Large mammals, including humans, save much of the energy needed for running by means of elastic structures in their legs and feet^1,2. Kinetic and potential energy removed from the body in the first half of the stance phase is stored briefly as elastic strain energy and then returned in the second half by elastic recoil. Thus the animal runs in an analogous fashion to a rubber ball bouncing along. Among the elastic structures involved, the tendons of distal leg muscles have been shown to be important^2,3. Here we show that the elastic properties of the arch of the human foot are also important.