Perspectives on bone mechanical properties and adaptive response to mechanical challenge.

The bones of the human skeleton serve a mechanical function besides providing a reservoir for calcium and hematopoietic homeostatis. When mechanically challenged, they usually respond and adapt; failure to do so can result in fracture. The mechanical behavior of bone is determined by bone mass and its material properties and by its geometry and architecture. Therefore, in vivo noninvasive measurements of bone mass, geometry, and structure can predict bone strength and are usually employed as a useful-if not always reliable-way to estimate bone fragility, whereas direct bone biomechanical testing in vitro can provide detailed information about mechanical strength. Because bone strains are likely to be regulators of bone mass and strength, exercise protocols designed to counteract the effects of osteoporosis should load the target bone with repeated high peak forces and high strain rates or high impacts on a long-term basis. Such a protocol creates varied strain distributions throughout the bone structure, producing short, repeated strains on the bone in directions to which it is unaccustomed. Exercise in this manner can maintain and perhaps increase bone mass and improve mechanical properties and neuromuscular competency, reducing skeletal fragility and the predisposition to falls.