrication and (C) after soaking in the simulated body fluids for 14 days; Macro-photographs showed the defects in the control group and the other two experimental groups ( D 1 : blank control without scaffolds, E 1 : pure β-TCP scaffolds, F 1 : Li 2 Ca 4 Si 4 O 13 scaffolds) at 12 weeks of post-surgery; (D 2 – F 2 ) showed 2D projection images of the three experimental groups at week 12; (D 3 – F 3 ) showed the transverse view of 3D reconstruction images of the three experimental groups at week 12; (D 4 – F 4 ) showed the sagittal view of 3D reconstruction images of the three experimental groups at week 12. Copyright 2019 Elsevier.

To achieve a breakthrough in osteochondral regeneration, we must focus on preparing integrated bionic scaffolds that accurately simulate the microenvironment of osteochondral regeneration, and solve the following key scientific problems: 1 ) accurate simulation of 3D shapes, multi gradient structures, regional specific matrix components, and the microenvironment factors of joints to prepare a bionic osteochondral scaffold ( Choe, et al., 2021 ; Lee, et al., 2021 ; Liu, et al., 2021 ); 2 ) achievement of osteochondral tissue regeneration and biological joint construction in vitro ; and 3 ) realization of the industrialization of integrated bionic stents and clinical transformations of the biological joints. The accumulation of separate tissue regeneration research for the cartilage and bone and the application of emerging cutting-edge technologies in recent years has facilitated breakthroughs regarding these technical problems.

At present, osteochondral scaffolds are commonly used in experimental research and clinical applications including natural biomaterial scaffolds, synthetic scaffolds, bioceramic scaffolds, bioactive glass, extracellular matrix scaffolds, and composite scaffolds.

The biological and mechanical properties of various tissue-engineered osteochondral scaffolds are different due to their different components and structures. For example, although natural biological scaffolds have the advantage of good biocompatibility, high cell affinity and degradability, which are conducive to adhesion and proliferation following the infiltration and recruitment of cells, they also have disadvantages, such as poor mechanical properties, rapid degradation rates, and limited sources. Synthetic scaffolds and bioceramics have good mechanical properties, strong plasticity, controllable degradation, and unrestricted wide sources. Their corresponding disadvantages are poor biocompatibility, low cell affinity, lack of hydrophilicity of some scaffolds, and their degradation products may have certain toxicities. With the development of tissue engineering, to overcome the shortcomings of single materials, two or more materials have been combined according to the principles of complementary characteristics and advantages, to design an ideal scaffold that can meet the needs of osteochondral tissue engineering.

Composite scaffolds combine the advantages of individual scaffolds, such as a controllable degradation rate, good cell compatibility, good hydrophilicity, and appropriate biomechanical strength. Su et al. (2021) prepared cartilage layers via collagen II and chitosan with a pore diameter of approximately 100 μm and a bone layer via the PLGA with a pore diameter of 500 μm. The chondrocytes labeled by nano magnetic particles were planted on this biphasic scaffold to observe their growth, proliferation, and distribution on the scaffold, to further investigate the effects of this method on the repair and regeneration of bone and cartilage. The experimental results showed that the combination of a scaffold structure and cells labeled by novel technology has good application prospects for repairing regenerated osteochondral defects.

Biomedical Material Fabrication in Osteochondral Regeneration

Rapid progress in biomedical material fabrication has been made in the field of osteochondral regeneration, including 3D bioprinting, electrospinning, aerogels, hydrogels, and drug loaded microspheres ( Deng, et al., 2021 ; Gao, et al., 2021 ; Jiang, et al., 2021 ). However, there are still some challenges, such as the accuracy and stability of 3D biological printing technologies and the flexibility and function of the products. A potential clinical application of bioprinting is to develop “ in vivo bioprinting” technology, which can accurately “print” cell materials on the damaged parts with the help of handheld print heads, to directly repair cartilage defects of different shapes and thicknesses. This technique has great potential for the development of individualized treatment plans and will help to eliminate the need for a secondary surgery.

