MUSCLE-FIBER TYPES OF WOMEN AFTER RESISTANCE TRAINING - QUANTITATIVE ULTRASTRUCTURE AND ENZYME-ACTIVITY

Muscle biopsies of the vastus lateralis muscle taken before and after 18 weeks of resistance training were compared by preparing frozen cros s sections for electron microscopy and using adjacent sections for fib er typing by myosin ATPase activity. Quantitative ultrastructural chan ges were observed in histochemically-identified muscle fiber types of twelve young women who underwent the training. The percentage of type IIB fibers decreased and IIA fibers increased. The cross-sectional are a of all major fiber types increased with training. The absolute volum e of myofibrils, intermyofibrillar space, and mitochondria increased w ith training for most major fiber types (type I, IIA and IIAB), but th e relative volume percentages were not significantly changed because o f corresponding fiber hypertrophy. Mean mitochondrial size for types I and IIA and myofibril size for types IIC and IIB increased significan tly with training. The capillary number per fiber and density did not change with training. Activity levels were measured for selected glyco lytic and oxidative enzymes. Cytochrome oxidase and hexokinase increas ed significantly with training, while creatine kinase, citrate synthas e, phosphofructokinase, glyceraldehyde phosphate dehydrogenase and hyd roxyacyl CoA dehydrogenase enzymes were not significantly altered. The results suggest that this type of high-repetition resistance training causes the intracellular components of all fiber types to increase pr oportionally with an increase in fiber size. In addition, the enzyme a nalysis indicates the muscle as a whole may increase its oxidative pho sphorylation capacity in conjunction with the decreased percentage of type IIB fibers.