Muscle forces or gravity: what predominates mechanical loading on bone?

Most mechanical forces acting on the skeleton are generated either through impact with the ground (i.e., gravitational loading) or through muscle contractions (i.e., muscle loading). If one of these conduits for activating mechanotransduction in bone is more effective than the other with respect to developing or maintaining bone strength, this would have important clinical implications for prescribing physical activity for the prevention or treatment of osteoporosis. This section of the symposium considered whether there is evidence from studies of humans that the effectiveness of physical activity to preserve bone health is dependent on whether the activities stimulate the skeleton primarily through gravitational or muscle loading. Conclusive evidence is lacking, but several lines of research suggest that physical activities that involve impact forces, and therefore generate both gravitation and muscle loading, are most likely to have beneficial effects on bone metabolism and reduce fracture risk.