Hamstring Stiffness and Strength Responses to Repeated Sprints in Healthy Nonathletes and Soccer Players With Versus Without Previous Injury

Background: The effect of 10 × 30 m repeated sprints on passive and active stiffness of semitendinosus (ST) and biceps femoris long head (BFlh), and knee flexor maximal voluntary isometric contraction (MVIC) and rate of force development (RFD), and whether athletes with previous hamstring injury have a different response, is unknown. Hypothesis: Repeated sprints would (1) increase BFlh stiffness and decrease ST stiffness and knee flexors MVIC and RFD in healthy participants; and (2) greater magnitude of response would be seen in athletes with previous hamstring injury. Study Design: Case series (experiment I) and case control (experiment II) study designs. Level of Evidence: Level 3. Methods: Healthy nonathletes attended 2 replicated sessions (experiment I, n = 18), while soccer players with (n = 38) and without (n = 67) previous hamstring injury attended 1 testing session (experiment II). Results: In both experiments, the knee flexors MVIC and RFD decreased after the sprints ( P < 0.05). In experiment I, the ST and BFlh passive stiffness reduced after the sprints ( P < 0.02), while a small BFlh active stiffness increase was noted ( P = 0.02); however, no correlation was observed between the 2 testing sessions for the postsprint muscle stiffness responses ( r = -0.07-0.44; P > 0.07). In experiment II, only an ST passive stiffness reduction was observed after the sprints ( P < 0.01). No differences were noted between injured and noninjured lower limbs for any variable ( P > 0.10). Conclusion: Repeated sprints are likely to decrease the knee flexor’s maximal and rapid strength, and to alter the hamstring stiffness in the nonathlete population. Previous hamstring injury does not apparently affect the footballer’s hamstring functional and mechanical responses to repeated sprints. Clinical Relevance: The responses of hamstring stiffness and knee flexor strength to repeated sprints are unlikely to be associated with hamstring injury.