Muscle-bone interaction as a prerequisite for bone health

Abstract

Bone loss is a well-established problem in astronauts, and so is loss of muscle mass and strength. Whilst involvement of dietary and endocrine factors is undoubted, the specific localization of bone losses in the lower extremity suggests a mechanical cause directly related to the absence of gravity. Ample evidence demonstrates that bones adapt themselves to forces imposed on them (so-called mechanoadaptation), and biomechanical analyses suggest that the greatest forces arise from muscle contractions. It is therefore proposed that microgravity-related bone loss is an effect of reduced musculoskeletal forces. In order to further elucidate this potentially important link, the division of space physiology of the DLR institute has undertaken three strands of experiments. Firstly, the Hephaistos, a novel orthotic device was developed in order to suppress calf muscle contractions, but maintain gravitational and mass-inertial loading of the tibia. Wearing this device leads to bone losses from the distal tibia within the order of magnitude of bed rest, suggesting that muscular contractions are relevant to the maintenance of bone. The second set of experiments assessed bone deformation in vivo in temporal relation to muscle contractions. Implanting bone screws armed with optical marker clusters and assessing tibia deformation with high-resolution optical motion capturing yielded that posterior bending and torsion were the most prominent deformation regimens. Moreover, hopping and jumping yielded substantially greater tibia deformation than walking, running or weightlifting. Thirdly, computer simulations demonstrate that torsion is of particular importance for mechanoadaptation in long bones. These recent studies have furthered our understanding of the mechanical environment that bones are exposed to. As expected, muscle contractions are the ultimate source for the greatest bone forces. More surprisingly, results from these experiments suggest that not only the magnitude, but also the direction of bone deformation regimens must be expected to play a meaningful role. With regards to countermeasure exercise, the present data suggest that treadmill running, bicycling and traditional weightlifting generate tibia deformation regimens that are inferior to plyometric and impact-based exercise, and that therefore countermeasures available on board the ISS are of limited value for bone maintenance.