Weber, S., Gehlert, S., Weidmann, B., Gutsche, K., Frese, S., Graf, C., Platen, P., & Bloch, W. (2011). Exercise induced slow and fast myofiber transitions in response to low intensive endurance exercise. Medicine and Science in Sports and Exercise, 43(5). Supplement abstract 1399.

This study tested whether long-term increases in training volume with sustained reductions in training intensity provide the basis for a corresponding Type I myofiber slow shifting in endurance-trained cyclists (N = 21). Ss participated in a training period of three months. The training was characterized by a constant training time (training volume) and low-intensive exercise. Resting biopsies were taken pre- and post-exercise intervention. Changes in fiber type distribution of Type I, IIa, and IIx fibers were determined by Myosin ATpase staining.

In all Ss, Type I myofibers increased while Type IIa fibers decreased. Total training time did not correlate to the amount of fiber type shifting of any fiber type. Basal Type I myofiber distribution correlated with the amount of Type I fiber shifting in a pre- to post-training comparison. Subgroups of Ss with different Type 1 myofiber content showed different dynamics in myofiber shifting.

Implication. Long-term elevations in training volume do not induce a corresponding type I myofiber shifting in every athlete. Myofiber shifting is influenced by the basal distribution of myofibers. A long-term reduction in power output because of lowered training intensity that results from increased training volume may reduce basal fiber distribution and the recruitment pattern of myofibers leading to partial disuse or enhanced functional usage of myofibers. This may explain in part the opposed changes in myofiber shifting observed in this study's Ss.

The belief that high volumes of low-intensity training is effective aerobic training generally is not supported by this study although in some individuals it might be. Training effects from this form of work are not similar across a group of seemingly similar athletes.

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