Skeletal muscle mitochondria and myoglobin, endurance, and intensity of training

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Abstract
Female rats were trained by treadmill running 40 min/day, 6 days/wk up to 11, 22 or 44 m/min for 14 wk. Low-intensity execise (11 m/min) increased the activities of the mitochondrial markers, citrate synthase and succinate dehydrogenase, by 50 and 58% in fast-twitch red vastus lateralis (FTR) muscles and by 32 and 15%, respectively, in slow-twitch red soleus (STR) muscles. Running up to 4-fold faster did not significantly elevate the activities of these enzymes beyond those obtained after running at 11 m/min. There was a small but direct trend of training intensity with citrate synthase activityin fast-twitch white vastus lateralis (FTW) muscles. Myoglobin concentration tended to increase as a linear function of training intensity in FTR muscles. In STR muscle, the concentration of myoglobin increased 15% in the 11 m/min group but was essentially the same as control values in the 22 and 44 m/min groups. The concentration of myoglobin was not changed with training intensity in FTW muscle. Exercise time to exhaustion increased in proportion to training intensity. Low-intensity running [.apprx. 50% maximal O2 uptake or less] apparently is a sufficient stimulus to induce most of the total possible mitochondrial increase in the red fiber types, whereas extremely fast running speeds are needed to stimulate significant increases in white muscle mitochondrial, and the mitochondrial differences in skeletal muscle between intensity groups do not appear to account for the differences in performance.