The effect of exercise-induced mechanical stimuli on cortical bone

Physical activity is capable of increasing bone mass. The specific osteogenic component of the mechanical milieu remains, however, unknown. Three experiments were designed to examine the response of the middiaphyseal rooster tarsometatarsus, to mechanical stimuli induced by running and high-impact drop-jumping. The mechanical environments produced by walking, running, and drop-jumping were quantified via in vivo strain gages, linear beam theory, and finite element modeling. Distributions of mechanical parameters proposed to drive adaptation, including strain magnitude and distribution, strain rate, and strain gradients, were spatially correlated with exercise-related changes across the middiaphysis.

In the first experiment, young adult (1.5 yr) roosters were subjected to high-speed running for 3 wk (1500 steps per day). Running activated previously quiescent periosteal surfaces in the tarsometatarsal middiaphysis, and the specific sites of activation were highly correlated with circumferential strain gradients (r² = 0.63). In the second experiment, 9 wk old roosters ran (2600 steps per day) for 8 wk. Running modestly altered the mechanical milieu compared to walking, but did not produce significant changes in tarsometatarsal morphology, mechanics, or mineral content. In the third experiment, 200 daily high-impact drop-jumps were applied to 17 wk old roosters for 4 wk. Drop-jumping induced very high strain rates and led to substantially increased bone formation compared to sedentary controls. Strain rates were spatially correlated (r ² = 0.440.67) with the specific sites of additional bone formation at the endocortical surface. No parameter, however, successfully explained increased periosteal bone formation.

These studies emphasize that the knowledge of the exercise-induced mechanical milieu may help to design exercise protocols that effectively increase bone mass in the growing and young adult skeleton. A previously proposed mechanical parameter (circumferential strain gradients) was confirmed as a good predictor of the specific sites of bone forming surfaces in the young adult skeleton. Comparing the mechanical milieus produced by high-speed running and drop-jumping suggests that the osteotropic efficacy of drop-jumping was accounted for by very high strain rates. The good correlation between the distribution of strain rates and the specific sites of increased lamellar bone formation within the middiaphysis accentuates the sensitivity of growing bone to this mechanical parameter.