Feet-Hurt.Com

Your heel swings from side to side as your foot moves. When the heel swings correctly, your foot can flatten and regain its arch as you walk. But if your heel swings too much, your foot may flatten more than it should. Over time, such excess movement may cause foot problems   During midstance, your heel is below the anklebone, and the front and back of your foot are aligned. Your foot easily bears your weight.   When the heel hits the ground, its outer edge touches first. Soft tissues (muscles, tendons and ligaments) relax. Your foot is able to flatten, adapt to uneven surfaces, and absorb the shock of touchdown.   As the heel lifts, it swings slightly to the inside. Muscles, tendons, and ligaments tighten. Your foot regains its arch, allowing your toes to push your weight off the ground.

From this site:   http://www.footcaredirect.com/walking.html