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

Abstract

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.

FIGURE 1

Associations of TBBMC with fat-free mass (top panel) and fat mass (bottom panel) in 685 women (triangles) and men (circles), aged 21 to 90 yr. Regression lines were generated separately for women (solid lines) and men (broken lines).

FIGURE 2

Changes in bone mineral density (%; left panel) and fat-free mass (kg; right panel) of postmenopausal women in response to 9 months of low-impact (i.e., weight lifting, rowing) or high-impact (i.e., walking, jogging, stair climbing/descending) exercise training versus a no-exercise control group. *Different from control, P < 0.05. [Adapted from Kohrt WM, Ehsani AA, Birge SJ Jr. Effects of exercise involving predominantly either joint-reaction or ground-reaction forces on bone mineral density in older women. J Bone Miner Res. 1997;12:1253–61. Copyright © 1997 American Society for Bone and Mineral Research. Used with permission.]

FIGURE 3

Point estimates of relative risk (±95% confidence intervals) of hip fracture from observational studies that examined multiple levels of physical activity (most active group vs least active group). [Adapted from Physical Activity Guidelines Advisory Committee. Physical Activity Guidelines Advisory Committee Report. Washington (DC): US Department of Health and Human Services (32).]

FIGURE 4

Age-related changes in fat-free mass (mean ± SD) in sedentary and physically active women (top panel) and men (bottom panel). [Adapted from Kyle UG, Genton L, Gremion G, et al. Aging, physical activity and height-normalized body composition parameters. Clin Nutr. 2004;23:79–88.]