Abstract
We analysed the importance of systemic and peripheral arteriovenous O(2) difference ( a-v¯O2 difference and a-v(f) O(2) difference, respectively) and O(2) extraction fraction for maximal oxygen uptake ( V˙O2max ). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with V˙O2max . Articles (n = 17) publishing individual data (n = 154) on V˙O2max , maximal cardiac output ( Q˙max ; indicator-dilution or the Fick method), a-v¯O2 difference (catheters or the Fick equation) and systemic O(2) extraction fraction were identified. For the peripheral responses, group-mean data (articles: n = 27; subjects: n = 234) on leg blood flow (LBF; thermodilution), a-v(f) O(2) difference and O(2) extraction fraction (arterial and femoral venous catheters) were obtained. Q˙max and two-LBF increased linearly by 4.9-6.0 L · min(-1) per 1 L · min(-1) increase in V˙O2max (R(2) = .73 and R(2) = .67, respectively; both P < .001). The a-v¯O2 difference increased from 118-168 mL · L(-1) from a V˙O2max of 2-4.5 L · min(-1) followed by a reduction (second-order polynomial: R(2) = .27). After accounting for a hypoxemia-induced decrease in arterial O(2) content with increasing V˙O2max (R(2) = .17; P < .001), systemic O(2) extraction fraction increased up to ~90% ( V˙O2max : 4.5 L · min(-1) ) with no further change (exponential decay model: R(2) = .42). Likewise, leg O(2) extraction fraction increased with V˙O2max to approach a maximal value of ~90-95% (R(2) = .83). Muscle O(2) diffusing capacity and the equilibration index Y increased linearly with V˙O2max (R(2) = .77 and R(2) = .31, respectively; both P < .01), reflecting decreasing O(2) diffusional limitations and accentuating O(2) delivery limitations. In conclusion, although O(2) delivery is the main limiting factor to V˙O2max , enhanced O(2) extraction fraction (≥90%) contributes to the remarkably high V˙O2max in endurance-trained individuals.