The vertical and the frontal components of the push exerted by the foot on the ground, walking at different speeds (3-12 km/h), have been measured by means of a sensitive platform, and the work against gravity, $W_{V}$, and the work due to velocity changes in forward direction, $W_{F}$, have been calculated. The work per minute against gravity, $\dot{W}_{V}$, increases up to 8 km/h and then decreases. The work per minute due to velocity changes in forward direction, $\dot{W}_{F}$, on the contrary, increases progressively with increasing speed. The characteristic patterns of $\dot{W}_{V}$ and $\dot{W}_{F}$ as a function of speed, have been analyzed. The external work per step $W_{TOT} = W_{V} + W_{F}$, must be sustained by muscular activity; both $W_{V}$ and $W_{F}$ alone on the contrary are not directly related with muscular activity, as the rigid skeletal structures make possible the transformation of kinetic energy into potential and viceversa. This gives reason of the fact that up to 8 km/h, $W_{T0T}$ is less than each of itscomponents $W_{F}$ and $W_{V}$. Two phases, in which muscles perform external positive work, are evidenced in the step cycle; these are separated by two interposed phases, in which negative work is performed.