Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O2 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 (DmO(2)). Non-invasive methods to determine DmO(2) in humans are currently unavailable. Muscle oxygen uptake (m.VO2) 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 DmO(2) when muscle oxygenation is low (k(LOW)), and hypothesized that: (i) k in well oxygenated muscle (k(HIGH)) is associated with maximalO2 flux in fibre bundles; and (ii) Delta 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 O2 flux in phosphorylating state was 37.7 +/- 10.6 pmol (s-1) mg(-1) (similar to 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). Delta k ranged -0.26 to -2.55 min-1, and correlatedwithCD(r = -0.68, P = 0.015). m. V-O2 k reflects muscle oxidative capacity only in well oxygenated muscle.Delta k, the difference in k between well and poorly oxygenated muscle, was associated with CD, a mediator of DmO(2). Assessment of muscle k and Delta k using NIRS provides a non-invasive window onmuscle oxidative and O-2 diffusing capacity.