Dose-response relationship between physical loading and mechanical competence of bone.

A dose-response relationship between mechanical loading and bone can be inferred by the fact that bone response is proportional to the applied load in cell and organ culture and in animal models where a known load at a given magnitude and frequency is applied. Proportional responses have been observed in second messengers, growth factors, bone matrix, and bone strength. In the human model, however, the determination of a dose-response relationship is hindered by the lack of appropriate technology to directly evaluate the mechanical load and the skeletal competence. In vitro loading of bone does not duplicate the in vivo physiologic conditions, in particular the neuromuscular responses to the loads applied to the skeleton. While animal studies aid in determining the mechanisms of bone response to mechanical loading and dietary interactions, they do not obviate the need for human clinical trials to evaluate the effects of physical activity on both skeletal competence and fall prevention. The literature reviewed shows that physical activity increases the competence of the skeleton to resist fracture by the maintenance and improvement of bone mineral density (BMD) and neuromuscular competency, thus reducing skeletal fragility, pre-disposition to falls, and fall impact.