At the start of exercise alveolar oxygen uptake (VO₂) lags behind the muscle energy requirement following a two phases transition: phase I is believed to be primarily due to the increased pulmonary blood flow, whereas phase II represents the rise of muscular O₂ consumption.

The present work was carried out to assess the role of the circulatory dynamics in determining the VO₂ kinetics at exercise onset.

VO₂ (Quark, Cosmed, IT) and cardiac output (Q; Portapres, TNO-TPD Biomedical Instrumentation, NL) were measured on 8 healthy volunteers (1.78±0.06m; 70.1±9.7kg) before and during moderate intensity (DVO₂=190±59mL/min) supine leg exercise. The two time constants (t_VO2 and t_Q) were calculated from the best fit of a mono-exponential function, excluding for VO₂ the first 15-20 s after the onset of exercise to evaluate phase II. Pearson correlation coefficient was calculated to assess correlations between variables, assuming p<0.05 as statistically significant.

t_Q was significantly shorter than t_VO2 (10.3±5.8 s vs. 39.2±3.7 s; paired t-test, p<0.001, n=8). VO₂ (mL/min) and Q (L/min) above rest were not related to both time constants. t_VO2 was linearly related to t_Q as follows: t_VO2=-0.47·t_Q+44.0 (R=0.730, p<0.05).

Results suggest that the O₂ uptake during phase II rises more rapidly (t_VO2 decreases) as blood flow adjusts more slowly (t_Q increases). Consequently, the conclusion that phase II VO₂ represents the increase in muscular O₂ uptake does not hold, at least in our conditions.