Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O2 diffusion capacity in humans

The final steps of the O2 cascade during exercise depend on the product of the microvascular-tointramyocyte PO2 difference and muscle O2 diffusing capacity (DmO2). Non-invasive methods to determine DmO2 in humans are currently unavailable. Muscle oxygen uptake (mVO2) 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 (kLOW), and hypothesized that: i) k in well-oxygenated muscle (kHIGH) is associated with maximal O2 flux in fiber bundles; and ii) Δk (kHIGH-kLOW) 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 (TSI) 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−1·100g−1). CD ranged 348 to 586 mm-2. kHIGH was greater than kLOW (3.15±0.45 vs 1.56±0.79 min-1, p\u3c0.001). Maximal O2 flux was correlated with kHIGH (r=0.80, p=0.002) but not kLOW (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). mVO2 k reflects muscle oxidative capacity only in well-oxygenated muscle. Δk, the difference in k between well- and poorly-oxygenated muscle, was associated with CD, a mediator of DmO2. Assessment of muscle k and Δk using NIRS provides a non-invasive window on muscle oxidative and O2 diffusing capacity