On-feet isometric bracing maintains cerebral arterial blood velocity during lower body negative pressure via preload...

Orthostatic stress reduces venous return and stroke volume (SV), risking cerebral hypoperfusion despite autonomic compensation. Although lower-limb counterpressure manoeuvres improve cerebral perfusion in upright posture, their effects on cerebral blood velocity (CBV) during lower-body negative pressure (LBNP) and the associated mechanisms are not fully defined. We therefore tested whether isometric lower-limb contraction is associated with preservation of CBV during LBNP, accompanied by attenuated effects of preload reduction. Thirteen healthy young adults (age: 25 ± 5 years; 5 women) completed randomized trials under two conditions: off-feet (saddle support, relaxed legs) and on-feet (isometric bracing against a footplate with slight knee flexion). Each condition included 6 min exposures to -30 and -50 mmHg. Systemic vascular conductance declined with increasing LBNP, whereas mean arterial pressure (MAP) was maintained in both conditions. At -50 mmHg, CBV decreased off-feet but was preserved on-feet; SV fell less and the compensatory rise in heart rate (HR) was attenuated on-feet. Repeated-measure correlations showed that CBV tracked SV (r_rm = 0.388, P = 0.002) and end-tidal CO₂ (r_rm = 0.318, P = 0.012), was inversely related to HR (r_rm = -0.448, P = 0.001) and was unrelated to MAP (r_rm = -0.003, P = 0.980) or systemic vascular conductance (r_rm = 0.193, P = 0.129). Thus, isometric lower-limb engagement is associated with preservation of CBV during LBNP, in a manner consistent with preload-mediated effects rather than augmented peripheral vasoconstriction. These findings are consistent with proposed mechanisms underlying physical counterpressure manoeuvres and support simple lower-limb isometric actions to improve orthostatic tolerance.