Effect of resistance exercise training on cortical and cancellous bone in mature male rats.

The effect of resistance training on tibial cancellous and cortical bone was evaluated in rats by using static histomorphometry and Northern analysis. Five-month-old male Sprague-Dawley rats were randomly assigned to exercise (Ex; n = 8) or control (Con; n = 4) groups. Animals were operantly conditioned to press two levers, facilitating full extension and flexion of the hindlimbs ("squats"), while wearing an unweighted vest. After an 8-wk familiarization period, Ex animals performed 3 sessions/wk for 17-19 sessions with progressively increased amounts of weight applied to the vest. Con rats completed the same exercise protocol without applied resistance. No difference in cross-sectional, medullary, or cortical bone area was observed between Ex and Con rats in the tibial diaphysis. In contrast, the cancellous bone area in the proximal tibial metaphysis was significantly larger in trained rats. Trabecular number, trabecular thickness, and the percentage of cancellous bone covered by osteoid were significantly greater in the Ex animals compared with Con animals. In addition, steady-state mRNA levels for osteocalcin for the Ex group were 456% those expressed in the Con group. The data demonstrate that resistance training increases cancellous bone area in sexually mature male rats and suggest that it does so, in part, by stimulating bone formation.