Alveolar oxygen uptake kinetics with step, impulse and ramp exercise in humans

The breath-by-breath V_ O2A of five male subjects (21.2 years ±3.2; 78.8 kg ±5.9; 179.6 cm ±5.8) was measured during a cycling exercise. Starting from a 10 W baseline, the subjects performed (i) ON and OFF step transitions (ST-ON; ST-OFF) to 50, 90, and 130 W; (ii) a ramp (R) exercise with work rate gradually increasing by 20 W min1; (iii) impulse transitions (I) to 250 and 410 W lasting 10 and 5 s, respectively. The V_ O2A data was modelled using non-linear weighted least square regressions. The amplitudes of the V_ O2A response turned out to be proportional to the input work rate intensities in all the modalities of exercise. Time constants (s) and time delays (td) of ST-ON and R responses were not significantly different, whereas those of ST-OFF were characterised by longer s values. s and td of I responses turned out to be identical to those of ST-ON when the V_ O2A responses were fitted using a five-component model. These results suggest that: (i) the system controlling alveolar gas exchange behaves linearly when it is forced by ST and R inputs (the ON and OFF phases being considered separate); (ii) the analysis of the I response depends strongly on the models selected to fit the V_ O2A data. The asymmetry between theONandOFF responses mirrors that found between the splitting and resynthesis rates of phosphocreatine, and these results support the notion that phosphocreatine could be the main controller of the skeletal muscle respiratory turnover in humans.