Impact of Prolonged Sitting on Lower and Upper Limb Micro- and Macrovascular Dilator Function

Abstract

Sedentary behavior in the workplace and increased daily sitting time are on the rise; however, studies investigating the impact of sitting on vascular function remain limited. Herein we hypothesized that 6 hours of uninterrupted sitting would impair limb micro- and macrovascular dilator function and this impairment could be improved with a bout of walking. Resting blood flow, reactive hyperemia to 5 min cuff occlusion (microvascular reactivity) and associated FMD (macrovascular reactivity) were assessed in popliteal and brachial arteries of young men at baseline (Pre Sit) and after 6 hours of uninterrupted sitting (Post Sit). Measures were then repeated after a 10 min walk (~1000 steps). Sitting resulted in a marked reduction of resting popliteal artery mean blood flow and mean shear rate (6-hr mean shear rate, −52±8 s⁻¹ vs. Pre Sit, p<0.05). Interestingly, reductions were also found in the brachial artery (6-hr mean shear rate, −169±41 s⁻¹ vs. Pre Sit, p<0.05). Likewise, following 6 hours of sitting, cuff-induced reactive hyperemia was reduced in both the lower leg (−43±7% vs. Pre Sit, p<0.05) and forearm (−31±11% vs. Pre Sit, p<0.05). In contrast, popliteal, but not brachial, artery FMD was blunted with sitting. Notably, lower leg reactive hyperemia and FMD were restored after walking. Collectively, these data suggest that prolonged sitting markedly reduces lower leg micro- and macrovascular dilator function but these impairments can be fully normalized with a short bout of walking. In contrast, upper arm microvascular reactivity is selectively impaired with prolonged sitting and walking does not influence this effect.

Introduction

Epidemiological data suggest that prolonged sitting time is an independent risk factor contributing to increased incidence of cardiovascular disease (CVD) (Katzmarzyk et al., 2009; Chomistek et al., 2013; Nosova et al., 2014). Indeed, the prevalence of sedentary behavior in the workplace and increased daily sitting time, common to many professions, has been associated with the development of CVD (Hamilton et al., 2007; Church et al., 2011). It is plausible that the link between prolonged sitting time and reduced cardiovascular health is attributable, at least in part, to direct detrimental effects of sitting on the vasculature. However, to date, limited experimental data exist establishing the impact of increased sitting time on measures of vascular function. One recent study by Thosar et al. indicated that 3 hrs of uninterrupted sitting reduced superficial femoral artery flow-mediated dilation (FMD) (Thosar et al., 2015). Although a blunted FMD is suggestive of impaired macrovascular endothelial function, a reduction in FMD following prolonged sitting could also be the result of an attenuated reactive hyperemia due to impaired microvascular function. However, little is known about the potential detrimental effects of prolonged sitting on microvascular reactivity and, to our knowledge, dilator function of vascular beds beyond the inactive legs (e.g. arms) have not been investigated.

Interestingly, Thosar et al. found that the effects of prolonged sitting on macrovascular function in the leg could be prevented with intermittent bouts of walking. These results are in general agreement with a previous study from our group demonstrating that 5 days of reduced daily steps (from >10,000 to <5,000 steps) impairs popliteal artery FMD and highlight the beneficial vascular effects of being physically active (Boyle et al., 2013). It is proposed that increased skeletal muscle blood flow and shear stress is a primary mechanism by which physical activity exerts favorable effects on the vasculature (Laughlin et al., 2008; Green, 2009; Tinken et al., 2010; Newcomer et al., 2011; Jenkins et al., 2012). However, it is unknown whether a bout of walking could serve as a restorative intervention for vascular function following a prolonged uninterrupted sitting period.

With this background in mind, we sought to examine the impact of 6 hours of uninterrupted sitting on micro- and macrovascular dilator function of the lower and upper limbs, as assessed by reactive hyperemia and the associated FMD response, respectively. A 6 hour sitting period was chosen to mimic a long bout of sedentary behavior commonly endured in a typical workday (Evans et al., 2012). Furthermore, in order to study plasticity of sitting effects on vascular function, measurements were repeated after subjects walked for 10 min at a self-selected pace. We hypothesized that prolonged sitting would impair micro- and macrovascular dilator function of the lower and upper limbs and that these impairments would be improved by a short bout of walking.

