Role of the hamstrings in human vertical jumping

In some human subjects performing maximum-height squat jumps, the EMG-pattern of semitendinosus is bi-phasic and that of biceps femoris is mono-phasic. The purpose of this study was to investigate the roles of biceps femoris and semitendinosus in squat jumping, and to explain why they are different. Because semitendinosus has a larger moment arm at the knee than biceps femoris, it generates a larger knee flexion moment. The results of experiments and forward simulations suggest that human subjects performing vertical jumps utilize semitendinosus to cause a forward-downward acceleration of the center of mass. This allows the knee extensors to build up force before starting to shorten, rather than during shortening, thus increasing muscle work and jump height.