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Partial Weight Bearing Does Not Prevent Musculoskeletal Losses Associated with Disuse

SWIFT, JOSHUA M.1*; LIMA, FLORENCE1*; MACIAS, BRANDON R.1; ALLEN, MATTHEW R.2; GREENE, ELIZABETH S.1; SHIRAZI-FARD, YASAMAN3; KUPKE, JOSHUA S.3; HOGAN, HARRY A.3,4; BLOOMFIELD, SUSAN A.1,5

Medicine & Science in Sports & Exercise: November 2013 - Volume 45 - Issue 11 - p 2052–2060
doi: 10.1249/MSS.0b013e318299c614
BASIC SCIENCES: Contrasting Perspectives in Exercise Science and Sports Medicine

Purpose 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.

Methods 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.

Results 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.

Conclusion 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.

1Department of Health and Kinesiology, Texas A&M University, College Station, TX; 2Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN; 3Department of Biomedical Engineering, Texas A&M University, College Station, TX; 4Department of Mechanical Engineering, Texas A&M University, College Station, TX; and 5Department of Nutrition and Food Science, Texas A&M University, College Station, TX

Address for correspondence: Susan A. Bloomfield, Ph.D., Department of Health and Kinesiology, MS 4243, Texas A&M University, College Station, TX 77843-4243; E-mail: sbloom@tamu.edu.

*J.M.S. and F.L. contributed equally to this article.

Submitted for publication December 2012.

Accepted for publication April 2013.

© 2013 American College of Sports Medicine