The Bone Gain Induced by Exercise in Puberty Is Not Preserved Through a Virtually Life-Long Deconditioning: A Randomized Controlled Experimental Study in Male Rats

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Abstract
To investigate the controversial issue whether exercise‐induced positive effects on bone can be maintained after cessation of exercise, 100 5‐week‐old male Sprague‐Dawley rats were used to assess the effects of long‐term exercise (EX, treadmill running) and subsequent deconditioning (DC, free cage activity) on the femoral neck and femoral midshaft. At entry, the rats were randomly assigned into eight groups: four control groups (C14, C28, C42, and C56), and four exercise groups (EX, EX + DC14, EX + DC28, and EX + DC42). Rats in the exercise groups were first subjected to a 14‐week period of progressively intensifying running, after which the rats of group EX were killed and the remaining exercise groups (EX + DC14, EX + DC28, and EX + DC42) were allowed to move freely in their cages for a subsequent deconditioning period of 14, 28, or 42 weeks, whereas control rats were kept free in their cages for the entire study period (0–56 weeks) and killed with their respective exercise group. At each time point, a comprehensive analysis of the femoral neck and midshaft characteristics (peripheral quantitative computed tomography analysis and fracture load [Fmax]) was performed. In comparison with their age‐matched controls, 14 weeks of treadmill training resulted in significant (p < 0.05) increases in all measured femoral neck parameters of the growing male rats (i.e., +25% in total cross‐sectional area [tCSA], +28% in total bone mineral content [tBMC], +11% in total bone mineral density [tBMD], and +30% in Fmax). On the contrary, no exercise‐induced positive effects were seen in femoral midshaft. The exercise‐induced benefits in the femoral neck were partially maintained during the deconditioning period of 14 weeks, the tCSA being +17%, tBMC +18% (both p < 0.05), and the Fmax +11% (p = 0.066) higher in the exercised group than control group. However, after 42 weeks of deconditioning, these benefits were eventually lost. In conclusion, exercise through the period of the fastest skeletal growth results in significant improvements in size, mineral mass, and strength of the femoral neck of male rats. However, these exercise‐induced bone benefits are eventually lost if exercise is completely ceased, and thus, continued training is probably needed to maintain the positive effects of youth exercise into adulthood. Further studies should focus on assessing the minimal level of activity needed to maintain the exercise‐induced bone gains.