An electromyographic study of the lower extremity muscles was undertaken in order to compare jogging, running, and sprinting. The study demonstrated that as the speed of gait increased, the support phase decreased, from 620 msec for walking to 260 msec for jogging to 220 msec for running to 140 msec for sprinting. The electromyographic data demonstrated that all muscle groups except the hip flexor and adductor longus were active during foot descent, floor contact, and midsupport. There was absence of muscle function during the late toe-off phase except that demonstrated by the adductor longus and the abdominal muscles during sprinting. The main muscle group that appears to increase the speed of gait is that of the hip flexors, which is closely linked to the knee extensors in order to propel the body forward in the line of progression. There was little or no activity in the gastrocnemius or in the intrinsic muscles of the foot about the time of toe-off, leading the authors to conclude that push-off per se does not appear to occur.