Hamstring muscle shortens actively during the late swing phase of high-speed running.

The purpose of this study was to investigate the muscle-tendon unit (MTU) and fascicle behavior of the biceps femoris long head (BFlh) along with the activation during the swing phase of high-speed running. Nine active males ran on a treadmill at 70, 80, and 90 % of their maximum running speed. Kinematics of the hip and knee joints were collected during running. The fascicle length of the BFlh was monitored using a B-mode real-time ultrasound imaging system. Muscle activation of the BFlh was simultaneously recorded using surface electromyography. During the swing phase of running, the changes in the MTU length for 80 % and 90 % of the maximum running speed was greater than 70 % (p < 0.05). The fascicle was actively lengthened with weak activation during the early to the middle swing phase but actively shortened with strong activation during the late swing phase. Despite the different changes in the MTU length with increasing running speeds (p < 0.05), the fascicle length change was not different. During the late swing phase of high-speed running, the BFlh fascicle was actively shortened, suggesting further investigation explaining the cause of high-speed running-related muscle strain injury while considering the functional characteristics of the muscle-tendon interaction.