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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>m</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$</mml:annotation></mml:semantics> </mml:math> ). 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>high</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> ), whole ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>whole</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> ) or shallowest ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>low</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> ) deoxygenation slope from deoxyhaemoglobin (HHb) and muscle O₂ saturation ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>S</mml:mi> <mml:mrow><mml:mi>m</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${S_{{\mathrm{m}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>high</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> [0.80 (0.54-0.92)], whereas poor to good reliability was observed for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>whole</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> [0.71 (0.38-0.89)] and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>low</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> [0.60 (0.19-0.83)]. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>high</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> led to lower MD compared to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>whole</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>low</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> (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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>high</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{high}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> :+32.2%, P < 0.001; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>whole</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{whole}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> :+ 40.1%, P = 0.025; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>k</mml:mi> <mml:mrow><mml:mi>low</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${k_{{\mathrm{low}}{.}{\mathrm{S}}}}$</mml:annotation></mml:semantics> </mml:math> :+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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>m</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$</mml:annotation></mml:semantics> </mml:math> .