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