Rod S. Barrett

Introduction & PurposeHamstring injuries are common in sprint-based sports, often occurring during eccentric contraction in the late swing phase of sprinting (Ahmad et al., 2014; Liu et al., 2012; Verrall et al., 2003). Neuromuscular fatigue is a key risk factor, as it compromises a muscle’s ability to absorb energy, increasing the likelihood of excessive fiber elongation (Danielsson et al., 2020; Mair et al., 1996; Thelen et al., 2005). The biceps femoris long head (BFLH), the most frequently injured hamstring (Thelen et al., 2005), is particularly vulnerable late in competition when fatigue is high (Ekstrand et al., 2011; Woods et al., 2004), a link also supported by post-exercise strength deficits (Bourne et al., 2021; Brocherie et al., 2015; Small et al., 2010). This study investigated how a repeated-sprint protocol affects force and work distribution across the hamstring muscles. We hypothesized that fatigue would induce a redistribution of mechanical loading among them. MethodsThis analysis used data from 38 participants in a prior study who performed overground sprints before and after a fatiguing repeated-sprint protocol (12 x 30m) (Gonçalves et al., 2023). 3D motion capture, force plates, and surface EMG data were used to drive an EMG-informed neuromusculoskeletal analysis. A generic musculoskeletal model (Rajagopal et al., 2016) was scaled to each participant’s anthropometry (Kainz et al., 2017) and optimized (Modenese et al., 2016) to derive dynamics for the BFLH, semitendinosus (ST), and semimembranosus (SM). Pre-to-post changes in eccentric muscle force and mechanical work were analyzed using ANCOVA, with running speed included as a covariate. ResultsThe protocol induced fatigue, confirmed by a significant running speed decline (−16.0%, p < 0.01). At the muscle level, post-fatigue SM force was significantly lower, while BFLH and ST forces were maintained. Peak fiber length increased significantly for the SM only. Although absolute eccentric work did not change, the SM's relative contribution to total eccentric work decreased from 68% to 60%, as the BFLH’s share simultaneously increased from 19% to 25%. This load redistribution remained significant after accounting for changes in running speed. DiscussionOur findings indicate that repeated sprinting redistributes eccentric demand across the hamstrings. Fatigue appears to preferentially affect the SM, reducing its eccentric work capacity. The BFLH compensates for this deficit with elevated relative loading. This synergistic shift offers a mechanistic explanation for the BFLH's high injury susceptibility when fatigued. A key limitation is that the models were not fully individualized for subject-specific factors like prior injury, fiber type, or muscle-tendon architecture (Hambly et al., 2025; Kellis et al., 2012; Kumazaki et al., 2012; Lievens et al., 2022; Millard et al., 2013; Pimenta et al., 2023). ConclusionFatigue from repeated sprints alters hamstring synergy, causing a compensatory shift in eccentric work from the semimembranosus to the biceps femoris long head. This finding provides a direct, muscle-specific biomechanical basis for the high incidence of BFLH injuries, explaining how fatigue creates mechanical vulnerability within a synergistic group. Future research should use fully individualized models to further explore these complex load-sharing mechanisms. Top of Form Bottom of Form References Ahmad, C. S., Dick, R. W., Snell, E., Kenney, N. D., Curriero, F. C., Pollack, K., Albright, J. P., & Mandelbaum, B. R. (2014). Major and Minor League Baseball Hamstring Injuries: Epidemiologic Findings From the Major League Baseball Injury Surveillance System. The American Journal of Sports Medicine, 42(6), 1464–1470. https://doi.org/10.1177/0363546514529083 Bourne, M. N., Pollard, C., Messer, D., Timmins, R. G., Opar, D. A., Williams, M. D., & Shield, A. J. (2021). Hamstring and gluteal activation during high-speed overground running: Impact of prior strain injury. Journal of Sports Sciences, 39(18), 2073–2079. https://doi.org/10.1080/02640414.2021.1917839 Brocherie, F., Millet, G. P., & Girard, O. (2015). Neuro-mechanical and metabolic adjustments to the repeated anaerobic sprint test in professional football players. European Journal of Applied Physiology, 115(5), 891–903. https://doi.org/10.1007/s00421-014-3070-z Ekstrand, J., Hägglund, M., & Waldén, M. (2011). Injury incidence and injury patterns in professional football: The UEFA injury study. British Journal of Sports Medicine, 45(7), 553–558. https://doi.org/10.1136/bjsm.2009.060582 Hambly, M. J., Worsey, M. T. O., Lloyd, D. G., & Pizzolato, C. (2025). Improving Calibration of EMG-Informed Neuromusculoskeletal Models Through Differentiable Physics and Muscle Synergies. IEEE Transactions on Biomedical Engineering, 72(12), 3453–3463. https://doi.org/10.1109/TBME.2025.3569682 Kainz, H., Hoang, H. X., Stockton, C., Boyd, R. R., Lloyd, D. G., & Carty, C. P. (2017). Accuracy and Reliability of Marker-Based Approaches to Scale the Pelvis, Thigh, and Shank Segments in Musculoskeletal Models. Journal of Applied Biomechanics, 33(5), 354–360. https://doi.org/10.1123/jab.2016-0282 Kellis, E., Galanis, N., Kapetanos, G., & Natsis, K. (2012). Architectural differences between the hamstring muscles. Journal of Electromyography and Kinesiology, 22(4), 520–526. https://doi.org/10.1016/j.jelekin.2012.03.012 Kumazaki, T., Ehara, Y., & Sakai, T. (2012). Anatomy and Physiology of Hamstring Injury. International Journal of Sports Medicine, 33(12), 950–954. https://doi.org/10.1055/s-0032-1311593 Lievens, E., Van Vossel, K., Van de Casteele, F., Wezenbeek, E., Deprez, D., Matthys, S., De Winne, B., McNally, S., De Graaf, W., Murdoch, J. B., Bourgois, J. G., Witvrouw, E., & Derave, W. (2022). Muscle Fibre Typology as a Novel Risk Factor for Hamstring Strain Injuries in Professional Football...

