Pronation refers to the inward roll of the foot during normal motion and occurs as the outer edge of the heel strikes the ground and the foot rolls inward and flattens out. A moderate amount of pronation is required for the foot to function properly, however damage and injury can occur during excessive pronation. When excessive pronation does occur the foot arch flattens out and stretches the muscles, tendons and ligaments underneath the foot.
Supination (or under-pronation) is the opposite of pronation and refers to the outward roll of the foot during normal motion. A natural amount of supination occurs during the push-off phase of the running gait as the heel lifts off the ground and the forefoot and toes are used to propel the body forward. However, excessive supination (outward rolling) places a large strain on the muscles and tendons that stabilize the ankle, and can lead to the ankle rolling completely over, resulting in an ankle sprain or total ligament rupture.