How You Walk

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.

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.
During midstance, your heel is below the anklebone, and the front and back of your foot are aligned. Your foot easily bears your weight.
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.

Too Much Movement Causes Strain!

When your foot flattens too much (overpronation), some bones are forced to support too much weight. The muscles pull harder on these areas, making it more difficult for tendons and ligaments to hold bones and joints in place. Over time, you may develop swelling or pain on the bottom of your foot or near the heel. Or a bony bump (bunion) may form at your toe joint.

When your foot flattens too much, the ankle and heel do not align during midstance. The foot strains under your body's weight.

MORE good information:
General Foot Care Tips
Finding the Right Shoes For You