Additional weight-bearing during exercise is more important than duration of exercise for anabolic stimulus of bone: a study of running exercise in female rats

Abstract

Mechanical loading is necessary for maintenance of skeletal integrity, but the most effective type, intensity, and duration of exercise are not known. In vivo experiments have indicated that the strain generated by the stimulus is more important than the duration of the stimulus. To elucidate this question, we studied 5-month-old female Wistar rats exercised on a motor-driven exercise belt for 17 weeks, 5 days per week (average velocity 20 m/min). Group 1 served as controls, group 2 was trained for 30 min, group 3 was trained for 30 min with a 50-g backpack, and group 4 was trained for 15 min with a 50-g backpack. Total body bone mineral content (BMC), bone mass of the lower extremities (LEBMC), total body lean soft-tissue mass (LSTM), and total body fat-tissue mass (FTM) were measured by dual-energy absorptiometry (DXA) at 0, 6, and 17 weeks. The BMC increased more in group 4 than in controls (15% vs. 8%, p < 0.03). In the other two intervention groups, no significant increases of total body BMC occurred compared with controls, although a trend was observed (12%). The LEBMC increased significantly in all exercising groups after 17 weeks, being 16% in group 2, 15% in group 3, and 20% in group 4, compared with 6% in controls ( p < 0.05). The increase in LSTM after 6 weeks was most pronounced in group 3, at 20%, compared with 10% in the control group ( p < 0.001). After 6 weeks of exercising, group 3 showed the most pronounced decrease in FTM, amounting to 33%, compared with a 1% increase in group 1 ( p < 0.001). Biomechanical testing of the femoral neck and cylinders of the femoral cortex yielded a pattern similar to that observed in BMC measurements of the total body and lower extremities. Biomechanical testing of the fourth lumbar vertebrae yielded no significant differences among the four groups. We conclude that a short duration of exercise with additional weight-bearing using a backpack leads to the most pronounced effects on total body BMC and LEBMC. In all exercising groups, the regional effect on the LEBMC was more pronounced than the effect on total body BMC; this was also reflected in the biomechanical properties.