BackgroundHamstring strain injuries are common during repeated-sprinting. Neuromuscular fatigue may be a risk factor but specific effects on muscle fiber dynamics remain unclear. This study investigated the effects of repeated-sprints on hamstring muscle dynamics. We hypothesized repeated-sprints would lead to a redistribution of hamstrings loads and an increase in fiber lengths. MethodsThirty-eight recreational athletes performed a repeated-sprints protocol (12x30m sprints). Pre- and post-protocol, 3D motion capture, ground reaction forces, and electromyography (EMG) signals from 12 muscles were recorded during overground sprints. A calibrated EMG-informed neuromusculoskeletal model was used to estimate muscle dynamics for the biceps femoris long head, semitendinosus, and semimembranosus. Muscle impulse, work, and force during mid-to-late swing were calculated to assess eccentric hamstring function. Statistical-Parametric-Mapping and repeated-measures ANCOVA were used to assess within-participant changes controlled for running speed changes. FindingsAfter repeated sprints, running speed significantly decreased (-16%) together with significant lower muscle forces and increased fiber lengths only in semimembranosus. Critically, the contribution to total muscle work significantly shifted from semimembranosus (pre:68%.vs.post:60%) to biceps femoris long head (pre:19%.vs.post:27%), even after adjusting for speed changes. InterpretationThe protocol induced a fatigue-related load redistribution among the hamstrings rather than a systematic increase in fiber lengths. Fatigue appears to selectively reduce the work contribution from the SM, increasing the relative eccentric load on the biceps femoris long head during the critical late swing phase. This shift in synergistic load-sharing may be a key mechanism explaining the high incidence of biceps femoris long head injuries, particularly when fatigued.

