Mechanical unloading has deleterious effects on the musculoskeletal system, and exercise offers a way to reduce or reverse these effects. Studies of crew members from the International Space Station have documented bone losses that do not fully recover even years after returning to Earth [1,2], and this raises concerns with repeated missions. We have used the adult hindlimb unloaded (HU) rat model [3] to simulate repeated exposure to microgravity, and to study densitometric and mechanical properties at multiple bone sites. This study focuses on mixed bone sites including proximal tibia metaphysis (PTM), and femoral neck (FN) region which also has a significant clinical relevance. Surprisingly, losses for the 2nd HU were milder than those for the 1st HU for bone mineral content (BMC) and volumetric bone mineral density (vBMD), suggesting a possible protective effect of the 1st HU [4]. Comparison to a separate group exposed to a single period of HU initiated at the same age as the 2nd HU group ruled out age effects contributing to the smaller deficit. BMC and vBMD values returned to baseline but remained below aging cage control (CC) values, raising the question of whether the milder losses for the 2nd HU might be attributable, wholly or partially, to lower levels at the start of the 2nd HU. The goal of the current study was to determine if adding a resistance exercise regimen during recovery from the 1st HU would restore bone properties to CC levels and/or significantly affect the response to the 2nd HU.

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