The effects of exercise training on bone development in growing rats were studied using the photon-absorption method and histomorophological analyses. Thirty-seven male Wistar strain rats at 4 wks of age were divided into five groups : sedentary control (C; N=7), 15 min/day training (T 15 ; N=6), 30 min/day training (T 30 ; N=8), 60 min/day training (T 60 ; N=8) and 120 min/day training (T 120 ; N=8) . All rats in these training groups were subjected to a treadmill running at a speed of 30 m/min, 5 days/wk for 11 wks. The results of our study were summarized as follows :
1) Fat-free dry weight of the tibia was significantly heavier in T 30, T 60 and T 120 than C and T 15, respectively.
2) Bone mineral content (mg/cm) of tibial midshaft in all T groups was significantly higher than that in C group, respectively, while no differences were found between any of the training groups.
3) On histological parameters of cross-sectional samples from tibia, such as cortical area, total area and appositional growth rate on periosteum, higher values were observed in T groups compared with C group.
These results suggest that physical training induce markedly facilitative girth growth associated with elevated bone formation of periosteum in the tibia, and moreover, the daily exercise duration does not change the degree of bone hypertrophy.

The effects of running exercise on bone histomorphometric parameters were examined in sham-treated (Sham) and ovariectomized (OVX) rats. The rats were either kept sedentary (C) or trained (T) on a rodent treadmill at 25 m/min, 30 min/day, 5 days/week for 10 weeks. Fat-free dry bone weight, bone mineral content (BMC), and histomorphometric parameters were assessed after excision of the tibiae at sacrifice. The results are summarized as follows
1) There were no significant differences in fat-free dry bone weight and BMC between OVX-C and Sham-C rats. However, BMC/body weight in OVX-C rats was significantly lower than in Sham-C animals. OVX-C rats had significantly lower trabecular bone volume at the proximal tibial metaphysis than Sham-C animals. Values of bone formation parameters were significantly higher in OVX-C rats than in Sham-C animals.
2) BMC/body weight in OVX-T rats was significantly higher than in OVX-C animals. OVX-T rats had a significantly higher trabecular bone volume at the proximal tibial metaphysis than OVX-C animals. There were no significant differences in bone formation parameters between OVX-T and OVX-C rats.
3) No significant differences were found in any parameters between sham-C and Sham-T rats.
These results suggest that (1) running exercise has a suppressive effect on bone loss after OVX, and (2) training-induced reduction of bone loss by OVX does not seam to be associated with facilitation of bone formation but with suppression of bone resorption.