Dissociation between the time courses of femoral artery blood flow and pulmonary VO2 during repeated bouts of heavy knee extension exercise in humans

It has frequently been demonstrated that prior heavy cycling exercise facilitates pulmonary VO2 kinetics at the onset of subsequent heavy exercise. This might be due to improved muscle perfusion via acidosis-induced vasodilating effects. However, it is difficult to measure the blood flow (BF) to the working muscles (via the femoral artery) during cycling exercise. We therefore selected supine knee extension (KE) exercise as an alternative, and investigated whether the faster VO2 kinetics in the 2nd bout was matched by proportionally faster BF kinetics to the exercising muscle. Nine healthy subjects (aged 21-44 years) volunteered to participate in this study. The protocol consisted of two consecutive 6-min KE exercise bouts in a supine position (work rate: 70-75% of peak power) separated by a 6-min baseline rest (EX1 to EX2). During the protocol, a pulsed Doppler ultrasound technique was utilized to continuously measure the BF in the right femoral artery. The protocol was repeated at least 6 times to characterize the precise kinetics. In agreement with previous studies using cycling exercise, the VO2 kinetics in the 2nd bout were facilitated compared with that in the 1st bout [mean +/- S.D. of the 'effective' time constant (tau): EXI, 68.6 +/- 15.9, versus EX2, 58.0 +/- 14.4 s. Phase II-tau: EX1, 48.7 +/- 9.0, versus EX2, 41.2 +/- 13.3 s. Empirical index of the slow component (DeltaVO(2(6-3))): EX1, 78 +/- 44, versus EX2, 57 +/- 36 ml min(-1) (P < 0.05)]. However, no substantial difference was observed for the facilitation of the femoral artery BE response to the 1st and 2nd exercise bouts [i.e. the (effective' tau of the femoral artery BF: EX1, 40.8 +/- 16.9, versus EX2, 39.0 +/- 17.1 s (P > 0.05)]. It was concluded that the faster pulmonary VO2 kinetics during heavy KE exercise following prior heavy exercise was not associated with a similar modulation in the BF to the working muscles.