There are two kinds of cells found in bone. The osteoclasts are constantly destroying old bone while osteoblasts are constantly building new bone. It is necessary for these two processes, bone destruction and bone building to be balanced in order to have solid healthy bones. There are two control mechanisms by which osteoblasts are stimulated to create new bone mass. One is hormonal regulation and the other is piezoelectric.
Bone consists of a protein matrix impregnated by minerals and penetrated by blood vessels.
The mineral components are piezoelectric crystalline structures. Piezoelectric means that when mechanical stress is applied to the bone, the crystalline structures generate an electric charge. This electric charge stimulates the deposition of more minerals along the lines of stress. It is because of this mechanism that physical exercise creates strong bones and lack of physical exercise causes loss of bone density. This is also the reason that astronauts lose bone density when exposed to periods of weightlessness.