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

Abstract: The final steps of the O2 cascade during exercise depend on the product of the microvascular-to-intramyocyte (Formula presented.) difference and muscle O2 diffusing capacity ((Formula presented.)). Non-invasive methods to determine (Formula presented.) in humans are currently unavailable. Muscle oxygen uptake (m (Formula presented.)) 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 presented.) when muscle oxygenation is low (kLOW), and hypothesized that: (i) k in well oxygenated muscle (kHIGH) is associated with maximal O2 flux in fibre bundles; and (ii) ∆k (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 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−1 100 g−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 < 0.001). 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). m (Formula presented.) 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 presented.). Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O2 diffusing capacity. (Figure presented.). Key points: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (m (Formula presented.)) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O2 availability, for comparison with biopsy variables. The m (Formula presented.) recovery rate constant in HIGH O2 availability was hypothesized to reflect muscle oxidative capacity (kHIGH) and the difference in k between HIGH and LOW O2 availability (∆k) was hypothesized to reflect muscle O2 diffusing capacity. kHIGH 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.