In addition to 3D printing technologies, tissue-engineered hydrogel scaffolds have also been utilized in cartilage repair. However, it is difficult for hydrogels to meet the advantages of high porosity, good mechanical properties, toxicity, biocompatibility, and a controllable degradation cycle. Most of the hydrogels can only satisfy one or two advantages. A composite hydrogel that can synchronize the degradation rate with the regeneration rate of cartilage tissue could be promising as a repairing material for the treatment of cartilage defects.

The applicational prospects for tissue engineering electrospinning scaffolds is optimistic, with high porosity and bionic extracellular matrix structures, but there are still many problems that must be addressed, such as: 1 ) electrospinning scaffolds seriously affecting the adhesion, proliferation, and differentiation of seed cells on materials; 2 ) solving the contradictions between the mechanical strength and degradation rates of materials; and 3 ) unclear the teratogenicity and tumorigenicity of materials in the human body.

While the tissue engineering of osteochondral integrated scaffolds can solve some of the existing problems in traditional treatments, there are also shortcomings. If there is no in-depth study on the mechanisms for the repair and regeneration of osteochondral integrated scaffolds, it is impossible to clarify the repair and regeneration mechanisms for the defect area from a microscopic cellular and molecular level. Moreover, the osteochondral integrated scaffold has double bionics for its structure and composition, which cannot be compared with the normal osteochondral structure at both biological and mechanical levels; further, special materials similar to the natural osteochondral structure-cannot be found. In addition, the calcified layer and tidal line play important roles in the structure of bone and cartilage, but the integrated bionic scaffold is still difficult to completely biomimic these unique structures. Therefore, at present, the problem of the calcification of the cartilage layer and easy separation between the two layers of biphasic and multiphasic scaffolds has not yet been solved. Nevertheless, use of new preparation technologies and methods, such as 3D printing and electrospinning technologies, discovery or synthesis of new scaffold materials, and cooperation between medicine, industry, materials, biological structures, biomechanics, and integrated bionic scaffolds can finally solve the clinical scientific problem of osteochondral defects.

Bone Regeneration

Bone defects, fractures or osteotomies, osteoporosis, and osteonecrosis in the locomotor system may require bone-structure restoration to achieve regeneration ( McFarland, et al., 2016 ; Khira and Salama, 2017 ; Choi and Rhee, 2017 ; Zhang Y. et al., 2018 ). Some examples of such cases are glenoid and humeral head bone defects occurring in shoulder dislocations, glenoid bone absorption in cases of severe shoulder osteoarthritis (OA), bone defects occurring in high tibial osteotomy, and tibial plateau depression fractures in cases of knee dislocation. Bone regeneration is defined as the process wherein bone-grafting materials are replaced by newly formed bone ( Wei et al., 2022 ). Till now, autologous grafts have been suggested as the gold standard for bone regeneration. However, the accompanying donor-site complications and the limited availability of autografts hamper their extensive use in clinical applications. Meanwhile, allogeneic bone grafts are challenged by vascularization issues and disease transmission risks ( Giannoudis, et al., 2005 ; Laurencin, et al., 2006 ). Thus, there is an urgent need for the development of biomaterials in sports medicine. There are different strategies used to treat bone defects, such as 1 ) simple artificial bone material, 2 ) artificial bone material with bioactive factors, and 3 ) artificial bone material with stem cells.

The ideal scaffold would have an appropriate hierarchical architecture that would permit normal metabolic activity as well as the migration, proliferation, and differentiation of cells together with angiogenesis and bone ingrowth. As an example, a highly porous scaffold would have a greater surface area that would promote improved osteogenic effects by allowing greater mass exchange and adsorption of growth factors ( Wu, et al., 2014 ; Tang, et al., 2016 ). The pore size of the scaffold is also critical for good bone regeneration, as the presence of smaller pores leads to hypoxic conditions that promote pre-osteogenic osteochondral formation while larger interconnected pores promote directional osteogenesis ( Karageorgiou and kaplan, 2005 ; Zhang, et al., 2013 ). Porous surfaces also stimulate interactions and linkages between the implant and the bone, and the pore size is critical for bone integration. For example, it was found that while 300-μm pores produced the most lamellar bone, the process of osseointegration was longer than with 200-μm pores ( Xu, et al., 2011 ).

