Influence of exercise on cancellous bone of the aged female rat

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Abstract
Dual-energy x-ray absorptiometry and dynamic histomorphometry were used to examine the effect of treadmill exercise on the bone density and cancellous bone formation and resorption in the proximal tibia and fifth lumbar vertebra (L5) of the aged female rat. Female rats aged 14 months were divided into four groups: 8 controls and 10 exercised for a 9 week study and 8 controls and 9 exercised for a 16 week study. Exercise consisted of running on a flat-bed treadmill, 17 m/minute, 1 h/day, 5 days/week. Tibial metaphysis and L5 vertebral density of each rat were measured in the 16 week study by DXA at weeks 0, 9, and 16. Compared to the control group, a significant increase in bone density in both metaphyseal tibia and L5 vertebra was apparent at 16 weeks after exercise training (P = 0.046 and 0.025, respectively, by two-way ANOVA). Histomorphometric analysis showed that the trabecular bone eroded surface and the ratio of eroded to mineralizing surface in tibial metaphysis were significantly lower in the exercised than in the respective control group in both the 9 and 16 week studies. In L5 vertebra, these decreases by exercise were apparent only in the 16 week study. A significant increase in the bone formation rate was apparent in the cancellous bone of the tibia but not of the vertebra after 16 weeks of exercise (P < 0.05). The trabecular architecture (bone number and separation) of the L5 vertebra in the exercised rats did not differ from that of the controls in either study. The trabecular bone volume and trabecular number of the tibial metaphysis were higher and the separation was less in the exercised rats but not significantly different from the control at each time point. In conclusion, it was demonstrated that the tibial metaphyseal and L5 vertebral bone density were higher in the treadmill-exercised than in the nonexercised adult rat. Increased bone formation and suppressed resorption could account for the beneficial effect of exercise on cancellous weight-bearing bone. In cancellous non-weight-bearing bone, it could be primarily in suppressing bone resorption and inhibiting age-related loss.