¹Massachusetts Institute of Technology, ²University of Massachusetts Medical School, and ³Harvard-MIT Division of Health Sciences and Technology.

   Many studies have been conducted investigating dynamic forces in microgravity environments, yet few have examined the result of partial gravity as found on Mars. Our novel Load Titration Unit is capable of quadrupedal unloading at a variety of partial weightbearing conditions.  In order to better understand the changes in musculoskeletal loading induced by this system, we quantitatively analyzed the gait of mice at multiple levels of reduced-g simulation. For this experiment, female BALB/cByJ mice were adjusted to a custom forelimb vest and tail wrap and adapted to partial weightbearing for a minimum of 48 hours. Using an AMTI HE-6x6-1 force plate, ground reaction forces in the x-, y-, and z-axes were collected from each mouse at NORMAL (100%), JACKET (100%), MARS (38%), LUNAR (16%) and 60% weight bearing conditions. Results showed 1) a decrease in peak z-forces with decreasing simulated g-level (e.g., MARS GRFz: -63.7% forelimb and -85.3% hindlimb, relative to NORMAL, p< 0.05) 2) a decrease in the rates of onset of these forces (MARS: -73.7% forelimb and -91.8% hindlimb, p< 0.05) and 3) an trend towards higher ratios of peak transverse to vertical forces. Additionally, an in vivo strain gage was surgically implanted on the anterior surface of the tibia. Strain data was collected under static and dynamic loads, alone and in combination with the AMTI force plate data. There were no significant differences in peak surface strains or strain rates across partial weightbearing conditions (p> 0.05). Joint dynamics were explored to explain this unexpected finding.