Muscle fiber composition and performance capacities of women.

The purpose of the present study was to determine whether the fast-twitch (FT) fiber composition in muscle was a) correlated with performance capacities, b) related to the trainability of the subjects, and c) whether the FT fiber composition could be predicted with standard laboratory tests. From twenty-four young women (ages 24.3 +/- 3.0 yrs) muscle biopsies were obtained from the vastus lateralis of both the left and right legs. In addition, each subject completed four laboratory tests: i) a maximal oxygen intake test on the cycle ergometer, ii) a high-kp power test, iii) a low-kp power test, and iv) a Sargent jump. Prior to training the correlations between %FT fibers and each of the tests (r equal to or less than 0.19) were not significant (P less than 0.05). After training anaerobically (N = 20) significant improvements (P less than 0.05) occurred in a) the high-kp test (delta = + 0.12 +/- 0.02 kpm/sec.kg) b) the low-kp test (delta = + 0.10 +/- 0.01 kpm/sec.kg) and c) the Sargent jump (delta = + 2.5 +/- 1.1 cm). However, these training-induced changes (delta) were not correlated with %FT fiber composition (r equal to or less than 0.35), nor were the post-training performances on the tests correlated with the %FT fibers (r equal to or less than 0.12). Performance capacities before and after training were not significantly different in groups with a low %FT fiber composition (35.8 +/- 1.6%) or a high %FT fiber composition (63.6 +/- 2.2%). None of the performance tests either singly, or in combination in a multiple regression equation, provided a suitable prediction of the FT muscle fiber composition. The results of this study indicate that athletic performances and/or susceptibility for training cannot be determined a priori from simple measurements of muscle fiber composition.