A running animal coordinates the actions of many muscles, tendons, and ligaments in its leg so that the overall leg behaves like a single mechanical spring during ground contact. Experimental observations have revealed that an animal's leg sti¡ness is independent of both speed and gravity level, suggesting that it is dictated by inherent musculoskeletal properties. However, if leg sti¡ness was invariant, the biomechanics of running (e.g. peak ground reaction force and ground contact time) would change when an animal encountered di¡erent surfaces in the natural world.We found that human runners adjust their leg sti¡ness to accommodate changes in surface sti¡ness, allowing them to maintain similar running mechanics on di¡erent surfaces. These results provide important insight into the mechanics and control of animal locomotion and suggest that incorporating an adjustable leg sti¡ness in the design of hopping and running robots is important if they are to match the agility and speed of animals on varied terrain.