Revista de histología molecular y fisiología médica

Estimating Human Skeletal Muscle Oxidative and Oxygen Diffusion Capacities Using Near-Infrared Spectroscopy

Abstract

Simone Pilotto

During exercise, the culmination of the O₂ cascade is contingent upon the interaction between microvascular to intramyocyte differences and muscle O₂ diffusion capacity. Presently, there is a lack of non-invasive techniques for determining in human subjects. Utilizing near-infrared spectroscopy (NIRS) and intermittent arterial occlusions to measure the recovery rate constant (k) of muscle oxygen uptake (m), we have established its correlation with in vivo muscle oxidative capacity. We postulated that k would be constrained by under conditions of low muscle oxygenation (kLOW). We proposed two hypotheses: (i) k in optimally oxygenated muscle (kHIGH) is linked to the maximum O₂ flux within fiber bundles; and (ii) the difference (Δk) between kHIGH and kLOW is associated with capillary density (CD). In a study involving 12 participants, we employed NIRS to assess the vastus lateralis k following moderate exercise. The timing and duration of arterial occlusions were manipulated to maintain the tissue saturation index within a 10% range either below (LOW) or above (HIGH) half-maximal desaturation, which was determined during sustained arterial occlusion. The maximal O₂ flux in the phosphorylating state was determined to be 37.7 ± 10.6 pmol s⁻¹ mg⁻¹ (∼5.8 ml min⁻¹ 100 g⁻¹), and CD ranged from 348 to 586 mm^–2. kHIGH surpassed kLOW (3.15 ± 0.45 vs. 1.56 ± 0.79 min^–1, P<0.001). While maximal O₂ flux correlated positively with kHIGH (r=0.80, P=0.002), there was no correlation with kLOW (r=–0.10, P=0.755). The range of Δk extended from –0.26 to –2.55 min^–1, and it exhibited a negative correlation with CD (r=–0.68, P=0.015). It is evident that solely reflects muscle oxidative capacity under conditions of optimal oxygenation. Moreover, the disparity (Δk) between well-oxygenated and poorly oxygenated muscle was found to be linked to CD, a key factor in. The assessment of muscle k and Δk through NIRS offers a non-invasive avenue for gaining insights into muscle oxidative and O2 diffusion capacities.