Sex differences in NIRS-derived values of reactive hyperemia persist after experimentally controlling for the ischemic vasodilatory stimulus

Men and women exhibit different near-infrared spectroscopy (NIRS) outcomes in response to vascular occlusion tests (VOT), which may be due to phenotypic characteristics or different degrees of desaturation during ischemia. The minimum skeletal muscle tissue oxygenation (StO ) observed during a VOT may be the primary determinant of reactive hyperemic (RH) responses. Our purpose was to determine the contribution StO and participant characteristics including adipose tissue thickness (ATT), lean body mass (LBM), muscular strength, and limb circumference to NIRS-derived indexes of RH. Also, we aimed to determine if matching StO would eliminate NIRS-VOT sex differences. Thirty-one young adults completed one or two VOTs during which the vastus lateralis was continuously assessed for StO . The men and women each completed a standard VOT with a 5-min ischemic phase. The men completed a second VOT with a shortened ischemic phase to produce a matching StO to the minimum of the women observed during the standard VOT. Mean sex differences were determined with tests, and relative contributions were assessed with multiple regression and model comparison approaches. During the 5-min ischemic phase, the men exhibited greater upslopes (1.97 ± 0.66 vs. 1.23 ± 0.59%·s ) and greater StO than the women (80.3 ± 4.17 vs. 76.2 ± 2.86%). Analysis revealed StO was a greater contributor to upslope than sex and/or ATT. For StO , sex was the only significant predictor ( = 0.26, men ∼4.09% > women). Experimentally matching StO did not eliminate the sex differences in upslope or StO , suggesting that characteristics other than the degree of desaturation primarily provoke sex differences in RH. Men exhibit greater values of reactive hyperemia than women even when controlling for the magnitude of desaturation during transient ischemia. Factors other than the ischemic vasodilatory stimulus, such as skeletal muscle mass and quality, likely provoke the commonly reported sex differences in reactive hyperemia measured by near-infrared spectroscopy.