Muscle O2 uptake kinetics in humans: implications for metabolic control

Abstract

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.

Kinetics of O2uptake, leg blood flow, and muscle deoxygenation are slowed in the upper compared with lower region of the moderate-intensity exercise domain

“…In the present study, constant-load KE exercise at ϳ33 W (for S2 and S3) resulted in end-exercise steady-state values of 118 (20) 114 (4) 121 (5) 8 (5) 129 (6) Values are means (SD) in mmHg. MAP, mean arterial pressure.…”

“…The close approximation between phase II pulmonary V O 2 and muscle O 2 consumption has been confirmed by using computer modeling simulations (3) and in direct comparisons of pulmonary V O 2 and leg muscle O 2 consumption (using LBF measured by thermodilution and measured arteriovenous O 2 content differences) during leg cycling exercise in humans (20).…”

“…The TD before an increase in HHb signal above pretransition BL (TD-HHb) was less (P Ͻ 0.05) for S2 [14 s (SD 7)] than for S1 [20 (Fig. 2D).…”

Pulmonary oxygen uptake and muscle deoxygenation kinetics during recovery in trained and untrained male adolescents

“…This hypothesis was based in part on the findings of similar studies in adults (Fukuoka et al 2002;Fukuoka et al 2006), and the assumption that pulmonary oxygen uptake kinetics reflects muscle oxygen consumption kinetics (Grassi et al 1996;Koga et al 2005;Krustrup et al 2009;Rossiter et al 2002), which has been shown to be faster during recovery in the trained state (as indicated by muscle phosphocreatine kinetics, Forbes et al 2008;Takahashi et al 1995;Yoshida 2002). Given the distinction in training status between our two groups, it is therefore perhaps surprising that there was no difference in the off-kinetics of pulmonary oxygen uptake between the trained and untrained adolescents.…”

“…moderate-intensity exercise), the kinetics of pulmonary oxygen uptake proceed via a time-course that can be effectively modelled as a single exponential function with a time delay reflecting the muscle-to-lung transit time (Ozyener et al 2001). The kinetics of the pulmonary oxygen uptake response at the on-and offset of exercise is of interest because they are generally interpreted to represent the kinetics of muscle oxygen consumption kinetics (Barstow et al 1990;Grassi et al 1996;Koga et al 2005;Krustrup et al 2009;Rossiter et al 2002), though how strong this relationship is at the offset of exercise has recently been questioned (Krustrup et al 2009). …”