The effect of swing leg retraction on running energy efficiency

Swing leg retraction reduces touchdown energy losses of running by decreasing foot speed at the moment of ground contact, but does the additional acceleration of the swing leg cost more energy than is saved? To determine whether swing leg retraction can increase the overall energy efficiency of running robots, we find the optimally efficient gaits of a McGeer-like runner over a range of retraction rates. Results show that overall energy usage, including that used to swing the legs, scales with energy loss at touchdown, which is minimized at the retraction rate that zeros foot tangential speed at ground contact. Other benefits of swing leg retraction, such as reduced foot slippage and damaging touchdown forces, are realized simultaneously with optimal energy efficiency.