Moderate intensity resistive exercise improves metaphyseal cancellous bone recovery following an initial disuse period, but does not mitigate decrements during a subsequent disuse period in adult rats.

Spaceflight provides a unique environment for skeletal tissue causing decrements in structural and densitometric properties of bone. Previously, we used the adult hindlimb unloaded (HU) rat model to show that previous exposure to HU had minimal effects on bone structure after a second HU exposure followed by recovery. Furthermore, we found that the decrements during second HU exposure were milder than the initial HU cycle. In this study, we used a moderate intensity resistance exercise protocol as an anabolic stimulus during recovery to test the hypothesis that resistance exercise following an exposure to HU will significantly enhance recovery of densitometric, structural, and, more importantly, mechanical properties of trabecular and cortical bone. We also hypothesized that resistance exercise during recovery, and prior to the second unloading period, will mitigate the losses during the second exposure. The hypothesis that exercise during recovery following hindlimb unloading will improve bone quality was supported by our data, as total BMC, total vBMD, and cancellous bone formation at the proximal tibia metaphysis increased significantly during exercise period, and total BMC/vBMD exceeded age-matched control and non-exercised values significantly by the end of recovery. However, our results did not support the hypothesis that resistance exercise prior to a subsequent unloading period will mitigate the detrimental effects of the second exposure, as the losses during the second exposure in total BMC, total vBMD, and cortical area at the proximal tibia metaphysis for the exercised animals were similar to those of the non-exercised group. Therefore, exercise did not mitigate effects of the second HU exposure in terms of pre-to-post HU changes in these variables, but it did produce beneficial effects in a broader sense.