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 ( V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ ). 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 ( k high . S ${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$ ), whole ( k whole . S ${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$ ) or shallowest ( k low . S ${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$ ) deoxygenation slope from deoxyhaemoglobin (HHb) and muscle O2 saturation ( S m O 2 ${S_{{\mathrm{m}}{{\mathrm{O}}_2}}}$ ) 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 k high . S ${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$ [0.80 (0.54-0.92)], whereas poor to good reliability was observed for k whole . S ${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$ [0.71 (0.38-0.89)] and k low . S ${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$ [0.60 (0.19-0.83)]. k high . S ${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$ led to lower MD compared to k whole . S ${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$ and k low . S ${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$ (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 ( k high . S ${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$ :+32.2%, P < 0.001; k whole . S ${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$ :+ 40.1%, P = 0.025; k low . S ${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$ :+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 V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ .