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Effect of Prolonged Sitting and Breaks in Sitting Time on Endothelial Function

Abstract: Sitting time (ST) is associated with cardiovascular disease risk factors whereas breaking ST has been reported to be beneficial for reducing cardiovascular risk.

PURPOSE: To examine the effects of breaking ST on superficial femoral artery (SFA) endothelial function. Hypotheses: 1) prolonged sitting would induce endothelial dysfunction and changes in shear forces and 2) breaking sitting time with brief periods of activity would prevent attenuation in endothelial function.

METHODS: Twelve non-obese men (24.2+/-4.2 yrs.) participated in two randomized 3hr sitting trials. In the sitting (SIT) trial, subjects were seated on a firmly cushioned chair for 3hr without moving their lower extremities. In the breaking sitting time trial (ACT), subjects sat similar to the SIT trial, but walked on a treadmill for five min at 2 miles[middle dot]hr-1 at 30min, 1hr 30min, and 2hr 30min during the sitting interval. SFA flow mediated dilation (FMD) was assessed at baseline, 1hr, 2hr, and 3hr in each trial. Statistical analyses were performed using dependent variables SFA FMD, and shear rates. Significance was set at p<=0.05.

RESULTS: In the SIT trial, there was a significant decline in SFA FMD from baseline to 3hr (baseline: 4.72+/-3.78%, 1hr: 0.52+/-0.85%, 2hr: 1.66+/-1.11%, 3hr: 2.2+/-2.15; p<0.05 by ANOVA) accompanied by a decline in mean shear rate (MSR) and antegrade shear rate (ASR), but no difference in SRauc. By 2-way repeated measures ANOVA, ACT prevented the sitting-induced decline in FMD, (baseline: 4.5+/-2.3%, 1hr: 5.04+/-2.85%, 2hr: 5.28+/-5.05%, 3hr: 6.9+/-4.5%) along with no decline in shear rates.

CONCLUSION: Three hours of sitting resulted in a significant impairment in shear rate and SFA FMD. When light activity breaks were introduced hourly during sitting, the decline in FMD was prevented.


Alex’s Notes: It is well-known that I resent chairs. The natural human resting position is squatting, not sitting, and in today's age, inactivity has warped almost every task we routinely do day-to-day. You sit when in lecture, you sit when you eat, you sit to watch TV, and you sit (in a car) to travel. And now we have yet another study showing how prolonged sitting is detrimental to health.

This study consisted of two screening visits and two sitting trials in random sequence, one with uninterrupted sitting (SIT) and one with breaks in sitting time (ACT). The subjects comprised 12 “apparently healthy inactive young men” (avg. 24-years; 23.7 BMI) who were nonsmokers, not on any anti-hypertensive, lipid lowering, or anti-diabetic medications, and performed less than 150 minutes per week of moderate intensity physical activity. Moreover, they were to maintain their usual diets and discontinue any OTC antioxidant supplements. The primary outcome measurement was blood vessel function as measured by flow-mediated dilation (FMD).

For both trials, the subjects were allowed to move their arms to use a computer or read or do pretty much anything that was not considered “vigorous.” This was to mimic how most people operate at their desk. However, the subjects were not allowed to move their legs and needed their feet to be firmly planted on the floor. I don’t quite understand why the researchers did this, as it would have been more realistic to let the people be “normal” and do what they normally do. Some individuals may move their legs when they sit while other may not, but requiring no movement is definitely a limitation of this study as the chances of someone not moving their legs at all seems slim. That said, the SIT trial was uninterrupted sitting for three hours whereas the ACT trial had subjects walk for five minutes at a 2mph pace on an adjacent treadmill at 30, 90, & 150 minutes.

And of course, from baseline to three hours there were significant hourly reductions in FMD in SIT but not ACT, as well as a decline in mean shear rate in SIT. The shear rate represents forward flow through the artery, and even in the absence of atherosclerosis, a chronic decline in shear-rate creates a pro-atherogenic environment that may contribute to vascular aging – at least in the legs. On this note, the authors hypothesize that,

“Perhaps repeated bouts of prolonged sitting result in chronic low shear rates in the lower extremities which impair endothelial function and accelerate the atherosclerotic process in the lower extremity. Because the upper extremity is constantly mobile for activities of daily living, even during the seated position, upper extremity endothelial function may be is relatively protected from the harmful effects of sitting.”

This of course gets back to the limitation of static feet mentioned earlier. It stands to reason that those individuals who annoyingly kick their feet against things and can’t stop moving may also be protected despite sitting. If anything, it would probably attenuate the detriments on endothelial function. Regardless, the bottom line is that you shouldn’t sit for prolonged periods. It is as simple as taking a brief walking or stretch (or both) break for a couple minutes every hour.


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