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

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 (T0.75) 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, T0.75 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.