Fig. 1. Adult E_p and E_k oscillations are plotted as a function of gait cycle duration. The shaded area indicates the periods of double support. The IP mechanism, as observed in adult gait, shows an out-of-phase oscillation of kinetic and potential energy, allowing energy exchange to occur from E_p to E_k and partially from E_k to E_p. Though 70% of the required mechanical energy can be recovered due to this energy saving mechanism, 30% of energy is lost and must be re-supplied by the muscles. It is primarily used to redirect the centre of mass from one pendular arc to the next. HC, heel contact; OTO, opposite toe-off; OHC, opposite heel contact; TO, toe-off.