Muscle substrate utilization from alveolar gas exchange in trained cyclists

The respiratory exchange ratio (R) during steady-state exercise is equivalent to whole-body respiratory quotient (RQ), but does not represent muscle metabolism alone. If steady-state values of carbon dioxide production (\(\dot VCO_2\)) and oxygen uptake (\(\dot VO_2\)) are plotted for different work rates, the slope of the line fitting these points should estimate muscle RQ. Twelve cyclists randomly performed five 8-min, constant work rate tests of 40, 80,120,160 and 200 W. Whole-bodyR, averaged over the final 2 min of each exercise bout, increased with increasing work rate. When\(\dot VCO_2\) was plotted as a function of\(\dot VO_2\), the regression lines through the five points displayed excellent linearity, had negative γ-intercepts, and a slope of 0.915 (0.043) [mean (SD)], which was greater than the whole-bodyR at any individual work rate [range 0.793 (0.027) at 40 W to 0.875 (0.037) at 200 W]. This slope was comparable to the lower slope of the\(\dot VCO_2\) versus\(\dot VO_2\) plot of an increasing work rate (ramp) protocol [0.908 (0.054)]. We conclude that, during mild and moderate exercise of relatively short duration, contracting muscle has a high and constant RQ, indicating that carbohydrate is the predominant metabolic substrate. WholebodyR does not accurately reflect muscle substrate utilization and probably underestimates muscle RQ at a given work rate.