In this review, we have focused on research over the past 5 years into the optimization of the architectural, chemical, and surface features of bone graft substitutes (BGS) for the promotion of bone regeneration and osteointegration. Three-dimensional printing permits the creation of BGS tailored for the individual patient by optimization of both mechanical and structural characteristics. The optimizing structure permits specific correspondence between the BGS and the patient’s body, leading to more rapid postoperative recovery ( Tan, et al., 2017 ). Although titanium is most commonly used for 3D printing, materials such as bioceramics and polymers such as polyetheretherketon (PEEK), which allow custom design, are only being investigated at the pre-clinical stage at the moment ( Mustafa, et al., 2011 ; Li, et al., 2017a ; Yang, et al., 2017 ). The issues that are being addressed in the use of these novel ceramic materials include optimal mechanical characteristics, architectural design, and chemical properties to enhance both porosity and degradability. Optimal surface characteristics are vital for osteogenic cell adhesion to the BGS, leading to the promotion of new bone growth ( Colquhoun and Tanner, 2015 ; Fernandez-Yague, et al., 2015 ; Babaie and Bhaduri, 2018 ). These properties can be manipulated by coatings that promote bone regeneration.

As bone repair is a complex process that is dependent on various growth factors, we discuss the application of active biomolecules for promoting bone repair ( Krishnan, et al., 2006 ; Carragee, et al., 2011 ; Polak, et al., 2011 ; Kim, et al., 2012 ; Santo, et al., 2013 ; Mumith, et al., 2017 ). These applications can be divided into three approaches are three approaches: 1 ) the application of recombinant growth factors, individually or as mixtures, together with a natural or calcium phosphate matrix, such as BMP-2 (Infuse bone graft), BMP-7 (OP-1 putty), and rhPGDF-BB (Augment bone graft ® ); 2 ) the use of ECM-derived peptides targeting cellular receptors, such as B2A (B2A2-K-NS) and P-15; 3 ) the use of small molecules targeting pathways that influence none mass, including parathyroid hormone (PTH), Nel-like molecule-1 (NELL-1), and LIM mineralization protein-1 (LMP-1). These molecules may affect bone mass directly or indirectly by inhibiting negative modulators of bone mass and thus promoting increased bone mass.

The third approach used stem cells as part of a cell-based construct. This requires the presence of progenitor cells allowing the formation of new tissue through interaction with host cells, stimulatory factors, and support providing cells with 3D cues for new tissue formation. The progenitor cells used include bone marrow stromal cells (BMSCs), adipose-derived mesenchymal cells (ASCs) and periosteum-derived stem cells (PDSCs) ( Ingber, et al., 2006 ; Bolander, et al., 2016 ; Bolander, et al., 2017 ). However, the necessity of pre-incubating the biomaterial with the cells complicates the engineering process considerably and also reduces the viability of the cells, leading to high production costs. In addition, the scaffolds may not have sufficient ability to promote vascularization in vivo and may thus be unable to maintain cell and tissue viability ( Lenas, et al., 2009 ). Because of these issues, this approach has not proved popular in clinical practice.

Bone tissue engineering approaches were devised to address the shortcomings of bone grafts and alloplasts and to promote the repair of bone defects and fractures. Both biological derivatives and synthetic materials have been used for scaffold fabrication, singly or in combination. The developments in the field include the use of scaffolds together with gene therapy and stimulatory factors ( Farberg, et al., 2012 ; Pountos, et al., 2016 ; El Bialy, et al., 2017 ; Yan, et al., 2019 ), while recent advances in the 3D printing of scaffolds open new directions for effective bone regeneration ( Ding, et al., 2013 ; Gong, et al., 2015 ; Lee, et al., 2017 ; Li, et al., 2018 ) ( Figure 7 ). Nevertheless, many challenges remain. Recent reports have emphasized the importance of local microenvironments for the success of these scaffolds. In addition, more understanding of the precise functions of the active biomolecular constituents, such as their influence on inflammation or bone precursor cells, is required. The precise control and delivery of these molecules in the correct doses are necessary to prevent undesirable side effects. There is intensive research in the form of pre-clinical studies to understand the underlying mechanisms of these therapies and their effective applications. Translation to clinical practice also requires many regulatory steps and costs.

