Response of the Femur to Exercise During Recovery Between Two Bouts of Hindlimb Unloading in Adult Male Rats

Mechanical unloading with microgravity exposure during spaceflight induces bone loss in weight bearing bones. Combined with loss of bone due to aging, this disuse bone loss puts astronauts at increased risk of fracture upon returning to 1G conditions. It is important to study countermeasures, such as exercise, to mitigate or prevent this bone loss. This study utilized the hindlimb unloaded (HU) rat model to characterize the effects of resistance exercise on recovery dynamics in-between two bouts of HU. In the larger project adult male Sprague-Dawley rats, six months of age, were divided into the following groups: baseline (sacrificed at 6 months of age); aging cage controls (did not undergo any treatment, sacrificed at 7, 8, 9, 10, and 12 months of age); 1HU7 (one month of HU at 6 months of age followed by three months of ambulatory recovery); 2HU10 (one month of HU at 6 months of age, ambulatory recovery for two months, one month HU at 9 months of age, and final two month ambulatory recovery); 1HU10 (one month HU at 9 months of age and two month ambulatory recovery); and 2HU+Ex (One month HU at 6 months of age, two month resistance exercise recovery, and a 2nd bout of HU at 9 months of age). This thesis focused on the 2HU+Ex data, while utilizing data from other groups for comparisons. The data in this thesis includes ex vivo densitometric and biomechanical properties at the femoral neck (FN), femur midshaft diaphysis (FD), and distal femur metaphysis (DFM). All compartments of BMC increased following exercise recovery above AC at the FN and DFM. Ambulatory recovery values revealed incomplete recovery in total and cortical BMC at the DFM and full recovery in other parameters. DFM and FD vBMD data indicated there were further benefits of exercise during recovery. Geometric data revealed periosteal apposition at the DFM and FN following exercise recovery. FD mechanical properties did not produce benefits of exercise. However, FN maximum force increased above all other groups after exercise recovery. Elastic modulus of the DFM showed benefits of exercise recovery in the response to the 2nd HU.