Abstract
We analysed the importance of systemic and peripheral arteriovenous O2 difference ( a-v¯O2 difference and a‐vfO2 difference, respectively) and O2 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 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‐vfO2 difference and O2 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 O2 content with increasing V˙O2max (R 2 = .17; P < .001), systemic O2 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 O2 extraction fraction increased with V˙O2max to approach a maximal value of ~90‐95% (R 2 = .83). Muscle O2 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 O2 diffusional limitations and accentuating O2 delivery limitations. In conclusion, although O2 delivery is the main limiting factor to V˙O2max, enhanced O2 extraction fraction (≥90%) contributes to the remarkably high V˙O2max in endurance‐trained individuals.
Highlights
Under resting conditions in humans, the oxygen uptake (O2) uptake (V O2) is 3-5 mL · kg–1 · min–1, and only a small fraction is consumed within the skeletal muscles.[1]
The systemic O2 extraction fraction increased with V pulmonary maximal oxygen uptake (O2max) until approximately 4.5-5.0 L·min−1
The measured leg O2 extraction fraction increased with leg and pulmonary V O2max to approach a maximal value at ~90-95%, strengthening the findings from the calculated systemic O2 extraction fraction
Summary
Under resting conditions in humans, the O2 uptake (V O2) is 3-5 mL · kg–1 · min–1, and only a small fraction is consumed within the skeletal muscles.[1]. Qmax multiplied by the arterial O2 content (CaO2) sets the upper limit of systemic O2 delivery, which is the principal limitation to V O2max during exercise recruiting in a large muscle mass, at sea level.[4,5,6] Despite extensive research since the 1950s on the factors limiting V O2max, it is still debated whether peripheral O2 extraction capacity contributes to limiting V O2max.[7,8] Several original studies[4,5,9,10,11,12] and review articles[6,13,14,15] have addressed this topic in recent decades, yet no study has aimed to statistically analyse all the existing data on the association between V O2max and its limiting factors This kind of analysis is warranted, as the original studies often used homogenous groups with a small number of subjects (
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