Influence of the fitting window on the O2 uptake kinetics at the onset of moderate intensity exercise

The O2 uptake (V'O2) data at the onset of an exercise are usually fitted with a mono-exponential function, after removal of the data pertaining to a conventional initial time period (ΔTr) lasting ~20s. We performed a thorough quantitative analysis on the effects of removing data pertaining to different ΔTr, aiming at identifying an objective method to establish the appropriate ΔTr. Breath-by-breath O2 uptake responses, acquired on 25 healthy adults performing a step moderate-intensity exercise, and 104 simulated bi-exponential responses, were analyzed. For all the responses, the kinetic parameters of a mono-exponential function, and the corresponding Asymptotic Standard Errors (ASE), were estimated by non-linear regression, removing the data pertaining to progressively longer initial periods (1s each) up to 60s. Four methods to establish objectively ΔTr were compared. The minimum estimated tau was obtained for ΔTr≅35s in both the V'O2 and simulated data, that was about 30% lower compared to that obtained for ΔTr≅0s. The average ASE values remained quite constant up to ΔTr≅35s, thereafter they increased remarkably. The tau used to generate the simulated response fell within the confidence intervals of the estimated tau in ~85% of cases for ΔTr=20s ("20s-w" method); this percentage increased to ~92% of cases when ΔTr was established according to both the minimum tau and its narrowest confidence interval ("Mixed" method). In conclusion, the effects of removing the V'O2 data pertaining to different ΔTr are remarkable. The "Mixed" method provided estimated parameters close to those used to generate the simulated responses and is thus endorsed.