With a normal footstep, the heel hits the ground first, followed by a rolling motion of the foot towards the forefoot and toes. Finally, the footstep is completed by pushing off of the ball area of the foot. During the motion of a normal footstep, the arch absorbs shock as it slightly flattens when the foot rolls toward the toes; the arch then springs back up into position. About half of the arch actually touches the ground, which is considered to be normal pronation.
In reality, many people do not have normal feet with normal pronation that complete normal footsteps. Overpronation and underpronation are two types of irregular footsteps: with overpronation, the foot rolls toward the inside more so than usual; with underpronation, the foot rolls toward the outside more so than usual. Overpronation, which indicates a flat foot and low or collapsed arch, can result in strained arches and pain along the inner area of the knee. Underpronation, which indicates a high arch, can lead to sprained ankles and stress fractures because the arch does not flatten enough to absorb shock.
To avoid the injuries that often result from overpronation and underpronation, it is important to check the pronation of your feet. This can be done using the simple water test: wet the entire bottom of your foot, then step your wet foot onto a piece of dark, heavy paper or a paper bag. See if your footprint shows about half of the arch (normal pronation), an oblong shape with no space left for the arch (overpronation), or only a thin line along the outside of the foot, next to the space left by the arch (underpronation).
Those with a normal pronation can wear almost any shoe type without compensation for a high or fallen arch. Overpronators can benefit from over-the-counter, molded-leather arch supports, stability shoes, or motion-control shoes. Underpronators should have a shoe with a neutral cushion and softer midsole to absorb shock; they should avoid motion-control footwear.