Fig. 4. Unloaded walking. (A) Average curves of the instantaneous recovery of mechanical energy r(t) for the African women (thick continuous line, mean of 32 steps by four subjects), the European women (thin continuous line, mean of 17 steps by five subjects) and all European subjects (males and females, broken line, average of 32 steps on ten subjects). The time-average of the standard deviation of the mean was less than 25% of r(t) during the lift of the centre of mass (t_tr,up in Fig. 3) and less than 35% during its descent (t_tr,down). (B) The area under the average r(t) curve divided by the step period attains a value of R_int()=R_int at the end of the step which is equal for all groups of subjects. The corresponding mean values of the parameters for all European subjects and for the African women are given in Table 1 (unloaded).