Supination is the opposite motion of pronation. A foot is in supination when the ankle appears to be 'tipped' to the outside so you are standing on the outside border of the foot. Supination allows the foot to be a more stable, rigid structure for when we push off on our next step.
The foot naturally supinates during the toe-off stage (when the heel first lifts off the ground until the end of the step) to provide more leverage and to help roll off the toes.
Excessive supination predisposes the ankle to injury because the stabilizing muscles on the outside of the lower leg (peroneals) are in a stretched position. It does in not take much force to cause the ankle to roll over, potentially causing ligament damage. It is the body's way to absorb shock and allow the foot to work as a lever.
