Abstract
Engaging in excessive amounts of sedentary behavior throughout the day is associated with a myriad of adverse cardiometabolic outcomes and early mortality. The independent mechanisms underlying the effects of sedentary behavior on health outcomes are not well understood; however, it is hypothesized that sitting influences the vasculature through both indirect and direct mechanisms. Recent research has demonstrated that after just one hour, prolonged sitting impairs vascular function in the legs. Although these findings have been replicated several times in young, healthy men, there is a paucity of data in women. In addition, other factors that may influence the acute response to a bout of sitting have not been well researched. Exercise training has beneficial effects on vascular function, but it is unknown whether participating in regular physical activity influences the acute physiological responses induced by sitting. PURPOSE: The aims of this research were to 1) investigate the effects of a 3-hour bout of uninterrupted sitting on hemodynamics and vascular and inflammatory biomarkers in middle-aged and older adults and 2) compare the responses over 3 hours of sitting between men and women and between adults classified as physically active or inactive. METHODS: Thirty adults (14 men and 16 women; mean ± SD: age, 46.6±8.8 years; body fat, 28.9±8.7%) performed a 3-hour bout of uninterrupted sitting in a quiet, temperature-controlled laboratory. Participants self-reported physical activity and sedentary behavior using the International Physical Activity Questionnaire and Sedentary Behavior Questionnaire, respectively. Participants were classified as inactive by reporting <150 minutes per week of moderate-to-vigorous physical activity (MVPA), or active by reporting ≥150 minutes per week of MVPA. Hemodynamic variables were measured at baseline, 1 hour, 2 hours, and 3 hours of sitting and included blood pressure, calf circumference, and superficial femoral artery diameter and blood velocity using Doppler ultrasound. Mean arterial pressure, blood flow, shear rate, and oscillatory shear index were calculated using standard equations. Vascular function (endothelin-1, intracellular adhesion molecule-1, vascular cellular adhesion molecule-1) and inflammatory (interleukin-6) biomarkers were measured pre and post sitting. Linear mixed-effects modeling was utilized to assess changes in hemodynamic variables and biomarkers over time and compare whether there were differences between men and women, and physically inactive and active adults. Bivariate correlations were used to assess the associations between the change in biomarkers and hemodynamics pre to post sitting. RESULTS: Twenty-seven participants, 14 men and 13 women, completed the study. Eleven were classified as inactive and 16 were classified as active. Middle-aged and older men and women, as well as those considered physically inactive or active, experienced detrimental changes to hemodynamics and vascular and inflammatory biomarkers over 3 hours of sitting. Specifically, participants showed increases in systolic blood pressure, mean arterial pressure, calf circumference, endothelin-1, and interleukin-6 over 3 hours of sitting, regardless of group. Participants also experienced decreases in heart rate and superficial femoral artery blood flow and velocity over 3 hours of sitting. There were no changes in diastolic blood pressure, intracellular adhesion molecule-1, or vascular cellular adhesion molecule-1 over time. Although both men and women experienced decreases in shear rate over time, the shear stress profile was influenced by physical activity status. Inactive adults showed a decline in mean shear rate, whereas active adults maintained mean shear rate over 3 hours of uninterrupted sitting. Additionally, inactive adults had a higher oscillatory shear index compared to active adults. Finally, the decrease in shear rate over 3-hours of sitting was associated with a decrease in blood flow, blood velocity and vascular cellular adhesion molecule-1, and an increase in endothelin-1 and interleukin-6. CONCLUSIONS: A single bout of uninterrupted sitting induced unfavorable changes in vascular function and inflammatory biomarkers, systolic blood pressure, mean arterial pressure, calf circumference, and superficial femoral artery blood flow and velocity in middle-aged and older adults regardless of sex or physical activity status. However, active adults demonstrated a more favorable shear profile. Unfavorable changes in hemodynamic variables from repeated exposure to uninterrupted sitting throughout the lifespan may help explain the increased risk for cardiometabolic disease with excessive amounts of sedentary behavior. Meeting the physical activity guidelines may attenuate some unfavorable changes within the vasculature associated with prolonged sitting.