Methods

Subjects

Eleven young healthy men recruited from the University of Missouri campus and surrounding Columbia MO area participated in this study (Age: 27 ± 1 yrs; Height: 179 ± 0.7 cm; Weight: 79 ± 1.3 kg; BMI: 25 ± 0.4 kg/m²). Due to previous evidence showing sex differences and hormonal influences on the vasculature, only men were recruited for this study (Sudhir et al., 1995; Adkisson et al., 2010). All experimental procedures and measurements conformed to the Declaration of Helsinki and were approved by the University of Missouri Health Sciences Institutional Review Board. Prior to participating in the study, each subject provided written informed consent. Subjects were recreationally active, non-smokers, with no history or symptoms of cardiovascular, pulmonary, metabolic, or neurological disease as determined from a detailed medical health history questionnaire. No subjects were using prescribed or over the counter medications.

Experimental Measurements

Subjects were instrumented with Lead-II surface ECG (Q710; Quinton, Bothell, WA) electrodes for continuous measures of heart rate (HR). Automated sphygmomanometry (Welch Allyn, Skaneateles Falls, NY) at the brachial artery of the left arm was used for basal measurements of blood pressure (BP). Popliteal and brachial artery diameter and blood velocity were measured using duplex-Doppler ultrasound (Logiq P5; GE Medical Systems, Milwaukee, WI) as previously described by our laboratory (Boyle et al., 2013; Fairfax et al., 2015). A 9MHz linear array transducer was placed just distal to the popliteal fossa for the popliteal artery and ~5cm proximal to the antecubital fossa for the brachial artery; probe placement was marked on the skin in order to ensure all repeated measurements were made in the same location. Simultaneous diameter and velocity signals were obtained in duplex mode at pulsed frequency of 5MHz and corrected with an insonation angle of 60 degrees. Sample volume was adjusted to encompass the entire lumen of the vessel without extending beyond the walls and the cursor was set mid vessel. To obtain an estimate of venous pooling during the course of the sitting period, calf circumference was measured using a tape measure at the widest point of the calf muscle.

Subjects were given a standardized meal (Weight Watchers Smart Ones breakfast quesadilla, 230 cal, 7 g fat, 29 g carbohydrates, 12 g protein; Dole pineapple juice, 6o z, 100 cal, 0 g fat, 24 g carbohydrates, 1 g protein) to be consumed 2 hours prior to arrival to the laboratory and a second exact meal 4 hours into the sitting period. The use of a standardized meal at these specified time points provided a similar postprandial environment between vascular function assessment periods.

Experimental Protocol

All study visits were performed in the morning beginning at 8:00AM in a temperature-controlled room kept at 21–22°C. A schematic of the study design is presented in Figure 1 illustrating the sequence of events and various positions in which measurements were made. Upon arrival to the laboratory subjects were placed in a semi-recumbent position (bed angle ~45°) and instrumented for all experimental measurements. A rapid inflating cuff was placed on the lower leg 5cm distal to the fibular head for assessment of lower leg reactive hyperemia and popliteal artery FMD. Similarly, a cuff was placed on the forearm 3cm distal to the antecubital fossa for assessment of forearm reactive hyperemia and brachial artery FMD. For these assessments, after ~30 minutes of resting quietly, 2 minutes of baseline hemodynamics were recorded and then the cuff was inflated to a pressure of 220 mmHg for 5 minutes. Continuous diameter and blood velocity measures were recorded for 30 seconds before and 3 minutes following cuff deflation and analyzed offline using a custom-designed wall-tracking software (Labview; National Instruments, Austin, TX), as previously described (Padilla et al., 2010; Boyle et al., 2013; Fairfax et al., 2015). These measures represent the Pre Sit baseline. A minimum of 15 minutes separated the popliteal and brachial artery assessments for each time point and the order was randomized.

