The muscular activity pattern, function and maximal contraction velocity during the starting dash and sprint running at maximal velocity were studied. The subjects were five adult male sprinters, whose sprint running movements in the sagittal plain were analyzed (100 fps) from videotape recordings made with a high-speed catera. The ground reaction forces and EMGs of eight leg muscles were also recorded. Changes in length from the origin to the insertion (muscle-tendon complex : MTC) of the eight leg muscles were calculated using several methods reported by Grieve et al. (1978), Hawkins and Hull (1990), Visser et al. (1990) and Jacobs and Van Ingen Schenau (1993). The muscular activity pattern and the maximal contraction velocity were investigate from the change in length of the MTC when the EMG activity was observed. The turnover velocity of the muscular activity pattern in the stretch-shortening cycle (SSC) was calculated from the acceleration of the contraction. Also the hip, knee and ankle joint torques were calculated, and the following results were obtained.1. Changes in muscular activity pattern and maximal contraction velocity during the starting dash During the first half of the swing period, the m.gluteus maximus exhibited shortening activity, the m.rectus femoris exhibited stretching activity and the m.iliopsoas exhibited SSC muscular activity. The m.vastus lateralis exhibited shortening activity during the last half of the swing period and stretching activity during the first half of the foot contact period. The m.biceps femoris exhibited SSC muscular activity from the middle of the swing period to the middle of the foot contact period. The m.gastrocnemius and m.soleus exhibited SSC muscular activity during the foot contact period. Above all, the maximal shortening stretching velocity, and the turnover velocity increased with increasing sprint running velocity after the start, except for the shortening velocity of the m.gastrocnemius and m.soleus, and the stretching velocity of the m.vastus lateralis, whose contraction velocity stayed almost constant. 2. Muscular activity and function during sprint running at maximal velocity During the first half of the swing period, the m.rectus femoris produced knee extension troque by stretching activity which functioned to reduce the knee flexion force which occurred through the joint force related to hip flexion torque. During the latter half of the swing period, knee flexion torque developed as a by-product from the m.biceps femoris, which serves as a hip extensor. On the other hand, the knee extension force which occurred through the joint force related to hip extension torque, the momentum required to extend the knee joint, and the knee extension torque which was produced by the shortening activity of the m.vastus lateralis were observed during the same period. Therefore, the knee extension movement observed from the outside occurred as a result of the total of these forces. The m.tibialis anterior acted to offset the ankle extension force which occurred through the joint force related to knee extension torque during the first half of the swing period, and the ankle extension torque developed during the latter half of the swing period preparation for foot contact by the m.gastrocnemius an m.soleus.