Oxygen uptake, cardiac output and muscle deoxygenation at the onset of moderate and supramaximal exercise in humans.

VO₂, Q and muscular deoxyhaemoglobin (HHb) kinetics were determined in 14 healthy male subjects at the onset of constant-load cycling exercise performed at 80% of the ventilatory threshold (80%(VT)) and at 120% of VO₂max (120%(Wmax)). An innovative approach was applied to calculate the time constant (τ₂) of the primary phase of VO₂ and Q kinetics at 120%(Wmax). Data were linearly interpolated after a semilogarithmic transformation of the difference between required/steady state and measured values. Furthermore, VO₂, Q and HHb data were fitted with traditional exponential models. τ₂ of VO₂ kinetics was longer (62.5 ± 20.9 s) at 120%(Wmax) than at 80%(VT) (27.8 ± 10.4 s). The τ₂ of Q kinetics was unaffected by exercise intensity and, at 120% of VO₂max, it was significantly faster (τ₂ = 35.7 ± 28.4 s) than that of VO₂ response. The time delay of HHb kinetics was shorter (4.3 ± 1.7 s) at 120%(Wmax) than at 80%(VT) (8.5 ± 2.6 s) suggesting a larger mismatch between O₂ uptake and delivery at 120%(Wmax). These results suggest that VO₂ at the onset of exercise is not regulated/limited by muscle's O₂ utilisation and that a slower adaptation of capillary perfusion may cause the deceleration of VO₂ kinetics observed during supramaximal exercise.