The purpose of this study was to investigate the immediate and short-term (0–3 h) effects of a single bout of eccentric plantarflexion exercise on three-dimensional (3D) Achilles tendon (AT) morphology in healthy and tendinopathic tendons. The eccentric exercise protocol consisted of six sets of 15 unilateral heel drops performed on a step. Freehand 3D ultrasound scans of the AT were acquired immediately before exercise and at 30-minute intervals over a 3-hour post-exercise recovery period to quantify tendon volume, length, and cross-sectional area (CSA). An exponential function characterized the recovery time course and calculated recovery time constants. Following eccentric exercise, both tendinopathic and healthy tendons exhibited immediate reductions in volume and CSA and increased tendon length. Tendinopathic tendons demonstrated a faster initial recovery of morphological properties, as indicated by shorter recovery time constants, compared with healthy tendons. However, this early recovery plateaued after one hour, and morphological recovery remained significantly lower than that of healthy tendons during the 2–3 h post-exercise period. These findings suggest that tendinopathic tendons exhibit an early but incomplete morphological recovery characterized by faster initial fluid reabsorption followed by a reduced capacity for sustained recovery, likely reflecting degeneration-related alterations in tendon structure and poroelastic behavior.

Anterior cruciate ligament (ACL) rupture is a serious sporting injury associated with impaired performance, high recurrence rates, early-onset knee osteoarthritis and substantial health and economic burden. In Australia, ACL injury incidence has increased over the past two decades, with particularly high rates observed among female and adolescent athletes participating in cutting and landing sports. Neuromuscular and structured warm-up programs can reduce ACL injury risk; however, implementation and adherence remain inconsistent, and evidence for effectiveness in males, elite athletes and several high-risk sports is unclear. Practitioners, therefore, require clear, evidence-based guidance to prescribe effective prevention strategies across diverse populations. This position statement provides contemporary, evidence-informed recommendations to guide exercise prescription for ACL injury prevention by (1) critically evaluating the effect of existing exercise interventions on ACL injury incidence, and (2) synthesising evidence on intrinsic risk factors and mechanistic pathways for non-contact ACL injury to inform targeted training strategies. Two systematic literature searches were conducted in Scopus, Embase and PubMed. The first identified randomised and non-randomised controlled trials and cohort studies evaluating the effect of exercise-based interventions on ACL injury incidence. The second identified prospective studies examining intrinsic risk factors for non-contact ACL injury. Evidence on ACL injury aetiology was synthesised by integrating injury risk factors, video analyses of injury events, in vivo and in vitro laboratory studies and in silico biomechanical modelling. We propose a tiered framework for exercise prescription. Tier 1 strategies emphasise population-level neuromuscular and structured warm-up programs, which meta-analyses indicate can reduce ACL injury risk by approximately 60% in adolescent female soccer, basketball and handball players. Effective programs typically combine plyometric, strength, balance/proprioception, mobility and agility exercises performed for 10–30 min, 2–3 times per week. Tier 2 strategies target specific neuromuscular deficits or high-risk movement patterns, including low knee flexion, dynamic knee valgus, lateral trunk flexion and excessive step width. Interventions may include plyometric and resistance exercises, motor control drills and neurocognitive challenges to increase transfer to game scenarios; however, direct evidence linking Tier 2 strategies to reduced injury incidence remains limited. This position statement provides an evidence-based framework to support the design and practical implementation of effective ACL injury prevention strategies in sport.

