Sex differences and acute intermittent isometric handgrip exercise effects on microvascular reactivity in young...

Near-infrared spectroscopy combined with a vascular occlusion test (NIRS-VOT) is a popular approach for investigating microvascular function across many settings. Although sex differences and exercise-induced changes in NIRS-VOT responses have been reported, limited evidence exists detailing whether these responses persist after accounting for confounding factors such as adipose tissue thickness (ATT) and the magnitude of the ischemic stimulus. Therefore, the purpose of this study was twofold: 1) to determine whether sex explains variability in NIRS-VOT outcomes beyond that accounted for by ATT and measures of the ischemic stimulus, and 2) to determine the extent to which acute isometric handgrip exercise influences NIRS-VOT responses in a sex-specific manner.

Twenty-four healthy adults (12 males) completed two experimental visits consisting of repeated NIRS-VOTs performed before and after either rhythmic handgrip exercise (30 contractions at 50% maximal voluntary contraction) or time-matched rest. Ischemia was terminated once 40% skeletal muscle tissue oxygenation (StO₂) was observed to standardize the ischemic stimulus. Model comparison analyses evaluated the predictive contributions of sex, ATT, and cumulative O₂ deficit to upslope, maximal saturation (StO_2max), and half-time to StO_2max (T_1/2). Linear mixed-effects models examined the effects of Sex, Condition, and Time on NIRS-VOT outcomes.

Sex was the strongest predictor of upslope, explaining approximately 15% of the variance, whereas cumulative O₂ deficit was the strongest predictor of StO_2max and T_1/2. Acute exercise increased downslope and upslope while reducing StO_2max (all p < 0.05). However, no Sex × Condition × Time interactions were detected (all p > 0.05). Because the study was not powered to detect higher-order interactions, the sex-specific exercise analyses should be considered exploratory rather than evidence that males and females respond equivalently.

Overall, these results demonstrate that determinants of NIRS-VOT responses are outcome-specific and influenced by both skeletal muscle metabolic characteristics and microvascular function. Cumulative O₂ deficit emerged as a primary determinant of StO_2max, supporting a muscle-centric framework for interpreting at least some NIRS-derived measures of reactive hyperemia.