Figure 1.

Schematic illustrating overall design of experimental protocol. Measurements taken at the time points of 2, 4, and 6 hours were made while the subject was in the seated position whereas the Pre Sit, Post Sit, and Post Walk measurements were taken while the subject was in a semi-recumbent position.

Following Pre Sit measures subjects were moved into a seated position. In order to capture the full hemodynamic effects of sitting on leg vascular function subjects were instructed to refrain from any leg movement during the sitting period. A study representative monitored the subject during the entire sitting period in order to ensure no leg movement occurred. Subjects were allowed to read and use a computer; thus, movements in the upper limbs that reflected desk-related activities were allowed. The chair used for the study was elevated with the legs of the subjects hanging and a foam platform was placed under the feet to ensure light support without causing any passive muscle contraction. This setup allowed for isolation of the gravitational effects on hemodynamics of the legs in the seated position. Subjects remained in the seated position for a total of 6 hours, a time frame reflective of a typical workday. Popliteal and brachial artery blood flow measurements and calf circumference were taken at 2, 4, and 6 hours during the sitting period (see Figure 1).

At the end of the 6 hour sitting period subjects were manually lifted by study personnel and placed back into the semi-recumbent position, in order to avoid any muscle contraction in the legs and to maintain the hemodynamic effects endured by the seated position. Two minutes of baseline hemodynamics in each artery were recorded and assessments of reactive hyperemia and FMD were then immediately repeated for the Post Sit time point. Measurements at this time point began with the popliteal artery in order to capture the immediate effects of sitting on the lower leg vasculature. Next, to determine if a short bout of physical activity could improve vascular function following sitting, subjects took a 10 min walk at a self-selected pace. The number of steps was assessed using a pedometer. Following walking, vascular function testing was repeated (i.e., Post Walk). Similar to Post Sit, measurements at this time point began with the popliteal artery.

Data Analysis

Mean blood flow was calculated from continuous diameter and mean blood velocity recordings at each of the experimental time points using the following equation: 3.14*(diameter/2)² * mean blood velocity * 60. For microvascular dilator function, popliteal and brachial artery mean blood flow and blood velocity responses to cuff release were calculated. These calculations encompassed the entire hyperemic response (i.e. to the point at which hemodynamics returned to resting values). Areas under the curve (AUC) for mean blood flow and blood velocity were also calculated for the entire post cuff release period using the sum of trapezoids method. For macrovascular dilator function, popliteal and brachial artery FMD percent change was calculated using the following equation: %FMD = (peak diameter-base diameter)/(base diameter) * 100. Shear rate was defined as 8*mean blood velocity/diameter. Shear rate AUC up to peak diameter was calculated as stimulus for FMD, as previously described (Thijssen et al., 2011; Boyle et al., 2013).

Statistics

Sitting measures of blood flow, cuff induced reactive hyperemic responses, and FMD data were analyzed using one-way repeated measures ANOVAs with Holm-Sidak posthoc test. Values for FMD are presented as percent change and corrected for shear rate AUC. %FMD was adjusted for shear rate AUC via ANCOVA in order to statistically control for the influence of shear stimulus on FMD response (Atkinson et al., 2009). ANOVA tests were completed using SigmaStat software (version 12.2) and ANCOVA tests were performed using SPSS software (version 22). Significance was accepted at p≤ 0.05. Data are expressed as means ± SEM.

Results

Hemodynamic Responses to Sitting

Mean resting HR and MAP of the subjects were 63 ± 0.4 bpm and 90 ± 1 mmHg, respectively. Over the course of the study period, HR data were as follows: Pre Sit 63±0.4, 2hr 66±0.7, 4hr 68±0.7, 6hr 72±0.8 (p<0.05 vs. Pre Sit), Post Sit 62±0.8, and Post Walk 62±0.8 bpm. During the 6 hours of uninterrupted sitting, popliteal (Figure 2A and Table 1) and brachial (Figure 2B and Table 2) artery resting mean blood flow, mean blood velocity, mean shear rate were all significantly reduced. As a result of the uninterrupted sitting period, calf circumference underwent a significant increase of 4.8 ± 0.3% (from 38 ± 1 cm at the immediate onset of sitting to 40 ± 1 cm at the 6 hr time point; n=10; p<0.05 vs. Pre Sit).

