Maintenance of bone mass and mechanical properties after short-term cessation of high impact exercise in rats

The effects of cessation of high-impact exercise on bone mass in female Wistar rats was assessed. Fifty 10-week old rats were randomly divided into five groups (n = 10): 4 weeks jump-exercise (4JEX), 4 weeks sedentary control (4S), 8 weeks jump-exercise (8JEX), 4 weeks jump-exercise followed by 4 weeks sedentary (4JEX4S) and 8 weeks sedentary control (8S). The rats were jumped trained 40 times/session, 5 days/week. After 4 weeks or 8 weeks the mass and breaking force in tibia and cross-sectional areas of the tibia were measured. The tibia in the 4JEX and 8JEX groups had significantly greater fat-free dry weight and maximum loads at the fracture tests than those in the 4S and 8S groups. The tibia of 4JEX and 8JEX also had significantly larger cortical area without a significant change in the medullary area at the cross-sectional analysis. Bone mass acquired in 4JEX4S group was retained after cessation of exercise. The results indicate that high-impact jump-exercise, which is osteotropic, leads to an increased cortical bone with enhanced periosteal bone formation, which is also, however, maintained after cessation of exercise. It is therefore suggested that high-impact jump-exercise may provide greater safety margin against disuse-related or/and age-related bone loss and skeletal fragility later in life.