Muscle O-2 uptake kinetics in humans: Implications for metabolic control

Muscle O-2 uptake (Vo(2)) 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 O-2 content difference across the leg [(Ca - Cv)o(2)leg], permitted the determination of the Vo(2) of the leg (Vo(2)leg) at 3- to 4-s time intervals. Vo(2)leg kinetics during the on-transition was taken as a close approximation of muscle Vo(2) (Vo(2)mus) kinetics. Alveolar Vo(2) (Vo(2)A), Qleg, leg O-2 delivery [(Q . Ca(o2)leg)], (Ca - Cv)o(2)leg, and Vo(2)leg kinetics were determined In six trained subjects [age 22.8 +/- 4.4 (SD) yr; maximal O-2 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), Vo(2)A, Qleg, and (Q . Cao(2))leg increased rapidly, whereas Vo(2)leg increased only slightly and (Ca - Cv)o(2)leg actually decreased. After phase I, all variables showed a monoexponential increase (phase II), with similar time courses [slightly faster for (Ca - Cv)o(2)leg]. In a consideration of both phases, the half times of the responses among variables were not significantly different: 25.5 +/- 2.6 s for Vo(2)A, 26.6 +/- 7.6 s for Qleg, 26.9 +/- 8.3 s for (Q . Cao(2)leg, 23.5 +/- 1.3 s for (Ca - Cv)(o2)leg, and 27.9 +/- 5.7 s for Vo(2)leg. We conclude that during the on-transition the kinetics of Vo(2)A and Vo(2)leg, 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 O-2 to the working muscles is not limiting Vo(2)leg kinetics. However, the present results cannot discriminate between maldistribution of blood flow/Vo(2) vs. inertia the intracellular oxidative machinery as the limiting factor.