Figure 2.

Mean resting blood flow of the popliteal (A) and brachial (B) arteries at experimental time points. Popliteal and brachial artery blood flow was reduced by sitting. However, popliteal, but not brachial, artery blood flow was restored after 10min of walking. Values are means ± SEM. *p<0.05 vs. Pre Sit; †p<0.05 vs. Post Walk.

Table 1.

Popliteal artery hemodynamics pre and post cuff release at each experimental time point

	Resting Baseline			Post Cuff Release
	Diameter (cm)	Mean Shear (s−1)	Mean Velocity (cm/s)	Peak Diameter (cm)	Delta Diameter (cm)	Hyperemic Velocity AUC (A.U.)	Shear AUC (A.U.)
Pre Sit	0.59 ± 0.02	59.7 ± 8	3.82 ± 0.62	0.62 ± 0.02	0.03 ± 0.006	4164 ± 645	54947 ± 8743
2hr	0.51 ± 0.03	12.4 ± 3*†	1.35 ± 0.19*†
4hr	0.52 ± 0.03	7.7 ± 4*†	0.78 ± 0.36*†
6hr	0.51 ± 0.03	7.4 ± 5*†	0.89 ± 0.49*†
Post Sit	0.57 ± 0.01	28.1 ± 3*†	1.92 ± 0.24*†	0.58 ± 0.01*	0.01 ± 0.005	2066 ± 221*†	27624 ± 3982*†
Post Walk	0.57 ± 0.02	60.5 ± 8	3.78 ± 0.42	0.61 ± 0.02	0.03 ± 0.008	4451 ± 465	60699 ± 7117
ANOVA: p value	0.013	<0.001	<0.001	0.025	0.063	<0.001	0.01

^*p<0.05 vs. Pre Sit;

^†p<0.05 vs. Post Walk

Table 2.

Brachial artery hemodynamics pre and post cuff release at each experimental time point

	Resting Baseline			Post Cuff Release
	Diameter (cm)	Mean Shear (s−1)	Mean Velocity (cm/s)	Peak Diameter (cm)	Delta Diameter (cm)	Hyperemic Velocity AUC (A.U.)	Shear AUC (A.U.)
Pre Sit	0.38 ± 0.01	324.3 ± 33.5	11.59 ± 1.61	0.41 ± 0.01	0.03 ± 0.009	8536 ± 797	163146 ± 15671
2hr	0.41 ± 0.02	120.1 ± 34.7*	7.42 ± 1.54
4hr	0.39 ± 0.02	118.7 ± 40.5*	6.58 ± 1.44*
6hr	0.39 ± 0.02	155.4 ± 41.2*	7.72 ± 0.91
Post Sit	0.38 ± 0.01	176.2 ± 37.1*	6.34 ± 0.84*	0.42 ± 0.01	0.03 ± 0.01	5469 ± 604*	109987 ± 12483*
Post Walk	0.37 ± 0.02	135.7 ± 23.8*	5.43 ± 0.55*	0.4 ± 0.01	0.03 ± 0.01	5924 ± 786*	115789 ± 19523*
ANOVA: p value	>0.05	<0.001	<0.05	>0.05	>0.05	<0.001	<0.05

^*p<0.05 vs. Pre Sit;

^†p<0.05 vs. Post Walk

Microvascular Hyperemic Responses

Time-course of popliteal and brachial artery mean blood flow following cuff release at each of the experimental time points is presented in Figure 3. Both peak responses and AUC of mean blood flow to cuff release were blunted following the 6hrs of sitting in the leg and arm (Figure 3). Similar findings were obtained when expressing the data as AUC of mean blood velocity (Tables 1 and 2), indicating that changes in reactive hyperemic blood flow were not driven by changes in conduit artery diameter.

