Skeletal muscle mitochondrial function and muscle oxygenation measured by near-infrared spectroscopy during brachial artery occlusion

Near-infrared spectroscopy (NIRS) is a widely used technique allowing the noninvasive measurement of skeletal muscle microvascular and mitochondrial function. However, the association between NIRS technology measurements and mitochondrial function analyzed from muscle biopsy by high-resolution respirometry is not clear. Healthy adults (n = 20; 10 females; age 25.3 ± 4.6 yr) participated in the study. A NIRS device was used to measure muscle oxygenation during a 5-min brachial occlusion. Muscle oxygen consumption (mV̇o₂) and the change in tissue saturation index (TSI) from baseline to the end of occlusion (ΔTSI Base-Min) were calculated. Muscle biopsy from the vastus lateralis was collected on a separate day to assess complex I-mediated mitochondrial respiration. mV̇o₂ (from 0.021 to 0.070 mLO₂/min/100 g), ΔTSI Base-Min (from 25.2% to 50.2%), and complex I-mediated mitochondrial respiration (from 9.7 to 57.3 pmol·mg^-1·s^-1) showed wide interindividual variation. Complex I-mediated mitochondrial respiration was correlated negatively with mV̇o₂ (r = -0.64, P = 0.002) and ΔTSI Base-Min (r = -0.80, P = 0.0003). In multivariate linear regression analysis (Enter method), complex I-mediated respiration was the only variable significantly associated with mV̇o₂ and ΔTSI Base-Min when sex, body fat percentage, and peak oxygen uptake (V̇o_2peak) during cycle ergometer test were included in the analysis as confounding factors (standardized coefficients β = -0.431, P = 0.017 and β = -0.691, P = 0.0006, respectively). Our findings suggest that during 5-min occlusion, higher mitochondrial respiratory capacity is associated with lower mV̇o₂ utilization. Lower oxygen utilization during occlusion may indicate a more efficient use of oxygen by mitochondria and better preserved energy metabolism.NEW & NOTEWORTHY This study evaluated the relationship between near-infrared spectroscopy (NIRS)-derived measures of muscle oxygenation and mitochondrial respiration from biopsy in healthy adults. During arterial occlusion, higher complex I-mediated mitochondrial respiration was independently associated with lower muscle oxygen consumption and smaller declines in tissue oxygenation. These findings suggest that greater mitochondrial capacity is linked to more efficient oxygen utilization, supporting the physiological relevance of NIRS-derived indices as indirect markers of mitochondrial function.