Over-pronation, or flat feet, is a common biomechanical problem that occurs when a person's arch collapses upon weight bearing. This flattening of the skeletal framework of the foot can place stress on the plantar fascia, the thick, fibrous band of tissue that reaches from the heel to the toes, causing painful inflammation. It can also lead to other foot problems, like heel spurs, tendonitis, or bunions. Often people with naturally low arches never experience discomfort. However, when symptoms develop and become painful, walking becomes awkward and causes increased strain on the feet and calves.

Source: foot.com