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 PO2 difference and muscle O(2) diffusing capacity ( DmO2 ). Non-invasive methods to determine DmO2 in humans are currently unavailable. Muscle oxygen uptake (m V̇O2 ) 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 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 V̇O2 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 . 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 V̇O2 ) 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 V̇O2 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.