Near‐infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O(2) diffusion capacity in humans

ABSTRACT: The final steps of the O(2) cascade during exercise depend on the product of the microvascular‐to‐intramyocyte [Formula: see text] difference and muscle O(2) diffusing capacity ([Formula: see text]). Non‐invasive methods to determine [Formula: see text] in humans are currently unavailable. Muscle oxygen uptake (m [Formula: see text]) 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 [Formula: see text] 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 [Formula: see text] 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 [Formula: see text]. Assessment of muscle k and ∆k using NIRS provides a non‐invasive window on muscle oxidative and O(2) diffusing capacity. [Image: see text] KEY POINTS: We determined post‐exercise recovery kinetics of quadriceps muscle oxygen uptake (m [Formula: see text]) 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 [Formula: see text] 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.