Figure 3.

Time course and area under the curve of hyperemic blood flow responses in the popliteal (A) and brachial (B) arteries at baseline (Pre Sit), after sitting (Post Sit) and after walking (Post Walk). Hyperemic blood flow responses to cuff occlusion, indicative of microvascular reactivity, were blunted by sitting. However, popliteal, but not brachial, artery hyperemic blood flow was restored after 10min of walking. Values are means ± SEM. *p<0.05 vs. Pre Sit; †p<0.05 vs. Post Walk.

Macrovascular Hyperemic Responses

Post Sit %FMD was reduced in the popliteal (Figure 4A), but not brachial (Figure 4B), artery compared to Pre Sit. Because shear rate AUC in response to cuff release was reduced Post Sit compared to Pre Sit (Table 1 and Table 2), statistical adjustments for differences in shear stimulus were made via ANCOVA. Popliteal artery %FMD remained significantly reduced with sitting with no main effect of shear rate AUC after statistical correction; this suggests reduced shear stress stimulus was not a primary contributor to the blunted popliteal artery FMD after sitting (main effect p>0.05).

Figure 4.

Popliteal (A) and brachial (B) artery %FMD and FMD corrected for shear rate AUC at time points Pre Sit, Post Sit, and Post Walk. Popliteal, but not brachial, artery %FMD, indicative of macrovascular dilator function, was reduced with sitting and restored after a 10min walk. The effect of sitting on popliteal artery FMD remained after correcting for shear rate with ANCOVA analysis. Values are means ± SEM. *p<0.05 vs. Pre Sit; †p<0.05 vs. Post Walk.

Hemodynamic and Vascular Response Post Walk

Subjects walked 1131 ± 14 steps during the 10 min walk. Post walk resting mean blood flow, mean blood velocity, mean shear rate, and popliteal artery diameter were increased to Pre Sit values (Figure 2A and Table 1). Popliteal artery mean blood flow (Figure 3A) in response to cuff release was also increased Post Walk and reached values similar to those found at Pre Sit. In contrast, brachial artery mean blood flow (Figure 3B) in response to cuff release was not affected by walking and responses remained impaired relative to Pre Sit. Finally, popliteal, but not brachial, artery %FMD, and FMD statistically corrected for shear rate AUC were improved Post Walk (Figure 4).

Discussion

The results of the present study demonstrate that prolonged sitting (6 hrs) impairs microvascular dilator function of the lower and upper limbs. In contrast, impaired macrovascular dilator function with sitting was noted in the lower, but not upper, limbs. Notably, the sitting-induced vascular impairments in lower limbs could be fully reversed after a short bout of activity in the form of a 10 min walk, whereas walking had no effect on impaired upper arm microvascular reactivity. Collectively, we demonstrate that in young healthy men prolonged sitting markedly reduces lower leg micro- and macrovascular dilator function but these impairments can be fully restored with a short bout of walking. In contrast, upper arm microvascular reactivity is selectively impaired with prolonged sitting and walking does not influence this effect.

We found that lower leg and forearm blood flow are both markedly reduced over the course of uninterrupted sitting, thus, exposing the vasculature to low levels of shear stress. Shear stress not only stimulates endothelium-dependent dilation but is also a required signal for sustaining an anti-atherogenic vascular cell phenotype. Indeed, the role of shear stress in maintaining vascular health is supported by endothelial cell culture models (Mohan et al., 2003a; Mohan et al., 2003b; DeVerse et al., 2013; Ishibazawa et al., 2013), studies in perfused isolated arteries (Yamawaki et al., 2003; Woodman et al., 2005; Gambillara et al., 2006; Padilla et al., 2014), in vivo animal experiments (Korshunov & Berk, 2003; Nam et al., 2009; Wang et al., 2011; Loyer et al., 2014), and studies in human subjects (Thijssen et al., 2009; Tinken et al., 2009; Ishibazawa et al., 2013; Jenkins et al., 2013; Johnson et al., 2013). For example, Woodman et al. (2005) showed that isolated rat skeletal muscle feed arteries exposed to low levels of shear stress for 4 hours results in a down-regulation of eNOS expression and reduced nitric oxide-mediated dilation. Recent studies in humans also demonstrate that experimentally induced low (and oscillatory) brachial artery blood flow and shear for 30 minutes by inflation of a distal cuff blunts FMD (Thijssen et al., 2009; Johnson et al., 2013), and is associated with increased markers of endothelial activation (CD62E⁺) and apoptosis (CD31⁺/CD42b⁻) (Jenkins et al., 2013).

