The effect of training on the recovery from immobilization-induced bone loss in rats.

We studied the effect of exercise on the recovery from osteoporosis induced by immobilization in growing laboratory rats. The right hind limb of male rats was immobilized by cast for 2 weeks. The rats were then divided into control and exercise groups. The controls were allowed to move freely in their cages and the exercise group was running for 1 hour every day. After 2 weeks immobilization, the ash weight of the immobilized tibia decreased to 85.2 +/- 0.6% (P less than 0.001) from that of contralateral tibia. After removal of the cast, the bone mass recovered and, in 9 weeks, the decrease of ash weight was reduced to 2.9 +/- 0.8% and 4.2 +/- 0.3% in the control and exercise groups, respectively. There was no statistical difference in ash weights between the running and control groups and the trabecular bone volume in the distal femur was also equal in both groups. Immobilization reduced the incorporation of [45Ca] into the tibia. After remobilization, the [45Ca]-incorporation was found to be significantly higher in the recovering leg than in the control leg. Exercise further stimulated [45Ca]-incorporation into the recovering tibia.