Background:Hamstring injury is one of the most common injuries in sports involving sprinting.Hamstring flexibility and strength are often consid-ered to be modifiable risk factors in hamstring injury.Understanding the effects of hamstring flexibility or strength training on the biomechanics of the hamstring muscles during sprinting could assist in improving prevention strategies and rehabilitation related to these injuries.The purpose of this study was to determine the effects of altering hamstring flexibility or strength on peak hamstring musculotendinous strain during sprinting.Methods:A total of 20 male college students(aged 18-24 years)participated and were randomly assigned to either a flexibility intervention group or a strength intervention group.Each participant executed exercise training 3 times a week for 8 weeks.Flexibility,sprinting,and isoki-netic strength testing were performed before and after the 2 interventions.Paired t tests were performed to determine hamstring flexibility or strength intervention effects on optimal hamstring musculotendinous lengths and peak hamstring musculotendinous strains during sprinting.Results:Participants in the flexibility intervention group significantly increased the optimal musculotendinous lengths of the semimembranosus and biceps long head(p ≦ 0.026)and decreased peak musculotendinous strains in all 3 bi-articulate hamstring muscles(p ≦ 0.004).Participants in the strength-intervention group significantly increased the optimal musculotendinous lengths of all 3 hamstring muscles(p ≦ 0.041)and sig-nificantly decreased their peak musculotendinous strain during sprinting(p ≦ 0.017).Conclusion:Increasing hamstring flexibility or strength through exercise training may assist in reducing the risk of hamstring injury during sprinting for recreational male athletes.