Abstract

Aim of this study was to directly determine the extent to which differences in muscle fiber composition and efficiency influence endurance in competitive cyclists. The percentage of Type I and II muscle fibers was determined from several biopsies from the vastus lateralis. During a laboratory performance test, 14 endurance trained cyclists cycled an ergometer for 1 h at the highest work rate they could tolerate. VO2 and RER were simultaneously measured using open circuit spirometry for calculating caloric expenditure. Subjects were divided into 2 groups of 7 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 similarity in VO2max, blood lactate threshold and average VO2 maintained during the 1 h performance test. Both groups averaged 4.5+-0.1 l/min during 1 h performance test (86-88% VO2max). However, the High % Type I Group, which possessed an average of 72+-3% Type I fibers, was able to maintain a 9% higher power output than the Normal % Type I Group which possessed an average of 48+-2% Type I fibers. Gross efficiency was thus significantly higher in the High % Type I Group compared to the Normal % Type I Group. We conclude that a high percentage of Type I muscle fibers improves endurance performance ability by significantly increasing the power generated for a given rate of oxygen consumption and energy expenditure. Verf.-Referat