Lower limb hemodynamics during lower body negative pressure and the impact of pre-conditioning exercise.

Lower body negative pressure (LBNP) simulates orthostatic stress by redistributing blood to the lower limbs, triggering cardiovascular compensatory responses without the influence of standing hydrostatic pressures. While central responses to LBNP are well-studied, lower limb vascular responses remain less characterized due to methodological constraints. This study directly quantified lower limb venous and arterial responses to graded LBNP with and without preceding exercise using ultrasound and measured calf volume changes using strain gauge plethysmography. Twenty healthy adults (10 females, 25 ± 4 years) completed a randomized crossover protocol with either 15 min of light intensity cycling exercise or supine rest prior to graded LBNP, applied at -20 and -40 mmHg. Calf volume increased by 2.0 ± 0.7% (p < 0.01), consistent with known fluid shifts during LBNP. LBNP at -40 mmHg decreased deep femoral vein (DFV) blood flow from 69 ± 48 to 43 ± 21 mL/min, and superficial femoral artery blood flow from 97 ± 75 to 40 ± 32 mL/min (both p < 0.01), but increased DFV diameter from 0.67 ± 0.12 to 1.02 ± 0.13 cm (p < 0.01). Preceding cycling had no substantive effects on cardiovascular responses (all p > 0.05). These findings demonstrate that lower limb arteries and veins exhibit distinct compensatory responses to central hypovolemia during LBNP. Prior exercise does not appear to alter peripheral blood flow regulation during LBNP, which may be due to its short duration (15 mins) and light intensity (90 ± 14 bpm). These results characterize lower limb compensatory responses to central hypovolemia and provide a framework for future studies on lower limb responses to LBNP exposure. OSF Registration: https://doi.org/10.17605/OSF.IO/TMGDJ.