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 (V over dot (o2)) kinetics at the onset of cycle ergometer exercise. Seven healthy subjects perf ormed rest-to-exercise transitions with a lower-body positive pressure (LBPP) of 45 Torr. Two work rates were selected for each subject: a m oderate intensity (V over dot (o2), similar to 1.9 l min(-1)); Delta[l actate], similar to 1 mequiv l(-1)) below the estimated lactate thresh old and a heavy intensity (V over dot (o2) similar to 2.6 l min(-1); D elta[lactate], similar to 3 mequiv l(-1)) above this threshold. Pulmon ary gas exchange variables and ventilatory (V over dot (E)) responses were computed breath-by-breath from mass spectrometer and turbine volu me meter signals, respectively, and mean response times (MRT) calculat ed. Samples of 'arterialized' venous blood were used for the determina tion of [lactate], pH and [K+]. While the application of 45 Torr LBPP had no effects on V over dot (o2) kinetics during moderate exercise (M RT: 33.5 +/- 1.2 s at 45 Torr vs. 32.8 +/- 1.3 a at 0 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 LBP P during heavy exercise did nor alter V over dot (o2) kinetics (MRT: 3 5.2 +/- 1.5 s at 45 Torr vs. 34.8 +/- 1.5 s at 0 Torr; P > 0.05), or [ lactate], pH or [K+]. Although both the V over dot (E) (via an increas ed f) and CO2 output (V over dot (co2) were significantly greater with LBPP by similar to 30 l min(-1) and similar to 500 ml min(-1) respect ively, end-tidal CO2 partial pressure was decreasing, suggesting an ad ditional ventilatory stimulus. These data can be interpreted to sugges t that oxygen delivery is not critically dependent upon blood flow to the working muscle at exercise onset, while LBPP-induced increases in V over dot (E) during suprathreshold exercise may be related to an acc umulation of metabolites at the working muscle or the effects of press ure per se.