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Few formulas have been used in exercise physiology as extensively as the Fick formula, which calculates the rate of oxygen consumption (i.e., VO2) as the product of cardiac output (Q) and the difference in oxygen content in arterial and mixed venous blood (ΔavO2). Unfortunately, the physiology of maximum VO2 (VO2MAX) is often misinterpreted due to a lack of appreciation for the limitations represented by the oft-ignored superscript annotations in the Fick formula. The purpose of this perspective is to explain the meaning of the superscript annotations and highlight how such annotations influence proper interpretation of VO2MAX physiology with the Fick formula. First, we explain the significance of the overdots aboveVO2 and Q which indicate a measure per unit of time. As we will show, the presence of an overdot above Q and lack of one above ΔavO2 denotes they are different types of ratios and should be interpreted in the context of one another--not in contrast to each other as is commonplace. Second, we discuss the significance of the overline above the "v" in ΔavO2, which indicates the venous sample is an average of blood that comes from mixed sources. The mixed nature of the venous sample has major implications for interpreting the influence of oxygen diffusion and blood flow heterogeneity on VO2MAX. Ultimately, we give recommendations and insights for using the Fick formula to calculate V.O2 and interpret VO2MAX physiology. [ABSTRACT FROM AUTHOR]

Few formulas have been used in exercise physiology as extensively as the Fick formula, which calculates the rate of oxygen consumption (i.e., V.O2) as the product of cardiac output (Q.) and the difference in oxygen content in arterial and mixed venous blood (Δav¯O2). Unfortunately, the physiology of maximum V.O2 (V.O2MAX) is often misinterpreted due to a lack of appreciation for the limitations represented by the oft-ignored superscript annotations in the Fick formula. The purpose of this perspective is to explain the meaning of the superscript annotations and highlight how such annotations influence proper interpretation of V.O2MAX physiology with the Fick formula. First, we explain the significance of the overdots above V.O2 and Q., which indicate a measure per unit of time. As we will show, the presence of an overdot above Q. and lack of one above Δav¯O2 denotes they are different types of ratios and should be interpreted in the context of one another—not in contrast to each other as is commonplace. Second, we discuss the significance of the overline above the “v¯” in Δav¯O2, which indicates the venous sample is an average of blood that comes from mixed sources. The mixed nature of the venous sample has major implications for interpreting the influence of oxygen diffusion and blood flow heterogeneity on V.O2MAX. Ultimately, we give recommendations and insights for using the Fick formula to calculate V.O2 and interpret V.O2MAX physiology.

When exercising with a small muscle mass, the mass‐specific O2 delivery exceeds the muscle oxidative capacity resulting in a lower O2 extraction compared with whole‐body exercise. We elevated the muscle oxidative capacity and tested its impact on O2 extraction during small muscle mass exercise. Nine individuals conducted six weeks of one‐legged knee extension (1L‐KE) endurance training. After training, the trained leg (TL) displayed 45% higher citrate synthase and COX‐IV protein content in vastus lateralis and 15%‐22% higher pulmonary oxygen uptake (V˙O2peak) and peak power output (W˙peak) during 1L‐KE than the control leg (CON; all P <.05). Leg O2 extraction (catheters) and blood flow (ultrasound Doppler) were measured while both legs exercised simultaneously during 2L‐KE at the same submaximal power outputs (real‐time feedback‐controlled). TL displayed higher O2 extraction than CON (main effect: 1.7 ± 1.6% points; P =.010; 40%‐83% of W˙peak) with the largest between‐leg difference at 83% of W˙peak (O2 extraction: 3.2 ± 2.2% points; arteriovenous O2 difference: 7.1 ± 4.8 mL· L−1; P <.001). At 83% of W˙peak, muscle O2 conductance (DMO2; Fick law of diffusion) and the equilibration index Y were higher in TL (P <.01), indicating reduced diffusion limitations. The between‐leg difference in O2 extraction correlated with the between‐leg ratio of citrate synthase and COX‐IV (r =.72‐.73; P =.03), but not with the difference in the capillary‐to‐fiber ratio (P =.965). In conclusion, endurance training improves O2 extraction during small muscle mass exercise by elevating the muscle oxidative capacity and the recruitment of DMO2, especially evident during high‐intensity exercise exploiting a larger fraction of the muscle oxidative capacity. [ABSTRACT FROM AUTHOR]