Résumé
Inconsistencies about dynamic asymmetry between the on- and off-transient responses in [Formula: see text] are found in the literature. Therefore the purpose of this study was to examine [Formula: see text]on- and off-transients during moderate- and heavy-intensity cycling exercise in trained subjects. Ten men underwent an initial incremental test for the estimation of ventilatory threshold (VT) and, on different days, two bouts of square-wave exercise at moderate (< VT) and heavy (> VT) intensities. [Formula: see text] kinetics in exercise and recovery were better described by a single exponential model (< VT), or by a double exponential with two time delays (> VT). For moderate exercise, we found a symmetry of [Formula: see text] kinetics between the on- and off-transients (i.e., fundamental component), consistent with a system manifesting linear control dynamics. For heavy exercise, a slow component superimposed on the fundamental phase was expressed in both the exercise and recovery, with similar parameter estimates. But the on-transient values of the time constant were appreciably faster than the associated off-transient, and independent of the work rate imposed (< VT and > VT). Our results do not support a dynamically linear system model of [Formula: see text] during cycling exercise in the heavy-intensity domain. Key words: oxygen uptake kinetics, on- and off-transients, slow component