Alveolar oxygen uptake and femoral artery blood flow dynamics in upright and supine leg exercise in humans

We tested the hypothesis that the slowerincrease in alveolar oxygen uptake(O₂) at the onset ofsupine, compared with upright, exercise would be accompanied by aslower rate of increase in leg blood flow (LBF). Seven healthy subjectsperformed transitions from rest to 40-W knee extension exercise in theupright and supine positions. LBF was measured continuously with pulsedand echo Doppler methods, andO₂ was measured breath bybreath at the mouth. At rest, a smaller diameter of thefemoral artery in the supine position(P < 0.05) was compensated by agreater mean blood flow velocity (MBV) (P < 0.05) so that LBF was not different in the two positions. At the end of6 min of exercise, femoral artery diameter was larger in the uprightposition and there were no differences inO₂, MBV, or LBF betweenupright and supine positions. The rates of increase ofO₂ and LBF in thetransition between rest and 40 W exercise, as evaluated by the meanresponse time (time to 63% of the increase), were slower in the supine[O₂ = 39.7 ± 3.8 (SE) s, LBF = 27.6 ± 3.9 s] than in the uprightpositions (O₂ = 29.3 ± 3.0 s, LBF = 17.3 ± 4.0 s;P < 0.05). These data support ourhypothesis that slower increases in alveolarO₂ at the onset of exercisein the supine position are accompanied by a slower increase in LBF.