Calluses are patches of rough, thick and yellowed skin that often develop on the soles of the feet.
Calluses can develop anywhere there is repeated friction. The common foot callus usually occurs at the ball of the foot, the heel, and/or the inside of the big toe.
A plantar callus is found on the bottom of the foot.
CAUSES:
The cause of the callus is going to be from some external pressure pushing against a boney prominence. The external pressure could be the ground (affecting the bottom of the foot), the shoe (affecting the sides or the top of the foot) or even and adjacent toe (affecting between the toes).
By definition, the build up of protective skin on the bottom of the foot is referred to as a callus. If the same problem occurs on the toes, it's called a corn.
Calluses are a symptom of some underlying problem which exists in the ball of the foot. The normal foot can carry our weight comfortably, without the formation of calluses and pain.
When a callus occurs, it signifies a change is occurring in the metatarsals (the long bones of the feet to which the toes connect). There are five metatarsals, and they each carry a portion of the body weight as we walk. When one or more metatarsals carry more weight then they are supposed to, the skin under these bones becomes thick, hard, and painful.
Some calluses on the feet develop from an improper walking motion, but most are caused by ill-fitting shoes.