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COYLE, E. F., L. S. SIDOSSIS, J. F. HOROWITZ, and J. D. BELTZ. Cycling efficiency is related to the percentage of Type I muscle fibers. Med. Sci. Sports Exerc., Vol. 24, No. 7, pp. 782–788, 1992. We determined that the variability in the oxygen cost and thus the caloric expenditure of cycling at a given work rate (i.e., cycling economy) observed among highly endurance-trained cyclists (N = 19; mean ± SE; O2max, 4.9 ± 6.1 1·min-1; body weight, 71 ± 1 kg) is related to differences in their % Type I muscle fibers. The percentage of Type I and II muscle fibers was determined from biopsies of the vastus lateralis muscle that were histochemically stained for ATPase activity. When cycling a Monark ergometer at 80 RPM at work rates eliciting 52 ± 1,61 ± 1, and 71 ± 1% O2max, efficiency was determined from the caloric expenditure responses (O2 and RER using open circuit spirometry) to steady-state exercise. Gross efficiency (GE) was calculated as the ratio of work accomplished · min-1 to caloric expenditure-min-1, whereas delta efficiency (DE) was calculated as the slope of this relationship between approximately 50 and 70% VO2max. The % Type I fibers ranged from 32 to 76%, and DE when cycling ranged from 18.3 to 25.6% in these subjects. The % Type I fibers was positively correlated with both DE (r = 0.85; P< 0.001; N = 19) and GE (r = 0.75; P < 0.001; N = 19) during cycling. Additionally, % Type I fibers was positively correlated with GE (r = 0.74; P < 0.001; N= 13) measured during the novel task of two-legged knee extension; performed at a velocity of 177 ± 6°-s-1‘ and intensity of 50 and 70% of peak O2 for that activity. Therefore, the relationship between muscular efficiency and muscle fiber type is not specific to cycling. We conclude that muscular efficiency varies greatly in highly endurance-trained cyclists and most of this variability appears to be related to differences in their % Type I muscle fibers. Therefore, when endurance-trained men exercise under these conditions, Type I muscle fibers appear to be substantially more efficient than Type II muscle fibers, possibly as a result of a lower rate of ATP turnover as reflected by a lower O2 while performing exercise at a given power output.