Functional Adaptability of Muscle Fibers to Long-Term Resistance Exercise

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SHOEPE, T. C., J. E. STELZER, D. P. GARNER, and J. J. WIDRICK. Functional Adaptability of Muscle Fibers to Long-Term Resistance Exercise. Med. Sci. Sports Exerc., Vol. 35, No. 6, pp. 944–951, 2003.PurposeWe compared the functional properties of muscle fibers from two groups of subjects that differed widely in their training history to investigate whether long-term resistance exercise alters the intrinsic contractile properties of skeletal muscle fibers.MethodsVastus lateralis muscle biopsies were obtained from six sedentary males (NT group, age = 23 ± 1 yr) and six males who had participated in regular resistance exercise training over the preceding 7.6 ± 1.6 yr (RT group, 22 ± 1 yr). Chemically skinned muscle fiber segments were activated with a saturating free [Ca2+] to quantify fiber peak Ca2+-activated force (Po), unloaded shortening velocity (Vo), and peak power. Fiber segment myosin heavy chain (MHC) isoform content was identified by gel electrophoresis.ResultsSlow and fast fibers from the RT group were larger in CSA and produced greater absolute Po and absolute peak power in comparison with fibers from the NT group. However, these differences were no longer evident after Po and peak power were normalized to fiber CSA and fiber volume, respectively. Vo/fiber length was dependent on fiber MHC content but independent of training status.ConclusionFiber hypertrophy was sufficient to account for intergroup differences in Po and peak power of slow and fast fibers. There was no evidence that the intrinsic contractility of slow or fast fibers, as evaluated by force, shortening velocity, and power normalized to the appropriate fiber dimensions, differed between RT and NT groups.

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