BACKGROUND: Sympathetic activity is known to be elevated in individuals with obesity, commonly demonstrated through cardiovascular tests, such as isometric handgrip exercise and cold-pressor activation. While prior research has identified a causal influence of adipose tissue on sympathetic tone, the relative contribution of fat-free mass (FFM) remains unclear. The purpose of this study was to determine the relative contributions of fat-mass (FM) and FFM to the hemodynamic responses to voluntary contractions and cold pressor activation. METHODS: 69 individuals participated in both visits of this 2-visit study. The first visit consisted of a prescreening, where height, weight, and body composition were evaluated using a calibrated scale, stadiometer, and bioimpedance spectroscopy (BIS), respectively. In the second visit, participants performed a 2-minute bout of isometric handgrip exercise at 35% of the predetermined maximal voluntary contraction, as well as a cold pressor test (CPT). Both tests began with a 2-minute baseline, followed by 2-minutes of isometric handgrip or cold-water hand immersion. During each protocol, heart rate (via electrocardiography) and blood pressure (via finger photoplethysmography) were monitored continuously. Responses to CPT and handgrip were quantified as the area under the curve for mean arterial pressure (blood pressure index; BPI) during each 2-minute stimulus period. Linear regression was then used to examined the relationships between each pressor variable (BPI responses to each test) and measures of body composition and anthropometrics (FFM, FM, and BMI). RESULTS: BPI during handgrip was significantly and positively correlated with FFM (r: 0.242, p: 0.04), but not BMI (r: 0.099, p:0.42) or FM (r: 0.034, p: 0.78). The association with FFM remained significant even after independently adjusting for FM (r: 0.288, p: 0.01) and BMI (r: 0.258, p: 0.03). In contrast, the BPI responses observed CPT were not significantly associated with BMI (r: 0.017, p: 0.89), FFM (r: 0.064, p: 0.60), or FM (r: 0.041, p: 0.74). CONCLUSIONS: Our results suggest that FFM, but not FM or BMI, functions as a primary driver of exercise pressor responses in humans. However, these findings do not extend to cold pressor responses.

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