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Black bears (Ursus americanus) maintain muscle to body weight ratio in unloaded soleus muscle during winter hibernation     Jonathan M Van Dyke¹, David V Baewer¹, Brian D Curry¹, Sandya R Govindaraju¹, James W Bain¹, Valentine Vogel², Michael Gappa³, Bruce Kohn³  Danny A Riley¹.   ¹Cell Biology, Neurobiology & Anatomy, Medical College of Wisconsin, 8701 Watertown-Plank Rd., Milwaukee, WI, 53227,  ²Wildlife Re-Creations, 10600 S. Howell Ave., Oak Creek, WI, 53154  ³Wisconsin Department of Natural Resources, Eau Claire and Rhinelander, WI 53703, 54501

   Humans unloaded in microgravity or during chronic bedrest exhibit significant muscle atrophy in as little as 11 days. They exhibit muscle weakness and increased susceptibility to muscle injury upon return to weightbearing activity. In contrast, black bears hibernate in dens for many months when food is scarce, and despite long periods of unloading, skeletal muscle reportedly exhibits little atrophy compared to that in humans unloaded for similar durations. The soleus muscle, the most sensitive muscle to unloading during spaceflight, has not been examined in bears. Our study compared soleus biopsies from summer-active adult bears to those taken after 4-5 months of hibernation. Muscle was removed from anesthetized bears and quick frozen for myofibrillar ATPase histochemical staining. Cross-sectional areas and fiber type percentages were assessed morphometrically. The mean cross-sectional fiber area to body weight ratio (mfa/bw) of summer bears was 24.1 ± 5.2 μm²/lb, and the soleus contained 89% slow, 1% intermediate and 11% fast fibers on average. Hibernating bears exhibited no significant change in mfa/bw (23.4 ± 6.8 μm²/lb) and a slight reduction (3%) in slow fibers. These data demonstrate that, during hibernation, black bears maintain muscle mass proportional to body weight and resist the negative effects of reduced weightbearing activity. For humans, muscle mass and body weight vary independently during unloading. Understanding the mechanism by which black bears resist musculoskeletal deterioration may provide insights for prevention of musculoskeletal wasting in astronauts. Supported in part by a Wisconsin Space Grant Consortium Fellowship to JMV.