As your foot lands at the end of a step, the vault flattens and expands, and your arches store energy which is released later as your foot lifts again. At that moment of lifting, an ingenious locking-unlocking mechanism also comes into play that transforms your springy foot into the stiff lever you need to push against the ground. Your foot rolls outward, away from the main arch–that is, it supinates. This rotation alters the arrangement of your foot bones so that they cinch together into a stiff lever.

The mechanism is a bit like those toys made from a jumble of parts threaded together with an elastic cord. When the cord is tight, the toy figure stands erect on its pedestal. When you push in the pedestal’s base, the cord loosens and the figure collapses.

What releases the tension in the cord that unifies the bones of your foot? As you step and the foot lands again, it rolls inward toward the arch; or pronates. That reversed rotation loosens the “cord” and unlocks the bony puzzle. Your foot becomes springy and malleable again, adapting to new surfaces and avoiding bruises or fractures from impacts. During running, the foot sustains forces as much as two-and-a half times your body’s weight.

Your foot is made up of many small bones. The elasticity of the ligaments and tendons that hold these bones together gives your foot flexibility. When you are standing motionless, the calcaneum bone and the first and fifth metatarsal bones are the endpoints of three arches­one along the inner edge of the foot, one along the outer edge, and one across the sole. When you run, these arches flatten and then return to their original shape, putting a spring in your step.

Some feet are equipped with special endowments that help them meet the challenges of particular sports. For instance, in some feet the first metatarsal bone–the “buttress” of the arch that connects to the big toe–hangs slightly lower than the other metatarsal bones, much as your thumb hangs down when you let your hand go limp. It’s a trait that probably dates from ancient days when our ancestors used their feet primarily to grasp branches.

This lowered bone, like a longer big toe, makes it easier to position the big toe for a hearty push-off. So, in every sport that involves sprinting or jumping, you can get faster acceleration. It also helps you edge better in skiing. But the trait comes with at least one disadvantage. Under repetitive pounding, as in long-distance running, the bone may bruise or fracture more easily because it protrudes slightly from the underside of the foot.

If your foot is unusually rigid, you also enjoy an advantage in sprinting and jumping. Rigid feet make consistently sturdy levers. By contrast, in sports like tennis, racquetball, and aerobics, where sideways motions predominate, feet that are unusually flexible work best. They can absorb the motion of a lot of short steps, lunges, and cuts, unlike rigid feet which are “terrible” at absorbing the stress of constant changes of direction, according to San Francisco sports podiatrist Richard Blake. “If you try to cut and your foot doesn’t give, you break a bone,” he says. In side-to-side sports, lower arches also help your play by providing a larger friction surface when you plant your foot to turn or brake.