The Effects of Lower Body Negative Pressure (LBNP) on the Near-Infrared Spectroscopy Vascular Occlusion Test (NIRS-VOT)

Introduction: The Near-Infrared Spectroscopy Vascular Occlusion Test (NIRS-VOT) is a novel method for assessing microvascular function in vivo. The influence of acute sympathetic nervous system (SNS) activation on its outcomes remains unclear. Lower Body Negative Pressure (LBNP) is a safe model for cardiopulmonary baroreceptor unloading that increases SNS activity. Understanding whether SNS activation alters NIRS-VOT kinetics is essential for interpreting data in populations with elevated sympathetic tone, such as aging or disease. Purpose: To determine whether SNS activation via LBNP (–20 mmHg) alters NIRS-VOT microvascular metrics in healthy adults and to identify potential sex or inter-arm differences. It was hypothesized that LBNP would promote greater deoxygenation but lower reoxygenation slopes due to greater sympathetic activity and vasoconstriction. Methods: Healthy men and women (N = 39; 17 men, 22 women) completed three randomized trials (Control Right, LBNP Left, LBNP Right). NIRS-VOT was performed on both forearms to measure muscle oxygen saturation (SMO 2 ). Central hemodynamics and blood pressure were measured continuously. Results: LBNP decreased heart rate (p < 0.001, η 2 ₚ = 0.400), cardiac output (p < 0.001, η 2 ₚ = 0.501), stroke volume (p = 0.031, η 2 ₚ = 0.179), and increased mean arterial pressure (p < 0.001, η 2 ₚ = 0.396). No sex × condition interactions were observed. RMSSD decreased significantly (p = 0.012, η 2 ₚ = 0.41), whereas RR interval was unchanged (p = 0.814, η 2 ₚ = 0.03). Baseline SMO 2 (p = 0.415, η 2 ₚ = 0.019), Slope 1 (p = 0.683, η 2 ₚ = 0.005), Slope 2 SMO 2 (p = 0.987, η 2 ₚ ≈ 0), time to halfway (p = 0.867, η 2 ₚ ≈ 0), and peak SMO 2 (p = 0.555, η 2 ₚ = 0.010) were unchanged. LBNP reduced time-to-peak SMO 2 (p = 0.015, η 2 ₚ = 0.154), indicating faster recovery. Adjusted skinfold influenced time-to-peak (p = 0.038, η 2 ₚ = 0.114) and peak SMO 2 (p = 0.035, η 2 ₚ = 0.118), with no effects on other outcomes. Regression analyses showed body fat % predicted baseline SMO 2 in both Control (p = .001, R 2 = 0.71) and LBNP (p = .017, R 2 = 0.48). Under Control, Slope 2 SMO 2 was predicted by baseline RMSSD (p = .038). During LBNP, Slope 2 SMO 2 was predicted by body fat % (p = .016) and diastolic BP (p = .026), while time-to-peak SMO 2 was predicted by skinfold thickness (p = .023). Time-to-halfway was predicted by grip strength alone (p < .001) and together with skinfolds (p < .001). All other variables were excluded from final models. Conclusion: LBNP did not change microvascular function as measured by Slope 1, Slope 2 SMO 2 , baseline SMO 2 , time-to-halfway, or peak SMO 2 . The only NIRS-VOT change was faster time-to-peak SMO 2 , suggesting enhanced recovery kinetics under sympathetic activation. Individual characteristics including body fat %, diastolic BP, grip strength, and skinfold thickness influenced specific NIRS-VOT outcomes. No sex differences were observed. Funding: This work was funded by the AHA awarded to SJI (#24AIREA1247045). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.