Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

We analysed the importance of systemic and peripheral arteriovenous O₂ difference ( $$ difference and a-v_f O₂ difference, respectively) and O₂ extraction fraction for maximal oxygen uptake ( $$ ). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with $$ . Articles (n = 17) publishing individual data (n = 154) on $$ , maximal cardiac output ( $$ ; indicator-dilution or the Fick method), $$ difference (catheters or the Fick equation) and systemic O₂ 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 O₂ difference and O₂ extraction fraction (arterial and femoral venous catheters) were obtained. $$ and two-LBF increased linearly by 4.9-6.0 L · min^-1 per 1 L · min^-1 increase in $$ (R²  = .73 and R²  = .67, respectively; both P < .001). The $$ difference increased from 118-168 mL · L^-1 from a $$ of 2-4.5 L · min^-1 followed by a reduction (second-order polynomial: R²  = .27). After accounting for a hypoxemia-induced decrease in arterial O₂ content with increasing $$ (R²  = .17; P < .001), systemic O₂ extraction fraction increased up to ~90% ( $$ : 4.5 L · min^-1 ) with no further change (exponential decay model: R²  = .42). Likewise, leg O₂ extraction fraction increased with $$ to approach a maximal value of ~90-95% (R²  = .83). Muscle O₂ diffusing capacity and the equilibration index Y increased linearly with $$ (R²  = .77 and R²  = .31, respectively; both P < .01), reflecting decreasing O₂ diffusional limitations and accentuating O₂ delivery limitations. In conclusion, although O₂ delivery is the main limiting factor to $$ , enhanced O₂ extraction fraction (≥90%) contributes to the remarkably high $$ in endurance-trained individuals.