RUNNING IN THE REAL-WORLD - ADJUSTING LEG STIFFNESS FOR DIFFERENT SURFACES

A running animal coordinates the actions of many muscles, tendons, and ligaments in its leg so that the overall leg behaves like a single me chanical spring during ground contact. Experimental observations have revealed that an animal's leg stiffness is independent of both speed a nd gravity level, suggesting that it is dictated by inherent musculosk eletal properties. However, if leg stiffness was invariant, the biomec hanics of running (e.g. peak ground reaction force and ground contact time) would change when an animal encountered different surfaces in th e natural world. We found that human runners adjust their leg stiffnes s to accommodate changes in surface stiffness, allowing them to mainta in similar running mechanics on different surfaces. These results prov ide important insight into the mechanics and control of animal locomot ion and suggest that incorporating an adjustable leg stiffness in the design of hopping and running robots is important if they are to match the agility and speed of animals on varied terrain.