The algorithm used for the calculation of gas exchange affects the estimation of O 2 uptake kinetics at the onset of moderate-intensity exercise.

At the start of a moderate-intensity square-wave exercise, after a short delay, breath-by-breath O 2 uptake at the mouth is approximated to a mono-exponential function, whose time constant is considered matched to that of the O 2 uptake of the working muscles. We compared the kinetic parameters obtained from the breath-by-breath gas exchange data yielded by the 'Independent-breath' algorithm (IND), which accounts for the changes in lung gas stores, with those obtained with the classical 'Expiration-only' algorithm (EXP). The two algorithms were applied on the same flow and gas fraction traces acquired on 10 healthy volunteers, performing 10 times the same moderate-intensity exercise transition. Repeated O 2 uptake responses were stacked together and the kinetic parameters of a mono-exponential function were estimated by non-linear regression, removing the data pertaining to 1-s progressively longer initial periods (ΔT r ). Independently of ΔT r , the mean response time (time constant + time delay) obtained for the IND data was faster compared to the EXP data (∼43 s vs. ∼47 s, P < 0.001), essentially because of shorter time delays. Between ΔT r  = 16 s and ΔT r  = 29s, the time constants of the IND data decreased (30.7 s vs. 28.0 s, P < 0.05; drop = 10%), but less than those of the EXP data (32.2 s vs. 26.2 s, P < 0.001; drop = 23%); with the same ΔT r , the time constants of the two algorithms' data were not different (P > 0.07). The different decrease in the time constant, together with the different mean response time, suggests that the data yielded by the two algorithms provide a different picture of the phenomena occurring at the beginning of the exercise. NEW FINDINGS: What is the central question of this study? Is the kinetics of breath-by-breath gas exchange at the start of a square-wave moderate intensity exercise faster when accounting for the changes in lung gas stores? What is the main finding and its importance? Accounting for the changes in lung gas stores, the time constant of the V ̇ O 2 ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}}}$ kinetics changed less when the data pertaining to the initial period were progressively removed; time delays and mean response times were faster. Consequently, using different gas exchange calculation algorithms, the physiological phenomena occurring at the beginning of moderate-intensity exercise are characterized differently.