Fig. 3. Typical experimental recordings of unloaded walking. The fraction of the mechanical energy recovered during unloaded walking at each instant of the step cycle [r(t), thick lines] is superimposed on the mechanical energy changes of the centre of mass (E_p, gravitational potential energy; E_k, kinetic energy; and E_cg=E_p+E_k, where E_cg is the total mechanical energy of the centre of mass, thin lines). Typical record obtained from (A) a European subject (male, 66.2 kg, 4.86 km h^-1) and (B) an African woman (Kikuyu, 83.3 kg, 4.85 km h^-1). The vertical broken lines on the E_k and E_p curves delimit the periods when the instantaneous recovery of mechanical energy r(t) is zero, indicated on the figure as t_pk+ when E_p and E_k increase simultaneously, and as t_pk- when E_p and E_k decrease simultaneously. The periods during which energy transduction between E_p and E_k occurs are indicated as t_tr,up during the lift of the centre of mass and t_tr,down during the descent of the centre of mass. As in the simulation, R_step is greater than R_int but, in contrast to the simulation, the r(t) curves recorded during the rise and fall of the centre of mass during walking are not symmetrical see (Fig. 1).

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