Kinetics of pulmonary \(\dot{V} \hbox{O}_{2}\) and femoral artery blood flow and their relationship during repeated bouts of heavy exercise

The mechanism that alters the pulmonary \(\dot{V}\hbox{O}_{2}\) response to heavy-intensity exercise following prior heavy exercise has been frequently ascribed to an improvement in pre-exercise blood flow (BF) or O₂ delivery. Interventions to improve O₂ delivery have rarely resulted in a similar enhancement of \(\dot{V}\hbox{O}_{2}.\) However, the actual limb blood flow and \(\dot{V}\hbox{O}_{2}\) dynamics in the second bout of repeated exercise remain equivocal. Seven healthy female subjects (21–32 years) performed consecutive 6-min (separated by 6 min of 10 W exercise) bilateral knee extension (KE) exercise in a semisupine position at a work rate halfway between the lactate threshold (LT) and \(\dot{V}\hbox{O}_{2}\)peak. Femoral artery blood flow (FBF) was measured by Doppler ultrasound simultaneously with breath-by-breath \(\dot{V}\hbox{O}_{2};\) each protocol being repeated at least four times for precise kinetic characterization. The effective time-constant (τ′) of the \(\dot{V}\hbox{O}_{2}\) response was reduced following prior exercise (bout 1: 61.0 ±10.5 vs. bout 2: 51.6±9.0 s; mean ± SD; P<0.05), which was a result of a reduced slow component (bout 1: 16.0±8.0 vs. bout 2: 12.5±6.7 %; P<0.05) and an unchanged ‘primary’ τ. FBF was consistently faster than \(\dot{V}\hbox{O}_{2}.\) However, there was no bout-effect on τ′ FBF (bout 1: 28.2±12.0 vs. bout 2: 34.2±8.5 s). The relationship between the exercise-associated \(\dot{V}\hbox{O}_{2}\) (i.e., \(\Delta \dot{V}\hbox{O}_{2}\)) and Δ FBF was similar between bouts, with a tendency (N.S: P>0.05) for \(\Delta \dot{V}\hbox{O}_{2}/\Delta\hbox{ FBF}\) to be increased during the transition to bout 2 rather than decreased, as hypothesized. The return of \(\dot{V}\hbox{O}_{2}\) kinetics toward first order, therefore, was associated with an ‘appropriate’, not enhanced, BF to the working muscles. Whether a relative prior-hyperemia in bout 2 enables a more homogeneous intramuscular distribution of BF and/or metabolic response is unclear, however, these data are consistent with events more proximal to the exercise muscle in mediating the \(\dot{V}\hbox{O}_{2}\) response during repeated heavy-intensity KE exercise.