Pronation is the inward (medial) roll of the foot, in particular the heel and arch which occurs naturally at the heel strike as a cushioning mechanism.
Over-pronation is when the feet roll inward too much.
Supination also known as under-pronation, is the opposite to pronation where the feet don't roll inward enough. Wearing the wrong type of shoe will lead to painful shins and joints, or even injury.
It is easy to find out if you're a pronator or supinator - Look at your trainers!
I) A pronator's outer soles wear down along the inside of the ball of the foot and they tend to have flat feet.
II) A supinator's outer soles wear down along the outer edge and their feet tend to have high arches. Shoes are designed with features to control these problems.
Pronation : When you run or walk, you land on the outside edge of your foot and roll inward. This entirely normal inward rolling is called pronation. For most runners, the pronation stops at a healthy point. However, some runners roll inward too much. This excessive inward rolling is called overpronation. Runners who overpronate should wear motion-control shoes, which contain special foams and devices that are designed to limit overpronation.
Here's the easiest way. Take off your shoes, whether your normal work-a-day shoes or your running shoes, and put them on a table with the heels facing out toward you. Now study the heels. If they are fairly straight and tall, you do not have an overpronation problem. If the heels tilt inward (toward the arches), on the other hand, you are probably an overpronator, and should try motion-control shoes. Many (but not all) overpronators are bigger, heavier runners with flat feet.
Supination : 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 lift 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.
Please note everybody pronates or supinates. It is the body's way to absorb shock and allow the foot to work as a lever. Excessive motion in either direction can be very problematic if not controlled
