Unaltered oxygen uptake kinetics at exercise onset with lower-body positive pressure in humans

The purpose of this study was to determine the influence of a reduced skeletal muscle blood flow on oxygen uptake (V̇(O₂)) kinetics at the onset of cycle ergometer exercise. Seven healthy subjects performed rest-to-exercise transitions with a lower-body positive pressure (LBPP) of 45 Torr. Two work rates were selected for each subject: a moderate intensity (V̇(O₂)) ~ 1.91 min^-1; Δ[lactate], ~ 1 mequiv l^-1) below the estimated lactate threshold and a heavy intensity (V̇(O₂)) ~ 2.61 min^-1 Δ[lactate], ~ 3 mequiv l^-1) above this threshold. Pulmonary gas exchange variables and ventilatory (V̇(E)) responses were computed breath-by-breath from mass spectrometer and turbine volume meter signals, respectively, and mean response times (MRT) calculated. Samples of 'arterialized' venous blood were used for the determination of [lactate], pH and [K⁺]. While the application of 45 Torr LBPP had no effects on V̇(O₂) kinetics during moderate exercise (MRT: 33.5 ± 1.2 s at 45 Torr vs. 32.8 ± 1.3 s at O Torr; P > 0.05) or on [lactate], pH or [K⁺], breathing frequency (f) was increased (P < 0.05) and tidal volume (V(T)) reduced (P < 0.05). The addition of LBPP during heavy exercise did not alter V̇(O₂) kinetics (MRT: 35.2 ± 1.5 s at 45 Torr vs. 34.8 ± 1.5 s at O Torr; P > 0.05), or [lactate], pH or [K⁺]. Although both the V̇(E) (via an increased f) and CO₂ output (V̇(CO₂)) were significantly greater with LBPP by ~ 30 l min^-1 and ~ 500 ml min^-1, respectively, end-tidal CO₂ partial pressure was decreasing, suggesting an additional ventilatory stimulus. These data can be interpreted to suggest that oxygen delivery is not critically dependent upon blood flow to the working muscle at exercise onset, while LBPP-induced increases in V̇(E) during suprathreshold exercise may be related to an accumulation of metabolites at the working muscle or the effects of pressure per se.