Every time your legs and feet land on the ground during a run, these tendons stretch as your hips, knees, and ankles bend and your arch flattens. When the tendons recoil, the energy they store is returned to help catapult you back into the air. All animals adapted for running, from kangaroos to deer, have legs with long, springy tendons, but these tendons are short in our close relatives the African apes.