Vascularized 3D printed scaffolds for promoting bone regeneration. (A) Schematic diagram of bridging deferoxamine (DFO) on the surface of 3D printed polycaprolactone (PCL) scaffold and its biological function for bone regeneration in bone defect model; (B) Micro-CT analysis of the effect of scaffolds on bone repair in vivo ; (C) Representative images of hematoxylin-eosin (HE) staining of the decalcificated bones slice, showing the new formed tissue including the fibrous tissue (F), newly mineralized bone tissue (NB) and the structure of scaffolds (S). Copyright 2019 Elsevier.

Tendon–Bone Interface Regeneration

Poor tendon–bone interface healing after tendon/ligament reconstruction.

One of the main problems in the functional reconstruction of tendons and ligaments is the poor healing of the tendon–bone interface ( Yang, et al., 2017 ; Xing, et al., 2020 , Zhu, C., et al., 2021). Natural tendon–bone interfaces can be categorized into four portions: tendon, uncalcified fibrocartilage, calcified fibrocartilage, and bone ( Weimin, et al., 2013 ; Liu, et al., 2019 ). The transition of the structures effectively prevents the structural damage caused by a sudden tension, by gradually distributing tension across the interface ( Lu and Thomopoulos, 2013 ). From a mechanical perspective, this structure perfectly connects ligament and bone tissue with an elasticity modulus of 200 MPa and 20 GPa, respectively, which also increases the strength of the insertion to avoid the avulsion of the tendon. However, after injury, even with proper surgical treatments, scar tissue takes the place of the transitional structure in the tendon–bone interface, which greatly decreases its mechanical properties (Zhu, J., et al., 2021). Hence, inducing the regeneration of natural tendon–bone interfaces is a major issue when treating tendon–bone interface injuries.

Scaffolds Fabricated for Tendon–Bone Interface Regeneration

A decellularized extracellular matrix is a common option for tendon–bone interface regeneration, as it possesses ideal biocompatibility and a natural microstructure. Through a combination of physical, chemical, and enzymatic treatments, Su et al . (2019) successfully fabricated decellularized triphasic hierarchical bone–fibrocartilage–tendon composites, with the preservation of natural microstructures and mineralization. After 8 weeks implantation occurred into the tibia bone tunnel, and the fabricated decellularized bone–fibrocartilage–tendon composite induced a notably larger amount of bone regeneration in the bone tunnel when compared with that of simple decellularized tendon tissue. However, it is still unclear whether the decellularized bone–fibrocartilage–tendon composite can induce cartilage regeneration between the tendon–bone interface, which is the main issue in tendon–bone interface regeneration. Additionally, the fabrication procedure of decellularized bone–fibrocartilage–tendon composite is relatively complicated. It is quite difficult to achieve a balance between the preservation of the natural microstructure and the elimination of remnant cell debris and consequently, there is no current gold standard method for the field.

Decellularized small intestinal submucosa (SIS) has been generated for use as a commercial medical implant as numerous studies have indicated that many bioactive factors can be preserved, even after the decellularization procedure ( Meng, et al., 2021 ; Singh, et al., 2021 ). However, unlike the decellularized bone–fibrocartilage–tendon composite, decellularized SIS scaffolds lack the required microstructures between the tendon–bone interface. Even though decellularized SIS has been successfully used in many other tissue repair processes, recent clinical studies jointly suggested that the use of decellularized SIS did not ( Su, et al., 2021 )result in better clinical results when used for rotator cuff repair surgery ( Iannotti, et al., 2006 ; Bryant, et al., 2016 ). The retear rate and American Shoulder and Elbow Surgeons shoulder score were not improved, which indicated that the tendon–bone interface regeneration was not properly induced by a decellularized SIS scaffold ( Sclamberg, et al., 2004 ).

