The aim of this study was to increase our understanding of the effect of exercise on cortical bone mass and turnover in aged female rats. Female Sprague-Dawley rats, 14 months of age, were divided into four groups: 8 controls and 10 exercised for the 9-week study, and 8 controls and 9 exercised for the 16-week study. Exercise consisted of treadmill running at 17 m/min for one h/day and 5 days/week for 9 and 16 weeks. All animals received double fluorochrome labeling of bone prior to sacrifice. Histomorphometric analysis was performed on 30-mu m-thick Villanueva-stained, undecalcified cross-sections of the tibial shaft. Tibial diaphyseal mineral density of each rat in the 16-week study was measured by dual energy x-ray absorptiometry in vivo at 0, 9, and 16 weeks. The diaphyseal mineral density of the exercised group was significantly greater than that of the control group (p < 0.05 by two-way ANOVA) and the individual slopes of the density vs. time was found to be higher in the exercised than in the control animals (mean +/- SE of exercised 0.56 +/- 0.13 vs. control 0.19 +/- 0.07 mg/cm(2)/week, p < 0.05) by the end of the experiment. The results of the histomorphometric analysis after 9 weeks of exercise showed that the periosteal labeled surface, mineral apposition rate, and bone formation rate were profoundly increased by 192% (p < 0.001), 35%, and 206% (p < 0.01), respectively. These values declined at 16 weeks (only the increase in labeled perimeter was significant, p < 0.05). Bone resorption on the endocortical surface was reduced (nonsignificantly) by 9 or 16 weeks of exercise compared with the control groups. However, the cross-sectional area and cortical bone area increased by 3% and 4%, respectively, after 9 weeks of exercise, reaching 8% (p = 0.053) and 10% (p = 0.057), respectively, at 16 weeks of exercise training. The marrow area of the exercised group did not differ from the aged-matched control group at each time point. In conclusion, running exercise created an accumulation of tibial cortical bone mass, which was due to stimulated periosteal bone modeling in the formation mode. A trend of reduced resorption (nonsignificant) at the exercised tibial endocortical surface at 9 and 16 weeks may suggest that exercise probably depressed endocortical BMU-based bone remodeling in the resorption mode. The benefit of exercise was not limited in aged female rats. The results were generally in agreement with the mechanostat theory proposed by Frost.
