Jumping on the balls of the feet, the contracted calf muscles (gastrocnemius, etc.) sustain the deceleration of the falling body; if the exercise is performed on a platform sensitive to the forces directed vertically, a damped oscillatory motion is observed which is due to muscle elasticity and viscosity. From the damping and from the frequency of the oscillations, the body mass being known, the constant of force $k = F_{el}/x$, and the friction coefficient $\mu = F_{el}/\dot{x}$, have been calculated, $F_{el}$ being the force exerted by the contracted muscles stretched by an amount $x$, and $\dot{x} = dx/dt$. For a subject of 70 kg body weight it has been found $k = 3,26$ kg/mm, $\mu = 0,057$ kg/mm/sec. In running the force exerted by the foot on the ground amounts to a maximum of 175 kg, which is sustained by the contracted muscles of the limb: the elastic energy stored at the end of the step can be calculated as 9,36 kgm. This energy is a very consistent fraction of the external positive work performed in a step cycle, and this gives reason of the high mechanical efficiency found in running.