Over-pronation and Supination
With pronation, when you walk the foot rolls inwards. This is actually a perfectly normal part of the gait cycle. The problems occur when your foot rolls inwards too far (over-pronation), or does not roll inwards enough (supination).
With over-pronation the foot arch flattens out and stretches the muscles, tendons and ligaments underneath the foot. The arch acts as a shock absorber and if it is not able to do this the force may travel to your legs, knees, hips, and spine. Your foot rolling inwards can cause your leg to twist inwards too, putting extra stress on the inside of your knee
With supination there is more of an outward roll of the foot and forces of impact are concentrated on the outside edge of the foot and are not distributed as efficiently.
This places a large strain on the muscles and tendons that stabilize the ankle, and can lead to additional strain to the lateral ankle ligaments and lateral knee.