HIGH-EFFICIENCY OF TYPE-I MUSCLE-FIBERS IMPROVES PERFORMANCE

We have recently demonstrated that people with a high percentage of Ty pe I muscle fibers display a relatively high muscular efficiency when cycling. These individuals generate a relatively high muscular power o utput at a given steady-state level of oxygen consumption and caloric expenditure. The purpose of this study was to directly determine the e xtent to which differences in muscle fiber composition and efficiency influence endurance performance in competitive cyclists. The percentag e of Type I and II muscle fibers was determined from several biopsies from the vastus lateralis which were histochemically stained for ATPas e activity. During a laboratory performance test, 14 endurance trained cyclists (mean+/-SE; VO(2)max, 5.2+/-0.11/min; body weight, 74+/-1 kg ) cycled an ergometer for 1 h at the highest work rate they could tole rate. VO2 and RER were simultaneously measured using open circuit spir ometry for calculating caloric expenditure. Subjects were divided into two groups of seven according to their muscle fiber type composition: High % Type I Group (> 56 % Type I fibers) Normal % Type I Group (38- 55 % Type I fibers). Each subject from High % Type I Group was paired with a subject from the Normal % Type I Group according to their simil arity in VO(2)max, blood lactate threshold and average VO2 maintained during the I h performance test. Both groups averaged 4.5+/-0.1 1/min during the 1 h performance test (i.e., 86-88 % VO(2)max). However, the High % Type I Group, which possessed an average of 72+/-3% Type I fib ers, was able to maintain a 9% higher power output (i.e., 342+/-9 vs 3 15+/-11 watts; p<0.001) than the Normal % Type I Group which possessed an average of 48+/-2 % Type I fibers. Gross efficiency was thus signi ficantly higher in the High % Type I Group compared to the Normal % Ty pe I Group (i.e., 21.9+/-0.3 % vs. 20.4+/-0.3 %; p<0.001). We conclude that a high percentage of Type I muscle fibers improves endurance per formance ability by significantly increasing the power output generate d for a given rate of oxygen consumption and energy expenditure.