The hamstrings are critical for athletic performance; however, no study has examined differences in hamstring muscle‐tendon geometry (cross‐sectional area/volume) and muscle typology (proportion of Type I/II fibers) between elite sprinters/jumpers and recreationally active individuals. This study aimed to compare hamstring geometry and typology between these groups and examine how these characteristics relate to sprint and strength performance. Elite sprint and jump athletes ( n = 15, 3 female, 21.7 ± 2.2 y, 180.6 ± 9.9 cm, 72.2 ± 9.6 kg) and recreationally active individuals ( n = 15, 4 female, 25.7 ± 3.0 y, 176.0 ± 9.5 cm, 76.3 ± 17.6 kg) completed sprint and eccentric knee flexor strength testing. Magnetic resonance imaging and spectroscopy were used to assess hamstring muscle‐tendon geometry and typology, respectively. Compared to recreationally active individuals, elite athletes had larger hamstring muscles (all muscles, mean difference: 59.75–150.45 cm ³ , p < 0.009), biceps femoris long head (BFlh) proximal aponeuroses (1.09 cm ³ , p < 0.001), BF short head distal aponeuroses (1.24 cm ³ , p = 0.002), semimembranosus proximal free tendons (0.75 cm ³ , p = 0.024) and aponeuroses (2.29 cm ³ , p < 0.001), semitendinosus distal free tendons (0.49 cm ³ , p = 0.01) and BFlh proximal aponeurosis interface areas (10.43 cm ² , p < 0.001). Elite athletes also had 1.5 times greater estimated proportion of Type II fibers ( p < 0.001). Medial hamstring geometry and muscle typology explained the greatest variance in maximal sprint speed ( R ² = 0.65), while BFlh and semimembranosus muscle volumes with semitendinosus tendon volume explained the greatest variance in eccentric knee flexor strength ( R ² = 0.59). Elite athletes had larger hamstring muscles, aponeuroses, and free tendons, and a greater estimated proportion of Type II fibers than recreationally active individuals. These structural and compositional differences likely contribute to their superior sprint and strength performance.

Targeted resistance training stimulates hamstring muscle hypertrophy, but its effect on tendon‐aponeurosis geometry is unknown. This study examined changes in hamstring muscle, free tendon, and aponeurosis geometry following a 10 week Nordic or hip extension exercise intervention. Thirty recreationally active males were randomly allocated (n = 10 per group) to a Nordic, hip extension, or control group. Magnetic resonance imaging of both thighs was acquired pre‐ and post‐intervention. Changes in free tendon and aponeurosis volume for each hamstring muscle, biceps femoris long head (BFlh) aponeurosis interface area and muscle volume‐to‐interface area ratio were compared between groups. Regional changes in muscle CSA were examined via statistical parametric mapping. The change in semimembranosus free tendon volume was greater for the Nordic than control group (mean difference = 0.06 cm³, 95% CI = 0.02–0.11 cm³). No significant between‐group differences existed for other hamstring free tendons or aponeuroses. There were no between‐group differences in change in BFlh interface area. Change in BFlh muscle volume‐to‐interface area ratio was greater in the hip extension than Nordic (mean difference = 0.10, 95% CI = 0.007–0.19, p = 0.03) and control (mean difference = 0.12, 95% CI = 0.03–0.22, p = 0.009) groups. Change in muscle CSA following training was greatest in the mid‐portion of semitendinosus for both intervention groups, and the mid‐portion of BFlh for the hip extension group. There was limited evidence for tendon‐aponeurosis hypertrophy after 10 weeks of training with the Nordic or hip extension exercises. For the BFlh, neither intervention altered the interface area although hip extension training stimulated an increase in the muscle volume‐to‐interface area ratio, which may have implications for localized tissue strains. Alternative muscle‐tendon loading strategies appear necessary to stimulate hamstring tendon adaptations.

Objectives: Hamstring strain injuries (HSIs) commonly affect the proximal biceps femoris long head (BFlh) musculotendinous junction. Biomechanical modeling suggests narrow proximal BFlh aponeuroses and large muscle‐to‐aponeurosis width ratios increase localized tissue strains and presumably risk of HSI. This study aimed to determine if BFlh muscle and proximal aponeurosis geometry differed between limbs with and without a history of HSI. Methods: Twenty‐six recreationally active males with ( n = 13) and without ( n = 13) a history of unilateral HSI in the last 24 months underwent magnetic resonance imaging of both thighs. BFlh muscle and proximal aponeurosis cross‐sectional areas, length, volume, and interface area between muscle and aponeurosis were extracted. Previously injured limbs were compared to uninjured contralateral and control limbs for discrete variables and ratios, and along the relative length of tissues using statistical parametric mapping. Results: Previously injured limbs displayed significantly smaller muscle‐to‐aponeurosis volume ratios ( p = 0.029, Wilcoxon effect size (ES) = 0.43) and larger proximal BFlh aponeurosis volumes ( p = 0.019, ES = 0.46) than control limbs with no history of HSI. No significant differences were found between previously injured and uninjured contralateral limbs for any outcome measure ( p = 0.216–1.000, ES = 0.01–0.36). Conclusions: Aponeurosis geometry differed between limbs with and without a history of HSI. The significantly larger BFlh proximal aponeuroses and smaller muscle‐to‐aponeurosis volume ratios in previously injured limbs could alter the strain experienced in muscle adjacent to the musculotendinous junction during active lengthening. Future research is required to determine if geometric differences influence the risk of re‐injury and whether they can be altered via targeted training.

