STRAIN GRADIENTS CORRELATE WITH SITES OF EXERCISE-INDUCED BONE-FORMING SURFACES IN THE ADULT SKELETON

Physical activity is capable of increasing adult bone mass, The specif ic osteogenic component of the mechanical stimulus is, however, unknow n. Using an exogenous loading model, it was recently reported that cir cumferential gradients of longitudinal normal strain are strongly asso ciated with the specific sites of periosteal bone formation. Here, we used high-speed running to test this proposed relation in an exercise model of bone adaptation, The strain environment generated during runn ing in a mid-diaphyseal tarsometatarsal section was determined from tr iple-rosette strain gages in six adult roosters (>1 year), A second gr oup of roosters was run at a high speed (1500 loading cycles/day) on a treadmill for 3 weeks, Periosteal surfaces were activated in five out of eight animals, Mechanical parameters as well as periosteal activat ion (as measured by incorporated fluorescent labels) were quantified s ite-specifically in 12 30 degrees sectors subdividing a mid-diaphyseal section. The amount of periosteal mineralizing surface per sector cor related strongly (R-2 = 0.63) with the induced peak circumferential st rain gradients. Conversely, peak strain magnitude and peak strain rate were only weakly associated with the sites of periosteal activation, The unique feature of this study is that a specific mechanical stimulu s (peak circumferential strain gradients) was successfully correlated with specific sites of periosteal bone activation induced in a noninva sive bone adaptation model. The knowledge of this mechanical parameter may help to design exercise regimens that are able to deposit bone at sites where increased structural strength is most needed.