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‐intramyocyteP O 2 ${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O 2 diffusing capacity ( D m O 2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non‐invasive methods to determineD m O 2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) 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 byD m O 2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ 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). Δ k ranged –0.26 to –2.55 min –1 , and correlated with CD ( r  = –0.68, P  = 0.015). mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$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 ofD m O 2$D{{\rm{m}}}_{{{\rm{O}}}_2}$ . 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 (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) 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 mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ 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 .