Ovariectomy-induced bone loss can be affected by different intensities of treadmill running exercise in rats.

Fifty-six Sprague-Dawley rats were either ovariectomized (OVX, n = 24), sham-operated (Sham, n = 24), or sacrificed (n = 8) at the beginning of the experiment to serve as a baseline group. The OVX and Sham groups were further randomly divided into control (CTRL), slow running (R10), and faster running (R18) groups. R10 and R18 groups ran for 2 x 30 min/day for 8 weeks at speeds of 10 m/min and 18 m/min, respectively. Exercise did not affect the mechanical or histomorphometric parameters of bone in the sham-operated rats. There was no effect of exercise on body weight gain in the OVX-R10 group, but in OVX-R18 it decreased the gain of body weight. In the OVX-CTRL group the maximal load and energy absorption of the femoral neck were 16.7% (P < 0.001) and 30.0% (P < 0.001) lower than in the Sham-CTRL group, respectively. In OVX animals, slow running had a positive effect on the maximal load of the femoral neck (86.5 N) when compared with OVX-CTRL rats (77.1 N, P < 0.07). 51.7% of the trabecular bone was lost in the distal femur as a result of OVX and exercise reduced this loss to 30.2% (R10) and 39.9% (R18). Ovariectomy increased the bone formation rate (BFR) and the mineral apposition rate (MAR) on the periosteum of the femoral shaft. Exercise decreased the periosteal BFR and MAR in OVX rats, but increased it at the endosteum. Osteoclast numbers in the femoral metaphysis were increased after OVX and running exercise inhibited this effect significantly. The maximal bending load of the humerus increased after OVX by 12.1% (P < 0.05). Exercise enhanced this effect, the slow running being more effective. These results suggest that bone in OVX rats is either more sensitive to exercise than in sham-operated rats or that the higher body weight with slow running induces optimal loading and strengthens the bones.