Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O 2 diffusion capacity in humans.

The final steps of the O 2 cascade during exercise depend on the product of the microvascular-to-intramyocyte P O 2 ${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O 2 diffusing capacity ( D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non-invasive methods to determine D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (m V ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) 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 D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ when muscle oxygenation is low (k LOW ), and hypothesized that: (i) k in well oxygenated muscle (k HIGH ) is associated with maximal O 2 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 O 2 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 O 2 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 V ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ 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 D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O 2 diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (m V ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O 2 availability, for comparison with biopsy variables. The m V ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ recovery rate constant in HIGH O 2 availability was hypothesized to reflect muscle oxidative capacity (k HIGH ) and the difference in k between HIGH and LOW O 2 availability (∆k) was hypothesized to reflect muscle O 2 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.