When compared with a natural extracellular matrix, artificial polymer scaffolds have advantages, including good quality control reliability and microstructure adjustability. Nevertheless, unlike a natural extracellular matrix, the artificial polymer scaffold lacks biocompatibility, biodegradability, and inducibility; highlighting the need to select appropriate materials. PLGA and PCL are common material options that have been deemed safe by the Food and Drug Administration ( Li, et al., 2015 ). By electrospinning, the fabricated scaffold is equipped with the similar microstructure when compared with that of a natural extracellular matrix ( Hua, et al., 2021 ). However, the microstructures of the tendon–bone interfaces are very different from the extracellular matrices of other tissues ( Rossetti, et al., 2017 ). The orientation of fiber in the extracellular matrix of the tendon–bone interface changes from aligned-to-random, suggesting that the fabricated scaffold should also mimic the transitional microstructure differences ( Deymier-Black, et al., 2015 ). Xie C. et al. (2021) fabricated electrospun scaffolds with aligned-to-random microstructures and morphologies of the tendon fibroblasts were cultured on the scaffold, and were organized and haphazardly oriented, respectively. In addition to the differences in the microstructures, there is also a gradient density for calcium in the tendon–bone interface. As PLGA and PCL are bioinert materials, which cannot induce bone regeneration, bioactive factors were commonly added to the scaffolds. In a previous study, we produced a PCL scaffold combined with gradient calcium phosphate silicate (CPS) content and found that the multilayer gradient composite effectively increased tendon–bone healing at the tear site, where better tissue cellularity and gradient mineralized cartilage formation were observed ( Su, et al., 2021 ). Wang D. et al. (2021) crosslinked nanofibrous scaffolds to fabricate a scaffold, which simulates the microstructure of the natural tendon-to-bone interface, and 3 months after implantation in rabbit, it fully regenerated the unique 3D structure of the tendon-to-bone interface ( Figure 8 ). However, the aforementioned studies were also too complicated for application in industrial production. Controlling the fiber orientation transition and gradient density of calcium is difficult using the current industry techniques.

Crimped nanofiber scaffold mimicking tendon-to-bone interface for fatty-infiltrated massive rotator cuff repair. (A) Schemata of fabrication of the crimped nanofibrous scaffold for massive rotator cuff tear repairing; (B) Representative micro-computed tomography (micro-CT) images of the proximal humerus and quantitative analysis. Copyright 2022 Elsevier.

Expectations in Tendon–Bone Interface Regeneration

To date, no medical implant for tendon–bone healing has been developed. Considering the difficulties in balancing the biocompatibility, biodegradability, inducibility, microstructure, and gradient density of the calcium, the time frame for a relevant implant is not short; however, it will be improved with time. Previous studies have either presented procedures that are too sophisticated or added bioactive materials not approved by the Food and Drug Administration. Scaffolds that can effectively induce tendon–bone interface healing, are easy to manufacture, and are highly reliable, are urgently required.

Materials for Sports Medicine Injury

Autografts are recommended as the “gold standard” for most common sports medicine injuries, including ACL tears and bone defects ( Cho, 1975 ; Clancy, et al., 1982 ; Howe, et al., 1991 ). Although autografts contribute to an increased healing rate, both structurally and functionally, donor site injury and limited tissue sources restrict their applications ( Piva, et al., 2009 ). Allografts can not only reduce surgery time but also reduce damage and complication at the donor site. However, a long-term follow-up study indicated that, when compared with autografts, allografts could prolong the post-surgery recovery time, increase the incidence of infection, and the risk of spreading infectious diseases ( Dahlstedt, et al., 1989 ; Good, et al., 1989 ; Greenberg, et al., 2010 ).

Natural biological materials are one of the major material sources of tissue engineering. SIS is a natural, acellular, degenerable, extracellular collagen matrix material which is mainly composed of helically interweaving type-I and -III collagen ( Badylak, et al., 1989 ; Badylak, et al., 1995 ). Studies confirmed that using SI as an ACL supplementary repair could contribute to neuro-vascularization and cell growth ( Nguyen, et al., 2015 ). However, Badylak et al. (1989) found that there was a reduced tensile resistance in the mucous membrane of the small intestine after 3 months in the experiment for goat ACL reconstruction, which could not meet the prerequisite in clinical biomechanics. Silk is a natural material and highly recommended for its desirable biocompatibility and mechanical strength ( Li, et al., 2014 ; Teuschl, et al., 2016 ). However, the uncontrollable speed of silk degeneration, limited cellular affinity, and unstable mechanical strength immediately after surgery all imposed restrictions on its application ( Soong and Kenyon, 1984 ).

