Steepest near‐infrared spectroscopy‐derived deoxygenation slopes during arterial occlusions provide more reliable assessments of muscle mitochondrial capacity

Assessment of near‐infrared spectroscopy (NIRS)‐derived muscle oxidative capacity relies on analysing deoxygenation slopes from NIRS signal versus time curves during brief arterial occlusions, which reflect the rate of post‐exercise recovery of muscle oxygen consumption (). However, current guidelines lack recommendations on the optimal selection of slopes for reliable measurement. The aim of the study was to compare a standardised partial‐segment approach against the conventional whole‐segment approach on the measurement and reliability of in vivo muscle oxidative capacity. Within the same session, 19 athletes ( n = 9 sprinters; n = 10 middle‐distance runners) completed two NIRS‐derived muscle oxidative capacity trials on the vastus lateralis. Rate constants ( k , min −1 ) were computed using the steepest (), whole () or shallowest () deoxygenation slope from deoxyhaemoglobin (HHb) and muscle O 2 saturation () signals. Test–retest reliability [(coefficient of variation (CV), intraclass correlation coefficient (ICC)] and minimum difference (MD) were assessed. For the HHb signal, ICC analysis revealed moderate to excellent test–retest reliability for [0.80 (0.54–0.92)], whereas poor to good reliability was observed for [0.71 (0.38–0.89)] and [0.60 (0.19–0.83)]. led to lower MD compared to and (0.64 vs. 1.12 vs. 1.54 min −1 , respectively). All three approaches led to significantly greater k values in runners compared to sprinters (:+32.2%, P < 0.001; :+ 40.1%, P = 0.025; :+49.6%, P = 0.001). Compared to the conventional whole‐segment approach, selecting the steepest intra‐occlusion slope improved the reliability and sensitivity of NIRS‐derived mitochondrial capacity, likely by better reflecting instantaneous changes in .