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Muscle biopsies were taken from female dancers, aged 24 +/- 2 years, first during an active period and then after 32 weeks of detraining. During the active period, the percentage of type I fibers was 63% +/- 12%, and of the subgroups IIA, IIB, and IIC, 26% +/- 9%, 8% +/- 7%, and 3% +/- 3%, respectively. There was no significant change due to detraining either in the proportion of type I fibers or in the type II subgroups. During the active period, the cross-sectional area of the type I fibers was 3798 +/- 586 micron 2. The areas of the subgroups IIA, IIB, and IIC were 3394 +/- 858, 2497 +/- 530, and 3296 +/- 638 micron 2, respectively. After the detraining period, the area of the type I fibers had increased significantly (P less than 0.05). The areas of the types IIA, IIB, and IIC tended to be increased although not significantly. It is suggested that the comparatively high percentage of type I fibers, which was found in the active dancers, is the result of selection rather than of training, while the small fiber areas seem to be due to training.
Dahlström et al. (Wed,) studied this question.