Hamstring muscle dynamics during accelerated sprints following repeated sprints

Abstract

Introduction & Purpose
Hamstring 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.

Methods
This 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.

Results
The 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.

Discussion
Our 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).

Conclusion
Fatigue 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.

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References

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