## 1 Introduction  
Cardiovascular disease (CVD) remains the global leading cause of death for men and women [1], which highlights the growing need to focus on a variety of applied methods and devices capable of providing early predictions of the risks associated with the onset and severity of CVD [2; 3; 4; 5; 6; 7; 8]. Devices and approaches are currently available to examine the health and function of the microvasculature, which have been identified as early indicatorsof CVD [2]. For example, a marker of microvascular function was shown to independently predict future (5-year follow-up) CVD in a large cohort of adult men who were, at the time, free of vascular disease [2]. Most commonly, reactive hyperemia (RH) has been used to define microvascular function and is represented by an exaggerated reperfusion immediately following transient ischemia, which is caused by various local myogenic, metabolic, and/or endothelial factors within arterioles [7; 9; 10; 11]. It is acknowledged that many approaches are available to capture associated RH responses [7; 12], but, here, strain-gauge venous occlusion plethysmography (VOP) [10] and near-infrared spectroscopy (NIRS) [3; 11; 13; 14; 15] are of particular interest. These two approaches seemingly garner substantial interest as demonstrated by their high usage among research scientists [16; 17; 18; 19]. Venous occlusion plethysmography uses a strain-gauge to record changes in circumference (i.e., limb volume) provoked by venous occlusion to provide measures of the arterial inflow rate [20; 21]. Indeed, VOP has demonstrated clinical importance as lower values (e.g., impaired peripheral endothelial function) were shown to be an independent predictor of mortality in patients with heart failure [22]. Although VOP is highly valid and reproducible, there have been some notable technological advancements since the advent of VOP [7]. One such technology includes NIRS, which is able to provide data describing oxygenated hemoglobin (O\({}_{2}\)Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb), hemoglobin difference (HbDiff) [7; 23]. Previously, it has been reported that changes in tHb were correlated with peak blood flow following fast and slow muscle contractions, which the investigators interpreted as evidence that this NIRS variable may be interpreted as changes in muscle blood flow provoked by active hyperemia [24]. However, there remain inconclusive data concerning whether or not NIRS-derived parameters, including tHb, can be used as surrogate measures to values obtained by a plethysmograph, like forearm blood flow (FBF). [PERSON] et al. [13] and [PERSON] et al. [5] previously suggested that NIRS and VOP correlated well during rest and RH. In contrast and more recently, [PERSON] et al. [3] reported that NIRS was not a suitable surrogate of strain-gauge VOP measures. Thus, there remains a gap in the literature regarding the match between NIRS and strain-gauge measures, which requires additional investigation, especially for investigators aiming to specifically capture blood-flow-related results that inform preventative measures and health promotion. Determining the match between NIRS and strain-gauge measures collected during VOP remains important, especially during studies that assess sex-specific relations and responses. When considering the routinely reported limitations of NIRS- and plethysmograph-derived measures, there is rationale to hypothesize that men and women may demonstrate different strengths of associations between these two techniques, which requires investigation. For example, [PERSON] et al. [15] hypothesized that greater adipose tissue thickness (ATT) of women may contribute to lower reliability in NIRS-based measures compared to men. However, it is interesting to note that ATT likely remained unchanged between their two reported visits [15], suggesting that some other factor primarily influenced the lower reliability for the women (e.g., metabolic rate, oxygen affinity, etc.).