Muscle fiber composition and enzyme activities of elite distance runners.

Muscle biopsies were obtained from the gastrocnemius of 14 elite distance runners, 18 middle distance runners, and 19 untrained men. The middle distance runners were all highly trained, but had significantly slower performance times than the elite runners at distances greater than 3 miles. Fiber composition and mean cross-sectional areas were determined from muscle sections incubated for histochemical activity. A portion of the specimen was used to determine succinate dehydrogenase (SDH), lactate dehydrogenase (LD/Y and phosphorylase activities. All subjects were tested for maximal oxygen uptake on a treadmill. As previously demonstrated by others, the elite runners' muscles were characterized by a high percentage (79%) of slow twitch (ST) fibers. On the average, the crosssectional area of their ST fibers was found to be 22% larger than the FT fibers (P less than 0.05). SDH activity of whole muscle homogenates from elicte and middle distance runners was 3.4- and 2.8- fold greater, respectively, than that measured in the untrained men. Since the LDH and phosphorylase activities were similar for the runners and untrained men, it appears that training for distance running has little influence on the enzymes of glycogenolysis.