Abstract

Our aim was to quantify the overload-induced hypertrophy and conversion of fiber types (type II to I) occurring in the medial head of the gastrocnemius muscle (MG). Overload of MG was induced by a bilaterla tenotomy/retraction of synergists, followed by 12-18 wk of regular treadmill locomotion (2 h of walking/running per day on 3 of 4 days). We counted all type I fibers and determined type I and II mean fiber areas in eight equidistant sections taken along thelength of control and overloaded MG. Increase in muscle weights (31), as well as in total muscle cross-sectional areas (37) and fiber areas (type I, 57; type II, 34), attested to a significant hypertrophic response in overloaded MG. An increase in type I fiber composition of MG from 7.0 to 11.5 occurred as a result of overload, with the greatest and only statistically significant changes (ca. 70-100) being found in sections taken from the most rostral 45 of the muscle length. Results of analysis of sections taken from the largest muscle girth showed that it significantly underestimated the extent of fiber conversion that occurred throughout the muscle as a whole. These data obtained on the MG, which possesses a compartmentalization of fiber types, support thenotion that all fiber types respond to this model with a similar degree of hypertrophy. Also, they emphasize the complex nature of the adaptive changes that occur in these types of muscles as a reuslt of overload. Verf.-Referat