The on-transient V O 2 response shows three functionally distinct phases as previously described (Whipp & Ward, 1990; see Fig. 1). The first phase, lasting approximately 20 s, has been termed the 'cardio-dynamic' phase (Whipp et al. 1982) as the increase in V O 2 is due primarily to an increase in pulmonary blood flow (Cummin et al. 1986). During phase 2, V O 2 increases towards a steady-state (phase 3) value in a manner which is exponential in nature, and is temporally aligned with the kinetics of phosphocreatine breakdown (the surrogate of adenosine triphosphate production) and thus is believed to reflect muscle O 2 consumption Whipp et al. 1999). If the work rate is of sufficient intensity to induce a sustained increase in blood lactate concentration, then a steady state is delayed or not seen, and a slow component of V O 2 becomes evident (Casaburi et al. 1989). This slow component, which is of delayed onset, will produce an end-exercise ∆V O 2 /∆WR relationship greater than that predicted from the V O 2 -power output relationship for moderate-intensity exercise (Poole et al. 1988). The purpose of this investigation was to examine differences in parameter estimates derived from different mathematical models and fitting procedures used for the analysis of V O 2 kinetics following the onset of moderate-and heavy-intensity cycle exercise. Exponential models with one or more components were used to describe the dynamics of gas exchange. Estimates of the parameters of the response, specifically the phase 2 time constant (r), and the slow component of V O 2 , were compared, together with a statistical analysis of how well each model fitted the data. We

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