Fig. 9. Preferred walking speed, W_tot, W_ext, W_int,k and work during double contact (expressed as a fraction of W_ext) are plotted as a function of walking experience. There is a slight increase in preferred walking speed as the children gain more experience in walking. W_tot and W_ext show a U-shaped relationship with walking experience. But walking experience showed no relationship with recovery values (and consequently the efficiency of the pendular exchange of energy), W_int,k and work during double contact.