The reason for the reduction in limb blood flow and shear stress with sitting is unclear and the mechanisms may be limb-specific. In the case of the lower extremities, increased hydrostatic pressure within the leg vasculature due to prolonged gravitational forces causes blood to pool within the venous circulation. The increased calf circumference we observed during sitting lends support to the idea that blood pooling is occurring in the lower limbs. This effect is exacerbated by reduced skeletal muscle activity during sitting, which eliminates any contribution of the muscle pump in facilitating venous return (Delp & Laughlin, 1998). Other mechanisms that have been proposed to contribute to an increase in leg vascular resistance during an orthostatic stress, such as sitting, are venous distention-induced arterial vasoconstriction and increased hydrostatic pressure-induced myogenic responses (Kitano et al., 2005). Furthermore, because muscle sympathetic nerve activity is higher in the upright compared to supine posture (Ray et al., 1993), adrenergic vasoconstriction may also be contributing to the reduced leg blood flow observed in our study. This may also contribute to the reductions in forearm blood flow observed during sitting. Likewise, it is plausible that circulating vasoconstrictor factors may be involved in mediating increased lower leg and forearm vascular resistance with sitting. Additional studies directed at understanding the mechanisms involved in reducing blood flow and shear rate of the lower and upper limbs are needed.

Consistent with the premise that vascular exposure to low levels of shear stress can lead to impairments in vasomotor function, here we found that 6 hours of sitting and associated sustained reductions in blood flow and shear stress impaired microvascular dilator function in the leg and arm. Specifically, we found that lower leg and forearm reactive hyperemia were more than 40% and 30% blunted, respectively after the sitting period indicating a reduced ability of downstream resistance arteries to vasodilate. Interestingly, however, we found that prolonged sitting reduced popliteal, but not brachial, artery FMD. The lack of impairment in brachial artery FMD after sitting is perplexing when considering the premise that reduced shear stress may be the primary mechanism of vascular dysfunction associated with sitting. This finding suggests that either the brachial artery is more resilient to reductions in shear compared to the popliteal artery and/or that the reduction in shear in the brachial artery during sitting, which was less than that of the popliteal artery, was not sufficient to trigger a vascular impairment. The lesser reduction in shear in the arm compared to the leg could also be related to the fact that subjects were allowed some upper limb movement during sitting. Alternatively, reduced shear stress may not be the only mechanism by which sitting induces vascular dysfunction. In this regard, it is possible that the blunted vascular dilatory responses following sitting were the result of persistent constrictive forces acting on the vasculature even after resuming the semi-recumbent position, independent of reduced shear stress. Future research is needed to elucidate the precise mechanisms by which sitting blunts microvascular dilator function in lower and upper limbs, and macrovascular dilator function in a limb-specific manner.

Our finding that popliteal artery FMD was reduced with sitting should be considered in light of the fact that the hyperemic shear stimulus for FMD was also reduced. To determine whether the reduction in FMD following sitting was attributed to the reduced dilatory shear stimulus following cuff release, statistical correction of FMD for shear rate AUC was performed via ANCOVA, as previously recommended (Atkinson et al., 2009). After statistical adjustment for shear, popliteal artery FMD remained significantly reduced following the sitting period. Collectively, our findings suggest that prolonged sitting impairs vasodilator function in upstream conduit arteries of the lower limbs; an effect that was not present in the brachial artery.