The distal semitendinosus tendon is commonly harvested for anterior cruciate ligament reconstruction, inducing substantial morbidity at the knee. The aim of this study was to probe how morphological changes of the semitendinosus muscle after harvest of its distal tendon for anterior cruciate ligament reconstruction affects knee flexion strength and whether the knee flexor synergists can compensate for the knee flexion weakness. Ten participants 8–18 months after anterior cruciate ligament reconstruction with an ipsilateral distal semitendinosus tendon autograft performed isometric knee flexion strength testing (15°, 45°, 60°, and 90°; 0° = knee extension) positioned prone on an isokinetic dynamometer. Morphological parameters extracted from magnetic resonance images were used to inform a musculoskeletal model. Knee flexion moments estimated by the model were then compared with those measured experimentally at each knee angle position. A statistically significant between-leg difference in experimentally-measured maximal isometric strength was found at 60° and 90°, but not 15° or 45°, of knee flexion. The musculoskeletal model matched the between-leg differences observed in experimental knee flexion moments at 15° and 45° but did not well estimate between-leg differences with a more flexed knee, particularly at 90°. Further, the knee flexor synergists could not physiologically compensate for weakness in deep knee flexion. These results suggest additional factors other than knee flexor muscle morphology play a role in knee flexion weakness following anterior cruciate ligament reconstruction with a distal semitendinosus tendon graft and thus more work at neural and microscopic levels is required for informing treatment and rehabilitation in this demographic.

The human semitendinosus muscle is characterized by a tendinous inscription separating proximal and distal neuromuscular compartments. As each compartment is innervated by separate nerve branches, potential exists for independent operation and control of compartments. However, the morphology and function of each compartment have not been thoroughly examined in an adult human population. Further, the distal semitendinosus tendon is typically harvested for use in anterior cruciate ligament reconstruction surgery, which induces long‐term morphological changes to the semitendinosus muscle‐tendon unit. It remains unknown if muscle morphological alterations following anterior cruciate ligament reconstruction are uniform between proximal and distal semitendinosus compartments. Here, we performed magnetic resonance imaging on 10 individuals who had undergone anterior cruciate ligament reconstruction involving an ipsilateral distal semitendinosus tendon graft 14 ± 4 months prior, extracting morphological parameters of the whole semitendinosus muscle and each individual compartment from both the (non‐injured) contralateral and surgical legs. In the contralateral leg, volume and length of the proximal compartment were smaller than the distal compartment. No between‐compartment differences in volume or length were found for anterior cruciate ligament reconstructed legs, likely due to greater shortening of the distal compared to the proximal compartment after anterior cruciate ligament reconstruction. The maximal anatomical cross‐sectional area of both compartments was substantially smaller on the anterior cruciate ligament reconstructed leg but did not differ between compartments on either leg. The absolute and relative between‐leg differences in proximal compartment morphology on the anterior cruciate ligament reconstructed leg were strongly correlated with the corresponding between‐leg differences in distal compartment morphological parameters. Specifically, greater between‐leg morphological differences in one compartment were highly correlated with large between‐leg differences in the other compartment, and vice versa for smaller differences. These relationships indicate that despite the heterogeneity in compartment length and volume, compartment atrophy is not independent or random. Further, the tendinous inscription endpoints were generally positioned at the same proximodistal level as the compartment maximal anatomical cross‐sectional areas, providing a wide area over which the tendinous inscription could mechanically interact with compartments. Overall, results suggest the two human semitendinosus compartments are not mechanically independent.