Although pronation is a normal, natural rolling motion of the foot that helps attenuate shock, severe overpronation occurs when the ankle rolls inward towards the center of the vertical line of the lower leg at more than 15 percent angle forcing the heel to lean inward. This means the foot and ankle cannot properly stabilize the body, weigh is carried on the inside edge of the foot and shock isn't absorbed as efficiently. This causes the big toe to do most of the push off at the end of the gait cycle. People with flat feet tend to overpronate.
