During the transition to loaded pedaling, the kinetics of alveolar and leg O2 uptake were similar (half times 25.5 vs 27.9 s), indicating bulk O2 delivery does not limit muscle O2 uptake kinetics.
During the on-transition of exercise, the kinetics of alveolar VO2 and leg muscle VO2 are similar, indicating that bulk delivery of O2 to working muscles does not limit muscle VO2 kinetics.
Muscle O2 uptake (VO2) kinetics in response to an augmented energetic requirement (on-transition) has never been directly determined in humans. We have developed a constant-infusion thermodilution technique that allowed rapid measurements of leg blood flow (Qleg) and, in conjunction with frequent serial measurement of arteriovenous O2 content difference across the leg (Ca - Cv)O2leg, permitted the determination of the VO2 of the leg (VO2leg) at 3- to 4-s time intervals. VO2leg kinetics during the on-transition was taken as a close approximation of muscle VO2 (VO2mus) kinetics. Alveolar VO2 (VO2A), Qleg, leg O2 delivery (Q.CaO2leg), (Ca - Cv)O2leg, and VO2leg kinetics were determined in six trained subjects age 22.8 +/- 4.4 (SD) yr; maximal O2 uptake 59.1 +/- 5.3 ml.kg-1.min-1 during the transition from unloaded pedaling to a workload (loaded pedaling; LP) (183 +/- 20 W) well below the previously determined ventilatory threshold. For all variables, two distinct phases were recognized. During the first 10-15 s of loaded pedaling (phase I), VO2A, Qleg, and (Q.CaO2)leg increased rapidly, whereas VO2leg increased only slightly and (Ca - Cv)O2leg actually decreased. After phase I, all variables showed a monoexponential increase (phase II), with similar time courses slightly faster for (Ca - CV)O2leg. In a consideration of both phases, the half times of the responses among variables were not significantly different: 25.5 +/- 2.6 s for VO2A, 26.6 +/- 7.6 s for Qleg, 26.9 +/- 8.3 s for (Q.CaO2leg, 23.5 +/- 1.3 s for (Ca - Cv)O2leg, and 27.9 +/- 5.7 s for VO2leg. We conclude that during the on-transition the kinetics of VO2A and VO2leg, as measured by these methods, are similar. The analysis of the early phase (first 10-15 s) of the on-transition indicates that bulk delivery of O2 to the working muscles is not limiting VO2leg kinetics. However, the present results cannot discriminate between maldistribution of blood flow/VO2 vs. inertia the intracellular oxidative machinery as the limiting factor.
Grassi et al. (1996) studied Healthy trained subjects (n=6). Transition from unloaded to loaded pedaling vs. Unloaded pedaling was evaluated on Half times of the responses among variables (VO2A, Qleg, leg O2 delivery, (Ca - Cv)O2leg, and VO2leg). During the transition to loaded pedaling, the kinetics of alveolar and leg O2 uptake were similar (half times 25.5 vs 27.9 s), indicating bulk O2 delivery does not limit muscle O2 uptake kinetics.