The off-transient pulmonary oxygen uptake (V(over dot)(O2)) kinetics following attainment of a particular V(over dot)(O2) during heavy-intensity exercise in humans

The oxygen uptake response to moderate-intensity exercise (i.e. < anaerobic threshold (theta(an))) has been characterised with a gain (i.e, response a mplitude per increment of work rate) and time constant that do not vary app reciably at different work rates or between the on- and off-transients. Abo ve theta(an), the response becomes more complex with an early component tha t typically projects to a value that has a gain similar to that of the < th eta(an) response, but which is supplemented by the addition of a delayed sl ow kinetic component. We therefore established a constant target (V)over do t(O2) ((V)over dot(O2)(1)) for each subject such that with different impose d work rates the contribution to (V)over dot(O2)(1) from the slow phase var ied over a wide range. Work rates were chosen so that (V)over dot(O2)(1) wa s attained at 2-24 min. Five subjects (aged 21-58 years) cycled at four to five different work rates. (V)over dot(O2) was measured breath-by-breath, a t (V)over dot(O2)(1) the work rate was abruptly reduced and the subject rec overed by cycling unloaded for 15 min. Unlike the on-transient, for which t he 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 significa ntly among the various tests. The off-transient kinetics far (V)over dot(O2 ) therefore was independent of the magnitude of the contribution to the slo w phase from the on-transient kinetics.