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

\( \dot{V}{\text{O}}_{2} \), \( \dot{Q} \) 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 \( \dot{V}{\text{O}}_{2\max } \) (120%_Wmax). An innovative approach was applied to calculate the time constant (τ₂) of the primary phase of \( \dot{V}{\text{O}}_{2} \) and \( \dot{Q} \) kinetics at 120%_Wmax. Data were linearly interpolated after a semilogarithmic transformation of the difference between required/steady state and measured values. Furthermore, \( \dot{V}{\text{O}}_{2} \), \( \mathop Q\limits^{ \cdot } \) and HHb data were fitted with traditional exponential models. τ₂ of \( \dot{V}{\text{O}}_{2} \) kinetics was longer (62.5 ± 20.9 s) at 120%_Wmax than at 80%_VT (27.8 ± 10.4 s). The τ₂ of \( \dot{Q} \) kinetics was unaffected by exercise intensity and, at 120% of \( \dot{V}{\text{O}}_{2\max } , \) it was significantly faster (τ₂ = 35.7 ± 28.4 s) than that of \( \dot{V}{\text{O}}_{2} \) 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 \( \dot{V}{\text{O}}_{2} \) 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 \( \dot{V}{\text{O}}_{2} \) kinetics observed during supramaximal exercise.