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1 Department of Physiology, St. Marianna University School of Medicine, Kawasaki City, Kanagawa 216-8511; 2 Department of Orthopaedic Surgery, University of Tokai, Isehara City, Kanagawa 259-1193; 3 National Center for Neurology and Psychiatry, Kodaira City, Tokyo 187-8551; 4 Department of Physiology and Biomechanics, National Institute of Fitness and Sports, Kanoya City, Kagoshima 891-2393; and 5 Aomori University of Health and Welfare, Aomori City, Aomori 030-8505, Japan
The effects of 2 and 4 mo of bed rest, with or without exercise countermeasures, on the contractile properties of slow fibers in the human soleus muscle were examined. Mean fiber diameters were 8 and 36% smaller after 2 and 4 mo of bed rest, respectively, than the pre-bed rest level. Maximum tetanic force (Po), maximum activated force (Fmax) per cross-sectional area (CSA), and the common-logarithm value of free Ca2+ concentration required for half-maximal activation (pCa50) also decreased after 2 and 4 mo of bed rest. In contrast, maximum unloaded shortening velocity (Vo) was increased after 2 and 4 mo of bed rest. After 1 mo of recovery, fiber diameters, Po, Fmax per CSA (P > 0.05), and pCa50 were increased and Vo decreased toward pre-bed rest levels. Effects of knee extension/flexion exercise by wearing an anti-G Penguin suit for 10 h daily, and the effects of loading or unloading of the plantar flexors with (Penguin-1) or without (Penguin-2) placing the elastic loading elements of the suit, respectively, were investigated during ~2 mo of bed rest. In the Penguin-1 group, mean fiber diameter, Po, Fmax per CSA, Vo, and pCa50 were similar before and after bed rest. However, the responses of fiber size and contractile properties to bed rest were not prevented in the Penguin-2 group, although the degree of the changes was less than those induced by bed rest without any countermeasure. These results indicate that long-term bed rest results in reductions of fiber size, force-generation capacity, and Ca2+ sensitivity, and enhancement of shortening velocity in slow fibers of the soleus. The data indicate that continuous mechanical loading on muscle, such as stretching of muscle, is an effective countermeasure for the prevention of muscular adaptations to gravitational unloading.
human soleus fibers; contractile properties; countermeasure
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