Gravitational load, sex, and age-sex effects on cardiovascular compensatory responses during sit-to-stand and lie-to-stand transitions

Active postural transitions impose rapid orthostatic stress that requires coordinated cardiovascular compensation. Whether the magnitude and timing of these responses differ by gravitational load, age, and sex during common daily movements remains unclear. This study compared short-term cardiovascular regulation during sit-to-stand (lower gravitational load) and lie-to-stand (higher gravitational load) transitions in younger and older adults. Fifty-six healthy participants (younger and older females and males; n = 14 per group) completed both transitions in a randomized, counterbalanced design. Beat-to-beat blood pressure and hemodynamic responses were assessed at baseline, immediately on standing, and across four predefined compensatory phases. General linear models examined the effects of condition, age, sex, and age-sex interactions, with cardiovascular medication included as a covariate. Lie-to-stand elicited larger immediate reductions in systolic, diastolic, and mean arterial pressure, alongside greater heart rate increases, compared with sit-to-stand (all p < 0.001). Older adults demonstrated larger drops in blood pressure and systemic vascular resistance, and blunted heart rate and cardiac output responses, particularly under higher gravitational load. In contrast, older adults maintained relatively higher stroke volume during lie-to-stand, suggesting greater reliance on mechanical compensation. Sex-related differences were condition-specific: younger males exhibited higher cardiac output during sit-to-stand, while older females showed higher late-phase mean arterial pressure and vascular resistance during lie-to-stand. These findings indicate that gravitational load strongly influences short-term cardiovascular compensation during active standing and amplifies age-related differences. Sit-to-stand and lie-to-stand transitions provide complementary information, highlighting the importance of considering transition type when evaluating orthostatic cardiovascular regulation.