Running at constant speed the contraction of the extensor muscles of the lower limbs (mm gastrocnemii, quadriceps of the thigh, glutei etc.), responsible for the push exerted at each step, follows immediately a stretch of the same muscles in sustained contraction; inertial forces plus the body weight are responsible for this stretch. This implies transformation of some of the kinetic and of the potential energy of the body into elastic energy; during the shortening of the muscles, immediately thereafter, the elastic energy is given back, which can be utilized for progression. During the acceleration of the body that takes place at a start of a run, the lengthening of the extensor muscles is very reduced until the speed is low; it increases thereafter with increasing speed. In the acceleration phase therefore elastic energy cannot be accumulated in the muscles, and it cannot therefore be taken into consideration to the end of progression.