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human
Why do hibernating animals not lose their muscles, when sleeping for longer periods?

Our muscles get cut out after a week of no use/training, but what about hibernating animals who sleep for longer periods? I know that they get their energy from fat storage and lower their temperature and cell activity to reduce energy demand, but do their muscles not need some kind of stimulus to create the reason for muscles to exist and be functioning when the animal wakes up?
gpt
Hibernators still tend to lose a good proportion of the protein (4 – 65%, depending on the species... less in bears, more in small hibernators) and strength (~25% over 130 days in bears) in their muscles during hibernation. Animals in fasting and hibernating states will always lose protein from muscles, which are catabolized to provide amino acids for gluconeogenesis and for obligate protein biosynthesis elsewhere in other tissues. These losses just tend to be much less than in bedridden humans (~90% loss of strength over a similar period).

The key adaptations that occur in hibernators is the reduction of 1) protein turnover (catabolism and anabolism is reduced), and 2) amino acid catabolism (via the urea cycle). Protein sparing tends to occur in fasting animals anyways, but the incredible reduction in the metabolic rates of hibernators leads to the further down regulation of these processes and the preservation of a large proportion of their pre-hibernating lean body mass.

Also, in hibernating bears specifically, the balance between protein catabolism and anabolism is maintained; proteins involved in skeletal muscle and liver protein biosynthesis are up regulated to balance out the catabolism of proteins that occurs. This is energetically expensive, but is likely the reason why bears do not undergo disuse muscle atrophy. There is also some degree of urea recycling to scavenge any amino acids that are degraded via the urea cycle, and this is presumably accomplished by the gut microflora because this biochemical pathway does not exist in vertebrates. These processes are likely the key factors that allow bears to conserve the vast majority of their lean body mass, muscle strength, and nitrogen over the several months that they hibernate.

Further reading/sources:

Carey, H. V., Andrews, M. T., Martin, S. L. Mammalian Hibernation: Cellular and Molecular Responses to Depressed Metabolism and Low Temperature. Physiol Rev 83 (2003) 1158-1181.

Fedorov, V. B., Goropashnaya, A. V., Toien, O., Stewart, N. C., Gracey, A. Y., Chang, C., Qin, S., Pertea, G., Quackenbush, J., Showe, L. C., Boyer, B. B., Barnes, B. M. Elevated expression of protein biosynthesis genes in liver and muscle of hibernating black bears (Ursus americanus). Physiol Genomics 37 (2009) 108-118.

Hellgren, E. C. Physiology of hibernation in bears. Ursus 10 (1998) 467-477.