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

The final steps of the O cascade during exercise depend on the product of the microvascular-to-intramyocyte difference and muscle O diffusing capacity ( ). Non-invasive methods to determine in humans are currently unavailable. Muscle oxygen uptake (m ) 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 when muscle oxygenation is low (k ), and hypothesized that: (i) k in well oxygenated muscle (k ) is associated with maximal O flux in fibre bundles; and (ii) ∆k (k  - k ) 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 flux in phosphorylating state was 37.7 ± 10.6 pmol s  mg (∼5.8 ml min  100 g ). CD ranged 348 to 586 mm . k was greater than k (3.15 ± 0.45 vs. 1.56 ± 0.79 min , P < 0.001). Maximal O flux was correlated with k (r = 0.80, P = 0.002) but not k (r = -0.10, P = 0.755). Δk ranged -0.26 to -2.55 min , and correlated with CD (r = -0.68, P = 0.015). m 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 . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (m ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O availability, for comparison with biopsy variables. The m recovery rate constant in HIGH O availability was hypothesized to reflect muscle oxidative capacity (k ) and the difference in k between HIGH and LOW O availability (∆k) was hypothesized to reflect muscle O diffusing capacity. k 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.