Fig. 6. Hypothetical propulsive impulses consistent with collision amelioration. If the mechanism for providing a powering impulse to the centre of mass (COM) also smoothes its path (A), powering is more efficient as collision losses are reduced. This may be achieved by two mechanisms (B): (i) hip torque while the hindleg is in early stance, or (ii) hindleg extension while the hindleg is late in stance. Either, or both, would have the effect of smoothing the path of the COM during forefoot placement, thus reducing the relatively high collision losses at this instant. Both of these mechanisms would be more effective, providing upward and forward impulses, with fore–hind phases of less than 25%.