Fig. 3. Mechanical energy changes of the centre of mass of the body during the running step. (A) 1 g, (B) 1.3 g. At each speed the curves show from bottom to top: the changes in the gravitational potential energy of the centre of mass of the body (E_p, dotted line), the sum of the kinetic energy of vertical motion (E_kv) plus E_p (continuous line), the kinetic energy of forward motion (E_kf) and the total translational energy of the centre of mass in the sagittal plane (E_cm=E_p+E_kv+E_kf). The records were obtained as described in the Materials and methods from the F_v and F_f signals for the steps indicated by the vertical interrupted lines in Fig. 2, expanded to the left to include the previous valley of E_p. At each speed, the zero line corresponds to the minimum attained by the E_p curve. The continuous line below each panel indicates the ground contact time.