Hyperkeratosis is the process in which a part of the skin develops into calluses, or dry, hard, thick skin, due to excessive pressure and constant friction. Calluses can develop on the sole of the foot, heel or the underside of the big toe. In addition to excessive walking and running, other factors can contribute to the constant pressure and friction that leads to calluses.
Abnormal metatarsal bones
People with flat feet or high arches typically have poorly aligned bones in their feet (the metatarsal bones), which results in uneven pressure when they walk.
Wearing high-heeled shoes or small, narrow shoes causes the skin to rub against the shoes, which creates a painful, constant friction.
Toe deformities, like a hammer toe, can increase the pressure on the toes.
The fatty tissues in the feet decrease as the body ages, which results in the loss of fat in the heels of the feet.
Carrying excess weight puts an increased amount of pressure on the feet.
Calluses may also develop as a part of the skin's natural response of protecting itself, as in the case of regularly walking barefoot.