The principle that bone undergoes changes in density and shape in response to the mechanical stresses imposed on it. Physical activity is essential to maintain healthy bones, whereas inactivity results in loss of mineral substance and weakening of bones, as evidenced by astronauts exposed to zero gravity for long periods. Mechanical loading creates hydrostatic pressure gradients within fluid-filled spaces in the bone tissue. These internal fluid pressures stimulate osteocytes and osteoblasts to promote bone metabolism and matrix production, thereby remodelling the bone. The law is named after German anatomist Julius Wolff (1836–1902).