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.