Non-degradable polyester materials, such as PET, are another option as they are equipped with high mechanical strength. Ligament advanced reinforcement system (LARS) is a representative product of non-degradable artificial ligaments and it was advantageous to the patients who needed to be back to the field with a high-level performance. Considering its disadvantages, such as poor hydrophilicity, inertness, and no osteogenic active ingredient, the LARS ligament would lead to the formation of a scar at the ligament-bone interface and loss of long-term effectiveness ( Li, et al., 2012 ; Tiefenboeck, et al., 2015 ). Some studies have focused on surface coatings to improve bioinert material defects. Studies have shown that coating the surface of LARS ligaments with fibroin or hydroxyapatite can effectively promote the degradation of LARS ligaments and the tendon–bone healing of inert material in the bone tunnel ( Ai, et al., 2017 ; Cai, et al., 2018 ; Wang, et al., 2018 ). However, there are still problems with the current technique, such as nonuniform surface coating on the ligament and binding force, which greatly limits the clinical applications of the products.

Single polymer/macromolecular based scaffolds (such as polylactic acid and poly (caprolactone)) have been recently developed and are being explored for potential clinical use (Correia Pinto, et al., 2017; Erisken, et al., 2008 ; Zhang, et al., 2005 ). Such scaffolds can be equipped with relatively good mechanical properties and structural machinability which can be adjusted for different requirements. Research has indicated that they can gradually degrade in 600 days and maintain a certain degree of strength, which provides stable conditions for tissue regeneration ( Blaker, et al., 2011 ). Most of the absorbable polymer materials have a low hydrophilicity and cellular affinity. Furthermore, polylactic acid can produce acid metabolites during the degeneration process and thus cannot form real tissue inductivity ( Lu, et al., 2005 ; Cardwell, et al., 2014 ). For successful tissue engineering, the single polymer/macromolecular based scaffolds should meet several criteria ( Zhao, et al., 2014a ; Zhao, et al., 2014b ; Zhao, et al., 2015 ; Li, et al., 2017b ). First, good biocompatibility and mechanical properties should be confirmed in vitro to lay the foundation for further in vivo implantation. During degeneration, the initial, middle-stage, and final grafts should cooperate with induced regenerative tissues to support sufficient strength. Finally, the toxicity of the degeneration product should be strictly limited.

There is a high demand for tissue regeneration techniques in the field of sports medicine, which make regenerative sports medicine crucial for the treatment of injuries and diseases in the locomotor system. There are several conundrums, however, that are limiting the application of these techniques ( Tables 1 , ​ ,2). 2 ). The first issue is how to restore like for like, i.e., how to recreate tissues that mimic the native tissues in both structure and function. The second issue is how to fabricate a structure that can overcome the defects of the native structure to ensure that its function is improved and the potential for reinjury is reduced.

Common sports medicine injuries and their promising treatment methods.

Promising treatments’ advantage and disadvantage.

For meniscus regeneration, tissue engineering strategies could potentially generate a meniscus like cellular or acellular structure with or without growth factors. Progress has already been made in restoring its native anisotropy and zonal organization. Further research should be conducted to understand how to restore its hooping effect from the collagen structure and the connection between the anterior and posterior horns and the tibia, as well as how to ensure its healing to the peripheral capsule ligament and how to obtain sustained vitalization of the neonatal structure to reduce the risk of reinjury.

For cruciate ligament, rotator cuff, and Achilles’ tendon regeneration, the goal is to obtain a viable type-I collagen dominated fibrous structure that can heal to the bone and soft tissue it connects. For the collagen regeneration-inducing technique that utilizes degradable synthetic material as the main construct, there is still a long way to go before it will be able to induce the regeneration of sufficient volumes of collagen. The collagen transformation-inducing technique is more practical with readily available collagen. However, vitalizing the structure in the intraarticular environment, obtaining a mature ligament or tendon structure with sufficient final strength, and achieving satisfactory ligament or tendon – bone healing requires further investigation.

Osteochondral regeneration induced with a biomimetic scaffold is a promising strategy to solve the problem of cartilage regeneration and cartilage–bone adhesion. Progress has been achieved in the development of a scaffold simulate native osteochondral construct with regard to its microstructure, components, and bioactive stimulator. Breakthroughs are still required, however, to obtain a scaffold that can induce bone, cartilage, and the bone–cartilage interface with desired biological and mechanical features throughout the regeneration induction process, and to obtain semi-finished or finished osteochondral products that can be directly used for implantation.

