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Fig. 1. Mechanical energy of the centre of mass of the body during running steps at the indicated speeds, in an old subject (A,C,E: 80 years, 73.5 kg mass, 1.71 m height, trained runner), and a young subject (B,D,F: 21 years, 58.5 kg mass, 1.79 m height, trained runner). 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 of the body in the sagittal plane (E_cm=E_p+E_k, black). The zero line corresponds to the minimum attained by the E_p curve. The horizontal bars indicate the time of contact during the step (from minimum to minimum of E_p). The time 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. The gap between red and blue bars indicates the duration of the aerial time. The records were obtained from the signals of a force platform. Note the lower vertical oscillation of the centre of mass in the old subject, indicated by a lower amplitude of the E_p and E_kv curves with a shorter aerial time, lower step duration and an higher step frequency.

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