No effect of muscle fiber type on mechanical efficiency during cycle exercise at 1.5 Hz

Abstract

The mechanical efficiency has been determined for 23 healthy young men to study possible effects of muscle fiber type on efficiency during cycle exercise. Each subject cycled for 10 min at in average 19 different powers ranging from ≈1.0 to 4.6 W kg^–1 (70–370 W) while the pedalling frequency was kept constant at 1.5 Hz. The rate of energy release was determined from the steady state O₂ uptake measured near the end of each 10 min exercise period. Delta efficiency was taken as the inverse of the slope of regression of O₂ uptake on power (dP/dnO₂). Gross efficiency at 3 W kg^–1 was established, and finally, the efficiency was taken from each subject’s slope of O₂ uptake versus power using a common, fixed Y-intercept. Muscle biopsies were taken from the lateral portion of the knee extensor muscle, and muscle fibers were classified as type I or type II. The proportion of type I fibers was 0.50 ± 0.13 (mean ± s), delta efficiency was 0.262 ± 0.010, and gross efficiency was 0.213 ± 0.005. There was no significant correlation between any efficiency measure and the proportion of type I fibers. A two-sided 95% confidence interval on the data suggests that if the efficiency of the two fiber types differed, the difference was less than 12%. For the same subject the efficiency did not differ more than a few percents between low powers where type II fibers may be little engaged and high powers where both fiber types are active. The data therefore support the idea that the efficiency does not differ between type I and type II fibers during cycling at 1.5 Hz.