Muscles exert torques which cooperate with the other forces present in determining the movements of the body and regulate energy exchange by transmitting, absorbing, releasing and dissipating energy. The instantaneous rates at which these processes take place in the leg muscles during walking have been calculated for a muscle system consisting entirely of one-joint muscles and for one con-taining only one three-joint muscle. The actual system of 1-joint and 2-joint muscles represents a compromise between the efficiency of the 3-joint muscle and the adaptability of those passing over only one joint.