Horowitz, J. F., Sidossis, L. S., & Coyle, E. F. (1994). High efficiency of type I muscle fibers improves performance. International Journal of Sports Medicine, 15, 152-157.

The extent to which differences in muscle fiber composition and efficiency influence endurance performance in competitive cyclists was determined. Biopsies of the vastus lateralis determined the percentage of Type I (slow-twitch) and Type II (fast-twitch) fibers. Endurance trained cyclists (N = 14) cycled on an ergometer for one hour at the highest tolerable work rate. Ss were divided into two groups: High Type I fiber group (>56% Type I), and Normal Type I fiber group(38-55% Type I). Physiological response measures during the cycling task were obtained.

The High group was able to maintain 9% higher power output than the Normal group. Gross efficiency was therefore higher in the High group than the Low group.

It was concluded that a high percentage of Type I muscle fibers improves endurance performance ability by significantly increasing the power output generated for a given rate of oxygen consumption and energy expenditure.

Implication. Endurance athletes have an "edge" if they have a higher proportion of Type I fibers in their musculature.

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