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High bone mass gained by exercise in growing male mice is increased by subsequent reduced exercise

¹Division of Applied Food Research and ²Division of Health Promotion, The National Institute of Health and Nutrition, Tokyo 162-8636; and ³School of Sport Sciences, Waseda University, Tokyo 359-1192, Japan

Submitted 31 October 2003 ; accepted in final form 12 April 2004

Exercise-induced bone gains are lost if exercise ceases. Therefore, continued exercise at a reduced frequency or intensity may be required to maintain these benefits. In this study, we evaluated whether 4 wk of reduced exercise after 4 wk of running exercise in growing male mice results in the maintenance of high bone mass. Five-week-old mice were divided into the following groups: 1) baseline control; 2) 4-wk control; 3) 4-wk exercise; 4) 8-wk control; 5) 4-wk exercise followed by 4-wk cessation of training; and 6) 4-wk exercise followed by reduced exercise at half the frequency. The regimen consisted of exercise 6 days/wk, and the reduced exercise regimen consisted of running 3 days/wk on a treadmill for 30 min/day, at 12 m/min on a 10° uphill slope. Running exercise significantly increased bone mineral density of the femur, periosteal mineral apposition rate, bone formation rate, percent labeled perimeter at the midfemur, and osteogenic activity of bone marrow cells. However, these parameters declined to the age-matched sedentary control after cessation of training. In contrast, the reduced exercise group had significantly higher mineral apposition rate compared with those of the sedentary control and cessation of training groups. Furthermore, bone mineral density for the reduced exercise group was significantly higher than those for the other groups. These results suggest that the high bone formation gained through exercise can be maintained, and bone mass was further increased by subsequent exercise even if the exercise frequency is reduced.

reduced exercise; bone mineral density; bone formation rate; bone marrow cells; growing mice

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