Hibernating bears conserve muscle strength and maintain fatigue resistance.

Black bears spend several months each winter confined to a small space within their den without food or water. In nonhibernating mammals, these conditions typically result in severe muscle atrophy, causing a loss of strength and endurance. However, an initial study indicated that bears appeared to conserve strength while denning. We conducted an in vivo, nonsubjective measurement of strength, resistance to fatigue, and contractile properties on the tibialis anterior muscle of six hibernating bears during both early and late winter using a rigid leg brace and foot force plate. After 110 d of anorexia and confinement, skeletal muscle strength loss in hibernating bears was about one-half that in humans confined to bed rest. Bears lost 29% of muscle strength over 110 d of denning without food, while humans on a balanced diet but confined to bed for 90 d have been reported to lose 54% of their strength. Additionally, muscle contractile properties, including contraction time, half-relaxation time, half-maximum value time, peak rate of development and decay, time to peak force development, and time to peak force decay did not change, indicating that no small-scale alterations in whole-muscle function occurred over the winter. This study further supports our previous findings that black bears have a high resistance to atrophy despite being subjected to long-term anorexia and limited mobility.