Exercising the skeleton: Beneficial effects of mechanical loading on bone structure

Physical exercise can strengthen the skeleton substantially, provided that one begins exercising at a young age. During skeletal growth, exercise augm ents weight-bearing bone structure by enhancing periosteal apposition. The largest gains in bone mass tend to be where hydrostatic pressures within ex tracellular fluid compartments are greatest. This finding suggests that ext racellular fluid pressures or fluid shear stress on bone cells pro vide a s timulus for bone formation. The best exercises for building bone are those involving impact loading of the skeleton. However, recent prospective studi es have shown that even impact exercise is not very effective for building bone in adults. In addition, impact exercise can increase the risk of osteo arthritis in older adults. Considering the meager benefits and potential ri sks df impact exercise, one might question whether it should be prescribed for prevention of osteoporosis. instead low-impact exercises designed to im prove muscle strength and balance may be preferred for prevention of fractu res. The goal of the low-impact exercise strategy is to prevent falls rathe r than to build bone mass. Several biochemical mediators of the mechanical loading effects on bone have been identified. Foremost among these are pros taglandins and nitric oxide. In addition, parathyroid and growth hormones p rovide permissive signals for bone mechanotransduction. finally, studies of inbred strains of mice suggest that the ability of bone to respond to mech anical loads is under genetic control.