In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy. Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.Bears are an exception to this rule; species in the family Ursidae are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation. During that time, bears go through a series of physiological, morphological, and behavioral changes. Their ability to maintain skeletal muscle number and size during disuse is of significant importance.During hibernation, bears spend 4-7 months of inactivity and anorexia without undergoing muscle atrophy and protein loss. A few known factors contribute to the sustaining of muscle tissue. During the summer, bears take advantage of the nutrition availability and accumulate muscle protein. The protein balance at time of dormancy is also maintained by lower levels of protein breakdown during the winter. At times of immobility, muscle wasting in bears is also suppressed by a proteolytic inhibitor that is released in circulation. Another factor that contributes to the sustaining of muscle strength in hibernating bears is the occurrence of periodic voluntary contractions and involuntary contractions from shivering during torpor. The three to four daily episodes of muscle activity are responsible for the maintenance of muscle strength and responsiveness in bears during hibernation.
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References
External links
 Media related to Muscle atrophy at Wikimedia Commons
Muscular atrophy at the US National Library of Medicine Medical Subject Headings (MeSH)