In bone regeneration, obtaining an appropriate scaffold and bioactive molecules does not seem to be a problem. However, precisely controlling the release of the bioactive molecules to avoid undesirable side effects requires further investigation.

For tendon–bone interface regeneration, the goal is to obtain a layer of fibrocartilage between the tendon and the bone. However, neither simple structure materials nor complicated scaffolds can be used clinically to achieve this goal.

The increasing incidence of sports medicine injury is posing clinical challenges to surgeons worldwide. Approaches in the field of regenerative sports medicine will present promising options for the structural and functional restoration of the locomotor system. Unlike tissue regeneration in other systems of the body, the to-be-regenerated structure in the locomotor system should be capable of bearing various types of forces during regeneration without exhibiting obvious deformation and should restore the native connection of the regenerated structure to the different surrounding tissues. The future of sports medicine injury repairing scaffolds relies on using optimal components for the structural materials, bionic 3D structures, which simulate natural structures, and long-lasting viability of the regenerated structure with bioactivity.

Author Contributions

LW: Reviewing articles, Writing and original draft, Format conducting.JJ: Reviewing articles, Writing and original draft. HL: Reviewing articles, Writing and original draft. TZ: Reviewing articles, Writing and original draft. JCa: Reviewing articles, Writing and original draft. WS: Reviewing articles, Writing and original draft. JCh: Reviewing articles, Writing and original draft. JX: Reviewing articles, Writing and original draft. YL: Reviewing articles, Writing and original draft. JW: Reviewing articles, Writing and original draft. KZ: Conceptualization, Supervision, Writing - review and editing, Project administration. JZ: Funding acquisition, Conceptualization, Supervision, Writing - review and editing, Project administration.

This work was supported by National Natural Science Foundation of China (Grant No. 81902186, 81671920, 31972923, 81871753, 81772341), National Key Research and Development Program of China (Grant No. 2018YFC1106200, 2018YFC1106201, 2018YFC1106202), and Technology Support Project of Science and Technology Commission of Shanghai Municipality of China (Grant No. 19441901700, 19441901701, 19441901702, 18441902800).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor, KL, and Reviewer, XY, declared a shared parent affiliation with the authors LW, JJ, JC, WS, JCh, JX, YL, JW, JZ, at the time of review.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Study of trans athletes concludes blanket sports bans are a mistake

The lead researcher of a landmark IOC-funded study looking at strength, power and aerobic capacity of trans athletes tells Outsports that sports federations should consider its findings carefully.

Results from a first-of-its-kind study comparing the strength, power and aerobic capacity of transgender athletes with cisgender athletes have been published in a major medical journal , with the authors cautioning sports bodies not to pass blanket bans without doing proper research.

Researchers invited 23 trans women and 12 trans men to undergo a series of performance tests in laboratory conditions, while also putting 21 cis women and 19 cis men through the same tests.

In certain cardiovascular tests, the trans women performed worse than the cis women, and were found to have less lower-body strength, according to the study published Wednesday in the British Journal of Sports Medicine.

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Lead researcher Professor Yannis Pitsiladis told Outsports “the main takeaway message is the requirement of international federations (and their “experts”) to treat trans women very differently to cis men.”

Pitsiladis is on the Medical and Scientific Commission of the International Olympic Committee, which helped to fund the research via a grant.

He added: “It follows that research conducted comparing biological men to biological women is almost irrelevant in this debate and evidence from such comparisons should not be used to inform policy as is the case by many “armchair professors” advocating the default ban position.

“Our study also stresses the need for studies to be conducted in trained individuals / athletes. The use of non-trained individuals / non-athletes (cis and trans) is also of limited to no use.”

All the athletes involved in the tests were required to participate in competitive sports or take part in physical training at least three times a week. The average age was 34 years old.

Meanwhile, all 35 trans athletes had to have completed more than one year of gender affirmation hormone therapy (GAHT).

Of the full cohort, 36% were active in endurance sports, 26% in team sports and 38% in power sports, with none of those involved competing at national or international level.

The study was designed by another of the lead researchers, Blair Hamilton of the University of Brighton, with support from colleagues at Heriot-Watt University in Edinburgh, the University of Rome and Hong Kong Baptist University. Pitsiladis is based at HKBU.

