Callouses are areas of skin that thicken in response to repeated pressure and friction. They are the body's way of protecting the skin and the structures beneath it from injury. Callouses can form on the skin on any part of the body that is exposed to excessive pressure or friction.
Sometimes they are useful. People who work hard with their hands typically develop thick callouses that protect their fingers and palms from nicks, scratches and cuts. Playing a stringed musical instrument is a painful process until strong callouses develop on the fingertips. In parts of the world where people commonly go barefoot for much of the year, a thick layer of callous will develop along the plantar (bottom) surface of the feet to provide protection. In areas of the world where wearing shoes predominates, callouses on the feet can cause a problem. Callouses typically occur on the sole of the foot, on heel or under the the five metatarsal heads (the area where the long inner bones of the toes extend into the foot). These areas typically bear most of the pressure and friction from standing and walking. As the callous thickens, it causes more pressure against the skin and inside of the shoe, causing pain.
Callouses on the hands reflect normal wear and tear, but callouses on the feet usually indicate a biomechanical problem, resulting in excessive pressure between the skin and bones underneath. Callouses can develop under the metatarsal head for two reasons. In many cases one or more of the metatarsal heads (usually the first or fifth) is too low, causing it to bear more pressure than the others. In other cases callouses indicate that one of the metatarsals is unstable and shifts weight to those adjacent to it. This is commonly seen in people with flat feet (pes planus). The arch is too low and the foot is unstable. The first metatarsal (the one connected to the big toe) is also unstable and when weight is applied to this area the first metatarsal drifts upward causing the second metatarsal to accept the extra weight. The second metatarsal is not capable of supporting this extra force and a callous forms where the skin tries to protect the bone. This process can happen with the other metatarsals as well. More than one callous often forms on the foot at the same time. In many cases a single large callous develops across the entire metatarsal pad on both feet.