Acute effects of static stretching to hamstrings on spatiotemporal variables and leg kinematics in late swing at maximal sprint speed phase and its relationship with Nordic hamstring strength

This study aimed to determine the acute effects of static stretching of the hamstrings on maximal sprint speed and its spatiotemporal variables, and lower limb kinematics during the late swing phase, and its relationship with Nordic hamstring strength. Sixteen healthy male college sprinters were asked to sprint 80 m without static stretching and with static stretching of the hamstrings for 120 s per leg before the sprint, and both conditions were counterbalanced. The knee flexion peak force and torque were measured using the Nordic hamstring. The differences between no static stretch and static stretch, and their relationship with Nordic hamstring strength were investigated. The results showed that the touchdown distance and support time increased, and flight distance decreased in under static stretch conditions with a decrease in maximal sprint speed. Moreover, under static stretch conditions, the angular velocity of knee extension at contralateral release was lower, while the theoretical hamstring length (difference between knee angle and hip angle) at ipsilateral touchdown was greater. In addition, the lower the peak force and torque of the Nordic hamstring, the more significant the decrease in maximal sprint speed, increase in support time, decrease in flight distance, and decrease in peak angular velocity of hip extension at static stretch. Furthermore, the more significant the decrease in maximal sprint speed at static stretch, the smaller the peak theoretical hamstring length at the no static stretch. Therefore, it is suggested that long- term static stretching immediately before sprinting in sprinters with poor Nordic hamstring strength and low hamstring compliance during the late swing phase may induce unfavorable kinematics to prevent hamstring strain injury and maximal sprint speed reduction.