MUSCLE SUBSTRATE UTILIZATION FROM ALVEOLAR GAS-EXCHANGE IN TRAINED CYCLISTS

The respiratory exchange ratio (R) during steady-state exercise is equ ivalent to whole-body respiratory quotient (RQ), but does not represen t muscle metabolism alone. If steady-state values of carbon dioxide pr oduction (V over dot CO2) and oxygen uptake (V over dot O-2) are plott ed for different work rates, the slope of the line fitting these point s should estimate muscle RQ. Twelve cyclists randomly performed five 8 -min, constant work rate tests of 40, 80, 120, 160 and 200 W. Whole-bo dy R, averaged over the final 2 min of each exercise bout, increased w ith increasing work rate. When V over dot CO2 was plotted as a functio n of V over dot O-2, the regression lines through the five points disp layed excellent linearity, had negative y-intercepts, and a slope of 0 .915 (0.043) [mean (SD)], which was greater than the whole-body R at a ny 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 V over dot CO2 versus V over dot O-2 plot of an increasing work rate (ramp) p rotocol [0.908 (0.054)]. We conclude that, during mild and moderate ex ercise of relatively short duration, contracting muscle has a high and constant RQ, indicating that carbohydrate is the predominant metaboli c substrate. Whole-body R does not accurately reflect muscle substrate utilization and probably underestimates muscle RQ at a given work rat e.