🚨NEW #OriginalResearch Strength, power and aerobic capacity of transgender athletes: a cross-sectional study 👉 A comparison of laboratory measures of transgender athletes to their cisgender counterparts 👉 What can we learn from this? #OpenAccess 👉 https://t.co/h2u05XymoZ pic.twitter.com/LfkcEkL9iM — British Journal of Sports Medicine (BJSM) (@BJSM_BMJ) April 11, 2024

The tests assessed body composition, lung function, cardiopulmonary exercise testing, strength and lower body power.

Among the results was a determination that the trans women athletes had decreased lung function compared to the cis women athletes.

In addition, the bone density of the trans women athletes was found to be equivalent to that of the cis women. Bone density is linked to muscle strength.

The researchers say their findings “reveal notable disparities in fat mass, fat-free mass, laboratory sports performance measures and hand-grip strength measures between cisgender male and transgender female athletes.

“These differences underscore the inadequacy of using cisgender male athletes as proxies for transgender women athletes.”

Pointing to guidance issued by the IOC in November 2021 , which moved away from a previous policy that required competing athletes who are trans to undergo “medically unnecessary” procedures or treatment, they added: “Based on these limited findings, we recommend that transgender women athletes be evaluated as their own demographic group, in accordance with the principles outlined in Article 6.1b of the IOC Framework on Fairness, Inclusion and Non-Discrimination based on Gender Identity and Sex Variations.”

The authors also concluded: “This research shows the potential complexity of transgender athlete physiology and its effects on the laboratory measures of physical performance.”

Noting the shortcomings of their cross-sectional, lab-based study, which had a limited sample size, they added: “A long-term longitudinal study is needed to confirm whether these findings are directly related to GAHT.”

Sports governing bodies will be invited to study the data, with the researchers also emphasizing the need for sports-specific studies to inform policy making.

Prof. Pitsiladis was a guest speaker Tuesday at SportAccord in Birmingham, U.K., a major conference for international sports federations. He spoke on a roundtable panel titled “Examining the implementation and development of rules for transgender athletes in International Federations.”

Academic + medical experts, sports leaders, and lawyers are now discussing the complex topic of rules for #transgender athletes in International Federations at LawAccord. #SportAccord #PowerOfSport #sportslaw pic.twitter.com/jougYcTm2F — SportAccord World Sport & Business Summit (@sportaccord) April 9, 2024

Asked by Outsports what might happen next after this week’s publication, Prof. Pitsiladis said he hoped it would help to convince the IOC to contribute funding that would allow for further research that takes a multicentre approach, involving many international federations.

He noted how smaller sports federations with less resources or expertise feel “somewhat lost” on the topic of trans inclusion and need more assistance. “Their only real option is to follow the big federations and hope for the best,” he said.

Pitsiladis believes the newly-published study can have an impact on inclusion, but is cautious over how others will interpret the results.

“It’s my expectation that it may become a little easier for some federations developing their policies to reject the default position adopted by some of the large federations to ban trans women athletes in the absence of scientific data to support such a position.

“But I suspect most will follow the positions of the large federations to ban. Also, it’s unlikely that those large federations will change their position as they are now too invested and they don’t really look at the science or evidence.

“Their wish is mainly to appease their membership and the decisions being taken are mainly justified by politics and dictates, rather than science.”

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MSU researchers find more action needed to prevent arthritis

April 11, 2024 - MSUToday

Originally published April 8, 2024 on MSUToday

The prevalence of early knee osteoarthritis (OA) symptoms faced by patients after anterior cruciate ligament (ACL) reconstruction is staggering — but not much is being done to address it according to  new research  published by scholars from Michigan State University’s  Department of Kinesiology .

The study – published by the  Journal of Athletic Training   in January of 2024 – shows these symptoms persist throughout the first year following surgery and need to be addressed with early intervention.

The co-authors of the study with MSU affiliations include  Ashley Triplett , assistant professor in the College of Education;  Sheeba Joseph , associate professor, Colleges of Human and Osteopathic Medicine;  Francesca Genoese , doctoral student in the Department of Kinesiology; Michael Shingles and Andrew Schorfhaar of Sparrow Hospital, alums of the College of Osteopathic Medicine.