[Developments of the theory of skeletal adaptation to mechanical loading]

The primary mechanical function of bone is to provide rigid levers for limbs to act against mechanical loading and gravity, and to remain as light as possible to allow efficient locomotion. There are increasing evidence that mechanical loading is an important, if not the most important, factor influences bone mass and architecture. Many bones are exposed to thousands of repetitive loads every day, which always cause strains. Unusual strain distributions, high strains, and high strain rates seem to be particularly osteogenic. Bone modeling can increase bone strength and mass, bone remodeling can conserve or reduce them, and each can be turned ON or OFF in response to its own threshold range of bone strains. During growth and development, the skeleton optimizes its architecture and strength by (re)modeling to adapt to the largest voluntary loads on bones. The loads come from muscles, not body weight. A new standard of defining osteoporosis might relate bone strength to muscle strength that concerns the osteopenias pathogenesis and its severity, and it would be useful for prevention and cure of osteoporosis.