Three Uses for Springs in Legged Locomotion

TLDR

Running animals and robots use springs—such as tendons and return springs—to store and release elastic energy, thereby saving energy and reducing heat during locomotion. Compliant foot pads absorb impact forces, reducing chatter and improving road holding by preventing repeated foot lift and settle.

Abstract

Running animals and robots can save energy and reduce unwanted heat production by bouncing along on springs, using the principle of the pogo stick. (The principal springs in animals are tendons.) They can make further energy savings by using return springs to halt the legs at the end of each foward or backward swing and start them swinging the other way. The forces at impact of feet with the ground can be moderated by compliant foot pads, which can improve road holding by preventing "chatter" (vibrations in which the foot repeatedly leaves and returns to the ground before settling). Animals use springs in all three ways but there seems to be a need for more use of springs in legged robots, especially in robots designed to run fast.