UCLA

The final steps of the O₂ cascade during exercise depend on the product of the microvascular-to-intramyocyte PO2${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O₂ diffusing capacity ( DmO2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non-invasive methods to determine DmO2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (m V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) recovery rate constant (k), measured by near-infrared spectroscopy (NIRS) using intermittent arterial occlusions, is associated with muscle oxidative capacity in vivo. We reasoned that k would be limited by DmO2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ when muscle oxygenation is low (k_LOW ), and hypothesized that: (i) k in well oxygenated muscle (k_HIGH ) is associated with maximal O₂ flux in fibre bundles; and (ii) ∆k (k_HIGH  - k_LOW ) is associated with capillary density (CD). Vastus lateralis k was measured in 12 participants using NIRS after moderate exercise. The timing and duration of arterial occlusions were manipulated to maintain tissue saturation index within a 10% range either below (LOW) or above (HIGH) half-maximal desaturation, assessed during sustained arterial occlusion. Maximal O₂ flux in phosphorylating state was 37.7 ± 10.6 pmol s^-1  mg^-1 (∼5.8 ml min^-1  100 g^-1 ). CD ranged 348 to 586 mm^-2 . k_HIGH was greater than k_LOW (3.15 ± 0.45 vs. 1.56 ± 0.79 min^-1 , P < 0.001). Maximal O₂ flux was correlated with k_HIGH (r = 0.80, P = 0.002) but not k_LOW (r = -0.10, P = 0.755). Δk ranged -0.26 to -2.55 min^-1 , and correlated with CD (r = -0.68, P = 0.015). m V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ k reflects muscle oxidative capacity only in well oxygenated muscle. ∆k, the difference in k between well and poorly oxygenated muscle, was associated with CD, a mediator of DmO2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O₂ diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (m V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O₂ availability, for comparison with biopsy variables. The m V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ recovery rate constant in HIGH O₂ availability was hypothesized to reflect muscle oxidative capacity (k_HIGH ) and the difference in k between HIGH and LOW O₂ availability (∆k) was hypothesized to reflect muscle O₂ diffusing capacity. k_HIGH was correlated with phosphorylating oxidative capacity of permeabilized muscle fibre bundles (r = 0.80). ∆k was negatively correlated with capillary density (r = -0.68) of biopsy samples. NIRS provides non-invasive means of assessing both muscle oxidative and oxygen diffusing capacity in vivo.