“…In previous studies, investigators have attributed the phenomenon of directional sensitivity to the sympathetic activation that accompanies rises in MAP, that are not observed when MAP falls. 2,3,7,11,19,33 Due to the observation that the 'strength' of directional sensitivity, expressed by ARI DIFF , did not change between sitting, standing and SSM maneuvers, 8 which should be accompanied by differences in sympathetic drive, we have proposed that directional sensitivity is an inherent characteristic of vascular smooth muscle mechanics, with contraction being much faster than relaxation. 8,34,35 Nevertheless, it should be kept in mind that the extent of sympathetic activation, and its temporal patterns in different physiological conditions, such as rest and SSM, are not known and further attempts to quantify it, as a determinant of directional sensitivity would be highly desirable.…”