Soleus muscle stability in wild hibernating black bears.

Based on studies of fast skeletal muscles, hibernating black and brown bears resist skeletal muscle atrophy during months of reduced physical activity and not feeding. The present study examined atrophy sparing in the slow soleus muscle, known to be highly prone to disuse atrophy in humans and other mammals. We demonstrated histochemically that the black bear soleus is rich in slow fibers, averaging 84.0 ± 6.6%. The percentages of slow fibers in fall (87.3 ± 4.9%) and during hibernation (87.1 ± 5.6%) did not differ ( P = 0.3152) from summer. The average fiber cross-sectional area to body mass ratio (48.6 ± 11.7 µm2/kg) in winter hibernating bears was not significantly different from that of summer (54.1 ± 11.8 µm2/kg, P = 0.4186) and fall (47.0 ± 9.7 µm2/kg, P = 0.9410) animals. The percentage of single hybrid fibers containing both slow and fast myosin heavy chains, detected biochemically, increased from 2.6 ± 3.8% in summer to 24.4 ± 24.4% ( P = 0.0244) during hibernation. The shortening velocities of individual hybrid fibers remained unchanged from that of pure slow and fast fibers, indicating low content of the minority myosins. Slow and fast fibers in winter bears exhibited elevated specific tension (kN/m2; 22%, P = 0.0161 and 11%, P = 0.0404, respectively) and maintained normalized power. The relative stability of fiber type percentage and size, fiber size-to-body mass ratio, myosin heavy chain isoform content, shortening velocity, power output, and elevated specific tension during hibernation validates the ability of the black bear to preserve the biochemical and performance characteristics of the soleus muscle during prolonged hibernation.

The effects of active and passive stretching on muscle length.

Active stretch is necessary for regulating muscle fiber length (ie, the number of series sarcomeres). Elevated cytoplasmic calcium is the proposed component of contractile activity required to activate signaling pathways for sarcomere number regulation. Passive stretch reduces muscle tissue stiffness, most likely by signaling connective tissue remodeling via fibroblasts. Passive stretch may induce sarcomere addition if the muscle fibers are lengthened sufficiently to raise cytoplasmic calcium through stretch-activated calcium channels. The magnitude of stretch in vivo is limited by the physiologic range of movement and stretch pain tolerance. The greatest effect of stretching muscle fibers is expected when the lengthening exceeds the optimum fiber length (Lo).