Differential bone and muscle recovery following hindlimb unloading in skeletally mature male rats

This study was designed to track the recovery of bone and muscle properties after 28 days of hindlimb unloading (HU) in skeletally mature male rats in order to quantify the degree and timing of the expected mismatch between bone and muscle properties. Outcome variables were in vivo plantarflexor peak isometric torque and proximal tibial volumetric bone mineral density (vBMD). Proximal tibia vBMD was significantly lower than age-matched controls (-7.8%) after 28 days of HU, continued to decrease through day 28 of recovery (-10%) and did not recover until day 84 of recovery. Plantarflexor peak isometric torque was significantly reduced after 28 days of HU (-13.9%). Further reductions of isometric torque occurred after 7 days of recovery (-15%), but returned to age-matched control levels by day 14. The functional relationship between bone and muscle (vBMD/isometric torque) tended to increase after 28 days of HU (+7.8%), remained elevated after 7 days of reloading (+9.1%) and was significantly lower than age-matched controls on day 28 (-13.6%). This relatively rapid return of muscle strength, coupled with continued depression of bone density at the proximal tibia metaphysis, may increase the risk for skeletal injury during recovery from prolonged periods of reduced mechanical loading.