Fig. 3. Energy recovery and mass-specific mechanical work per unit distance as a function of walking speed in different age groups. The pendular recovery of mechanical energy defined by equation 1 (R; circles) and by equation 2 (R_c; crosses) and the external work (W_ext) are presented in the two upper rows. The internal work done by one leg against the other (W_int,dc) is presented in the third row. The kinetic internal work (W_int,k; fourth row) is measured, allowing energy transfer between the segments of the same limbs but not between limbs. The internal work (W_int; fifth row) is measured, allowing energy transfer between W_int,dc and W_int,k (see Materials and methods). The bottom row shows the total work (W_tot=W_ext+W_int). The broken lines represent the weighted mean of the adult data (other indications are as in Fig. 2). Note that above 1 m s^–1, W_ext, W_int and W_tot are greater in children than in adults; these differences are greater the younger the subject and tend to disappear after the age of 10.