Another important finding of the present study was that the leg vascular impairments caused by an extended period of sitting were alleviated following a relatively low intensity bout of activity in the form of a 10 min walk. This was quite remarkable given the modest level of activity imposed by the intervention. In line with the possibility that sustained low levels of shear stress associated with sitting contribute to the impairment in vascular function, it is conceivable that the favorable effects of walking on micro- and macrovascular function of lower limbs are related to the re-establishment of blood flow and shear stimulus within this vasculature. In addition, although the walking was of low intensity, it is plausible that the relative increase in metabolic activity compared to the reduced muscle activity during prolonged sitting could improve basal dilator tone within the leg vasculature. Importantly, it should be noted that impaired forearm microvascular dilator function was not improved with walking. These data suggest that the beneficial effects of increased activity were only conferred within the vasculature perfusing the active muscles. Indeed, relative to the leg, the forearm vasculature is certainly exposed to less of an increase in blood flow and shear during light walking. The finding that forearm microvascular reactivity was blunted with sitting but not reversed by walking is significant in view of existing data indicating that impaired forearm microvascular function is predictive of cardiovascular events in subjects with and without cardiovascular disease (Heitzer et al., 2001; Anderson et al., 2011).

The finding that the lower extremities exhibited increased vulnerability to vascular dysfunction with sitting should be considered in light of the body of evidence demonstrating that the vasculature of the lower limbs is more susceptible to atherosclerotic disease compared to the vasculature of the upper limbs (Ross et al., 1984; Stary et al., 1995; Kroger et al., 1999; Aboyans et al., 2011; Li et al., 2014). However, given that leg vascular dysfunction induced by prolonged sitting appears to be largely reversible with small amounts of activity, at this time it is unclear if reduced leg vascular function associated with sitting contributes to the increased propensity of the lower extremities to atherosclerotic disease. In this regard, although vascular dysfunction induced by sitting can be normalized by a bout of walking, a question that remains to be answered is whether one would be at greater risk for cardiovascular disease by spending a longer single period in a “dysfunctional” state versus oscillating between shorter periods of functional (while walking) and dysfunctional (while sitting) arterial function. Further research is warranted to determine the impact of daily prolonged sitting, with and without incorporation of walking, on micro- and macrovascular outcomes and explore whether such long-term vascular adaptations are systemic or limb-specific. In addition, since our study only included young healthy men, future research should determine whether sitting-induced changes in vascular function are influenced by sex or age.

In conclusion, the present study revealed, in healthy young men, that microvascular dilator function is impaired in the lower and upper limbs following a prolonged bout of uninterrupted sitting which is analogous to what many people experience in a typical workday. Because microvascular dysfunction was observed in the lower leg and the forearm, our data suggest a systemic effect of prolonged sitting. In contrast, we found that sitting impaired macrovascular dilator function in the lower leg only suggesting that sitting produces both systemic and local (i.e., limb-specific) vascular effects. Notably, we showed that sitting-associated vascular impairments in the lower limbs could be fully reversed after a short bout of walking; however, this reversibility in function does not appear to occur in the forearm microvasculature. Collectively, these findings provide evidence of a marked vulnerability of the vasculature to prolonged sitting and highlight the importance of physical activity in restoring normal leg vascular function.

New Findings.

What is the central question of this study?

The prevalence of sedentary behavior in the workplace and increased daily sitting time has been associated with the development of cardiovascular disease; however, studies investigating the impact of sitting on vascular function remain limited.

What is the main finding and its importance?

We demonstrate that there is a marked vulnerability of the vasculature in the lower and upper limbs to prolonged sitting and highlight the importance of physical activity in restoring vascular function in a limb specific manner.

Author Contributions

Task	RMR	SWH	DPC	PJF	JP
Conception and Design	X		X	X	X
Performed Experiments	X	X	X
Analyzed Data	X
Interpreted Results	X	X	X	X	X
Prepared Figures	X
Drafted Manuscript	X
Edited/revised Manuscript	X	X	X	X	X
Approved Manuscript	X	X	X	X	X