In previous generations flat feet kept many men out of the army. Now we know that flat feet are usually an asset; they help you run faster and jump higher. Most people with flat feet have normal arches. Their arches appear to be flat because during running and walking, their feet roll inward so far that they touch the ground and you can't see them. During running, your foot strikes the ground with a force equal to more than three times body weight. Landing flat footed transmits this force up your leg to damage muscles, bones and joints.
Feet are supposed to be shock absorbers. Instead of landing on the whole bottom of your foot, you land on the outside bottom of your foot and roll inward toward your arch. This is called pronation, which helps to prevent injuries by dissipating the force of your footstrike and keeping it from being directed at any one part of your leg or foot. The more you roll in, the harder you foot hits the ground to drive you forward with greater force and help you run faster. Almost all sprinters and halfbacks have pronated flat feet.