Abstract: Objective To observe the effect of rapid reduction of physical activity on bone mass (BMC), bone mineral density (BMD)and bone metabolism in growing rats. Methods 48 male SD rats were divided into the high-intensity exercise group (T1, n=12), high tolow-intensity exercise group (T2, n=12), low-intensity exercise group (T3, n=12) and control group (Con, n=12). During the first 6 weeks,the rats in T1 and T2 group ran on treadmill with high intensity, while rats in T3 group took low-intensity treadmill exercise. 6 rats were sacrificedfrom each group at the end of 6th week. BMC, BMD of femur were measured with DEXA. Alkaline phosphatase (ALP), osteocalcin(BGP), tartrate-resistant acid phosphatase (TRACP) were detected with ELISA. The exercise intensity of T2 group was reduced rapidly to alow level as in T3 group, but remained in the other 2 groups. At the end of the second 6 weeks, the other rats were investigated again. ResultsCompared with the first measurements, ALP and BGP of the T2 group decreased significantly (P<0.05), and TRACP increased (P<0.05). There was no significant difference in BMC and BMD of femur (P>0.05). Conclusion Rapid reduction of physical activity can suppressbone formation and stimulate bone resorption of long bone among growing rats.

Key words: physical activity, bone mass, bone mineral density, bone metabolism, rats