Without taking off your shoes, try to remember: Is your big toe longer or shorter than your second toe? The answer, according to foot experts who study sports performance, can give a telling clue to athletic ability.

On most people's feet the second toe is longer. But, if your big--or first--toe extends further, you possess a natural advantage in skiing, sprinting, and certain other sports. Having a longer big toe, you can more readily lean your full body weight onto it. In skiing, planting the big toe is a must for cutting an edge. Sprinters also need a firmly grounded big toe to accelerate quickly. Your big toe can exert about twice as much force as your second toe.

When you consider the elemental role of feet in human motion, it's not surprising that even such small variations in their structure affect sports performance. After all, footwork forms the foundation of most sports. In football, track, soccer, and other running sports, your feet are the springs and levers that cushion and propel you. In basketball and volleyball, they're launching pads for leaps. In tennis and aerobics, they act as brakes and pivots for side-to-side motions. They're formidable weapons in martial arts, dainty pillars in ballet, and grippers and levers for climbing.

By their unique structure, your feet simultaneously support your weight, balance and propel you, and safely absorb the shocks of your motion. Central to their success is the arch, which is actually a complex of three bony arches: the familiar, tall one along the inner edge of the foot; a less lofty one that runs along the foot's outer edge; and a third one that curves the sole across its width from ball to heel.

Together, the three form a supporting vault that distributes your weight much as the walls of a medieval church redirect the mass of its enormous roof onto giant buttresses. The buttresses in this case are the heel and two shaft-like bones, called the first and fifth metatarsals, that connect to the first and fifth toes.

Unlike an architectural vault, however, your foot's structure must respond to constant changes in the strength and direction of downward force as you move. Each arch consists of numerous bones bound together by tough but somewhat elastic ligaments and tendons. Because of these elastic connections, arches can flex.