The off-transient pulmonary oxygen uptake (VO(2)) kinetics following attainment of a particular VO(2) during heavy-intensity exercise in humans.

The oxygen uptake response to moderate-intensity exercise (i.e. < anaerobic threshold (an)) has been characterised with a gain (i.e. response amplitude per increment of work rate) and time constant that do not vary appreciably at different work rates or between the on- and off-transients. Above an, the response becomes more complex with an early component that typically projects to a value that has a gain similar to that of the < an response, but which is supplemented by the addition of a delayed slow kinetic component. We therefore established a constant target VO2 (VO21) for each subject such that with different imposed work rates the contribution to VO21 from the slow phase varied over a wide range. Work rates were chosen so that VO21 was attained at 2-24 min. Five subjects (aged 21-58 years) cycled at four to five different work rates. VO2 was measured breath-by-breath, at VO21 the work rate was abruptly reduced and the subject recovered by cycling unloaded for 15 min. Unlike the on-transient, for which the slow component shows a long delay, the off-transient was best fitted as two simultaneous exponential components. The slower off-transient component had a small amplitude and long time constant, but did not differ significantly among the various tests. The off-transient kinetics for VO2 therefore was independent of the magnitude of the contribution to the slow phase from the on-transient kinetics.