Partial weight bearing does not prevent musculoskeletal losses associated with disuse

The purpose of this study was to investigate whether partial weight-bearing activity, at either one-sixth or one-third of body mass, blunts the deleterious effects of simulated microgravity (0G) after 21 d on muscle mass and quantitative/qualitative measures of bone.

Using a novel, previously validated partial weight-bearing suspension device, mice were subjected to 16% (G/3, i.e., simulated lunar gravity) or 33% (G/6, i.e., simulated Martian gravity) weight bearing for 21 d. One gravity control (1G, i.e., Earth gravity) and tail-suspended mice (0G, i.e., simulated microgravity) served as controls to compare the effects of simulated lunar and Martian gravity to both Earth and microgravity.

Simulated microgravity (0G) resulted in an 8% reduction in body mass and a 28% lower total plantarflexor muscle mass (for both, P < 0.01) as compared with 1G controls, but one-sixth and one-third partial weight-bearing activity attenuated losses. Relative to 1G controls, trabecular bone volume fraction (-9% to -13%) and trabecular thickness (-10% to -14%) were significantly lower in all groups (P < 0.01). In addition, cancellous and cortical bone formation rates (BFR) were lower in all reduced weight-bearing groups compared with 1G controls (-46% to -57%, trabecular BFR; -73% to -85%, cortical BFR; P < 0.001). Animals experiencing one-third but not one-sixth weight bearing exhibited attenuated deficits in femoral neck mechanical strength associated with 0G.

These results suggest that partial weight bearing (up to 33% of body mass) is not sufficient to protect against bone loss observed with simulated 0 g but does mitigate reductions in soleus mass in skeletally mature female mice.