Shorter, more frequent mechanical loading sessions enhance bone mass

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Abstract
The beneficial effects of exercise on bone mass and strength can be attributed to the sensitivity of bone cells to mechanical stimuli. However, bone cells lose mechanosensitivity soon after they are stimulated. We investigated whether the osteogenic response to a simulated high-impact exercise program lasting 4 months could be enhanced by dividing the daily protocol into brief sessions of loading, separated by recovery periods. The right forelimbs of adult rats were subjected to 360 load cycles·d−1, 3 d·wk−1, for 16 wk. On each loading day, one group received all 360 cycles in a single, uninterrupted bout (360 × 1); the other group received 4 bouts of 90 cycles/bout (90 × 4), with each bout separated by 3 h. After sacrifice, bone mineral content (BMC), and areal bone mineral density (aBMD) were measured in the loaded (right) and nonloaded control (left) ulnae using DXA. Volumetric BMD (vBMD) and cross-sectional area (CSA) were measured at midshaft and the olecranon by using pQCT. Maximum and minimum second moments of area (IMAX and IMIN) were measured from the midshaft tomographs. After 16 wk of loading, BMC, aBMD, vBMD, midshaft CSA, IMAX, and IMIN were significantly greater in right (loaded) ulnae compared with left (nonloaded) ulnae in the two loaded groups. When the daily loading regimen was broken into four sessions per day (90×4), BMC, aBMD, midshaft CSA, and IMIN improved significantly over the loading schedule that applied the daily stimulus in a single, uninterrupted session (360×1). Human exercise programs aimed at maintaining or improving bone mass might achieve greater success if the daily exercise regime is broken down into smaller sessions separated by recovery periods.