Perspectives on bone mechanical properties and adaptive response to mechanical challenge

The bones of the human skeleton serve a mechanical function besides providi ng a reservoir for calcium and hematopoietic homeostasis. 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 i ts material proper ties and by its geometry and architecture. Therefore, in vivo noninvasive measurements of bone mass, geometry, and structure can pr edict bone strength and are usually employed as a useful-if not always reli able-way to estimate bone fragility, whereas direct bone biomechanical test ing in vitro can provide detailed information about mechanical strength. Be cause bone strains are likely to be important regulators of bone mass and s trength, exercise protocols designed to counteract the effects of osteoporo sis should load the target bone with repeated high peak forces and high str ain rates or high impacts on a long-term basis. Such a protocol creates var ied strain distributions throughout the bone structure, producing short, re peated strains on the bone in directions to which it is unaccustomed. Exerc ise in this manner can maintain and perhaps increase bone mass and improve mechanical properties and neuromuscular competency, reducing skeletal fragi lity and the predisposition to falls.