Fig. 4. (A) The mass-specific work per distance (W_int,dc; J kg^-1 m^-1; filled circles) done by one leg against the other during the double contact (DC) phase and the mass-specific external work per distance (W_ext; J kg^-1 m^-1; open circles) done to maintain the centre of mass (COM) motion in the sagittal plan are given for each age group as a function of the walking speed. (B) The ratio W_int,dc/W_ext is given for each age group as a function of speed in the lower panels. Other indications are as in Fig. 3.