Figure 7. Model showing the effect of gravity on the pendular transfer of energy during the walking step.

The forward velocity change of the centre of mass of the body due to the impact against the ground (ΔV_ground; right inset), and to the action of gravity during the pendular transfer of kinetic energy into potential energy (ΔV_gravity; left inset), are given as a function of walking speed (Appendix). ΔV_ground is determined geometrically from the orientation of the limb and the velocity of the centre of mass (V_c) when the heel strikes the ground. ΔV_gravity is calculated by equalizing the differential of the kinetic energy of forward motion with the differential of the potential energy. In the bottom equation, the finite forward velocity change, due to the shift of kinetic energy into gravitational potential energy, is called ΔV_gravity, and the finite change in height is the vertical displacement of the centre of mass S_v. This is calculated geometrically as described in the Appendix from the forward displacement of the centre of mass during single contact (L_sc) and the length of the leg (l_leg). The continuous lines refer to 0.4 g, the dotted lines to 1.0 g and the dashed lines to 1.5 g. Note that, at each gravity level, ΔV_ground equals ΔV_gravity at the speed at which the pendular recovery of mechanical energy is at a maximum, as indicated by the arrows on the abscissa and in Fig. 5.