Uninterrupted sitting can impair popliteal flow-mediated dilation (FMD) responses in young, premenopausal females when endogenous or exogenous estrogen levels are low. However, it is unknown if sitting-induced FMD responses are altered when estrogen levels are elevated in females who naturally menstruate (NAT) or those using combined, monophasic oral-contraceptive-pills (OCP). This study tested the hypothesis that the decline in popliteal FMD following an acute bout of prolonged sitting would be attenuated during the later versus earlier phases of a natural menstrual or OCP cycle. Popliteal FMD was measured before and after 3-h of sitting in NAT-females (n=9; 23±3 years) and OCP-females (n=9; 23±3 years) during both of their respective phases. At pre-sit, relative FMD was greater in the later phase versus earlier phase in NAT (4.6±1.6% to 5.8±1.5%; P=0.002), but not between pill phases among OCP (4.4±1.2% to 4.8±1.6%; P=0.32). Both groups exhibited similar prolonged sitting-induced impairments in popliteal FMD (Pre-Post Sitting Time: P<0.001; Group ΔFMD: P=0.66; Phase ΔFMD: P=0.42; Interaction ΔFMD: P=0.72), regardless of menstrual cycle phase (earlier: -2.5±1.5%; later: -2.4±1.0%) or pill phase (inactive-pill: -2.4±1.4%; active-pill: -2.1±1.1%). Our findings demonstrate that lower-limb arterial endothelial-dependent vasodilatory function was enhanced in the later versus earlier menstrual phase among NAT, but unaffected by combined, monophasic pill phases in OCP. Furthermore, healthy, young females exhibited pronounced negative lower-limb vascular effects in response to prolonged sitting regardless of whether they were in the earlier or later phases of a natural menstrual or contraceptive pill cycle.

Sedentary time (ST) has recently been included in the 24-h activity guidelines. However, the impact of habitual sedentary patterns on autonomic cardiovascular regulation are unclear. We tested the hypothesis that more ST and fewer sedentary breaks were associated with lower cardiovagal baroreflex sensitivity. More frequent sedentary breaks, but not total ST, was independently and positively associated with vagally-mediated blood pressure control. Breaking up ST could be more important than total ST for cardiovascular health. Novelty: Breaks in sedentary time is an independent predictor of cardiovagal baroreflex sensitivity, with more frequent breaks associated with better vagally-mediated blood pressure regulation.

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