Effect of strength and tightness of lower extremity muscles on biceps femoris kinematics during sprinting

BACKGROUND: hamstring strain injuries often occur during sprinting. Many factors have been hypothesized to cause these injuries. the purpose of this study was to investigate the effects of the strength ratios in the hip and knee joints, contralateral hip flexor tightness, and hamstring flexibility on musculotendon length of the biceps femoris long head during sprinting for providing a better understanding of this multifactorial injury. METHODS: Twelve male sprinters performed maximal effort overground sprinting. A three-dimensional musculoskeletal model was used to compute musculotendon length of the biceps femoris in a sprinting gait cycle. the straight leg raising (SLR) test, Thomas test of the contralateral limb, and hamstrings-to-quadriceps and hip extensors-to-flexors strength ratios were measured. Stepwise linear regression analysis was used to assess whether factors measured could predict the biceps femoris length during terminal swing of sprinting. RESULTS: Decreased hip extensors-to-flexors ratio and SLR scores and increased contralateral Thomas test scores predicted 53.2% of variance in biceps femoris muscle length, at peak knee extension during terminal swing (P<0.05). CONCLUSIONS: The results indicate that a composite factor including a combination of hamstring flexibility, contralateral hip flexor tightness, and hip extensors-to-flexors strength ratio can predict biceps femoris musculotendon length during the terminal swing phase of sprinting.