The role of fitness on VO2 and VCO2 kinetics in response to proportional step increases in work rate

Summary The purpose of this study was to determine the effect of fitness and work level on the O₂ uptake and CO₂ output kinetics when the increase in work rate step is adjusted to the subject's maximum work capacity. Nine normal male subjects performed progressive incremental cycle ergometer exercise tests in 3-min steps to their maximum tolerance. The work rate step size was selected so that the symptom-limited maximum work rate would be reached in four steps at 12 min in all subjects. Oxygen consumption (VCO₂) and carbon dioxide production VCO₂ were calculated breath by breath. For the group, the time (mean, SEM) to reach 75% of the 3-min response (T _0.75) for VO₂ increased significantly (P<0.01) at progressively higher work rate steps, being 53.3 (5.5) s, 63.5 (4.6) s, 79.5 (5.0) s, and 94.5 (5.8) s, respectively. In contrast, T _0.75 for VCO₂ did not change significantly 74.9 (7.4) s,. 75.6 (5.0) s, 85.1 (5.3) s, and 89.4 (6.3) s, respectively]. VCO₂ kinetics were slower than VO₂ kinetics at the low fractions of the subjects' work capacities but were the same of faster at the high fractions because of the slowing of VO₂ kinetics. The first step showed the fastest rise in VO₂. While VO₂ kinetics slowed at each step, they were faster at each fraction of the work capacity in the fitter subjects. The step pattern in VO₂ disappeared at high work rates for the less fit subjects. The heart rate response paralleled that of VO₂. We conclude that VO₂ and VCO₂ kinetics are slower in the less fit subjects but only VO₂ kinetics are significantly attenuated in response to proportional step increases in work rate.