Background The hamstring muscles are susceptible to strain injury during sprinting, and the biceps femoris long head (BFlh) is reportedly the most frequently injured muscle. Therefore, elucidation of the risk factors is of particular concern for injury prevention.
Objective To clarify the potential risk factors for the incidence of muscle strain injury in each hamstring muscle by comparing the time of occurrence of peak musculotendon length and electromyography (EMG) activation during the late swing phase of sprinting.
Design Descriptive laboratory study.
Setting Male college track and field athletes.
Participants 13 male sprinters (age 20.2±0.6 years).
Intervention Full-body kinematics and EMG of the BFlh and semitendinosus (ST) were recorded during overground sprinting at maximum effort. The hamstring musculotendon lengths were computed by using a three-dimensional musculoskeletal model.
Main outcome measurements The time at the peak musculotendon length, in terms of percentage of the running gait cycle, was calculated and compared with that of the peak EMG activity.
Results The peak length of the BFlh and ST was observed during the late swing phase, at 86.7% [2.5%] and 81.1% [2.5%] gait cycle, respectively (mean [SD]). A significant difference was observed in the time of the peak musculotendon length (P<.001). Peak EMG activation time of the BFlh occurred significantly later (86.0% [7.9%] gait cycle, P<.01) than that of the ST (76.2% [7.8%] gait cycle). The peak musculotendon length was synchronous with the peak EMG activation in the BFlh, while a statistically significant difference was observed between these values in the ST (P<.05).
Conclusions The results indicate that the BFlh is exposed to instantaneous high tensile force during the late swing phase of sprinting, suggesting a higher potential risk for strain injury among the hamstring muscles.