Fig. 1. Mechanical energy of the centre of mass of the body during running steps at the indicated speeds. In each panel the curves show the gravitational potential energy (E_p, blue), the kinetic energy of vertical motion (E_kv, red), the kinetic energy of forward motion (E_kf, brown), the kinetic energy of motion in the sagittal plane (E_k=E_kv+E_kf, green), and the total translational energy of the centre of mass in the sagittal plane (E_cm=E_p+E_k, black). The records were obtained as described in the Materials and methods from the signals of a force platform. In each panel, the zero line corresponds to the minimum attained by the E_p curve. Note that the maximum of E_k occurs in close proximity to the maximum of E_kv during the downward displacement, whereas it follows the maximum of E_kv during the lift (vertical dotted lines). The horizontal bars indicate the time of contact during the step (from minimum to minimum of E_p). The time of contact during which positive external work is done (increment of E_cm) is indicated in red, whereas the time of negative external work (decrement of E_cm) is indicated in blue. Subject: 72.5 kg mass, 1.79 m height, 23 years old.