Endothelial dysfunction following prolonged sitting is mediated by a reduction in shear stress

Brief periods of inactivity reduce leg microvascular, but not macrovascular, function in healthy young men

NEW FINDINGS

What is the central question of this study? We aimed to examine leg vascular responses to brief periods of inactivity. What is the main finding and its importance? We demonstrate that a mere 10 min of sitting is sufficient to impair leg microvascular function (reactive hyperaemia). However, conduit artery vasodilatation (flow-mediated dilatation) was unaffected, indicating maintained macrovascular function. Interestingly, immobile supine rest also resulted in a reduction in microvascular function alone that was prevented when calf muscle contractions were performed. Collectively, these data highlight the susceptibility of the microcirculation to short periods of inactivity and the beneficial role of skeletal muscle contraction for vascular health.

ABSTRACT

Prolonged sitting for 1-6 h has been shown to impair leg macrovascular [i.e. reduced flow-mediated dilatation (FMD)] and microvascular (i.e. reduced reactive hyperaemia) function. These impairments appear to be mediated through reductions in shear stress. Interestingly, a reduction in shear rate has been observed as early as 10 min into sitting. However, it is unknown whether this acute reduction in shear stress is sufficient to affect vascular function. Accordingly, we studied 18 young men and assessed popliteal artery FMD and reactive hyperaemia before (Baseline) and after (PostSit) a 10 min sitting period. Popliteal artery shear rate was significantly reduced during sitting (Baseline, 62 ± 35 s^-1 ; 10 min sitting, 27 ± 13 s^-1 ; P < 0.001). Macrovascular function was unaffected by 10 min of sitting (Baseline, 4.4 ± 2.1%; PostSit, 4.3 ± 2.3%; P = 0.97), but microvascular function was reduced (Baseline, 4852 ± 2261 a.u.; PostSit, 3522 ± 1872 a.u.; P = 0.02). In a subset of individuals, we extended the recovery period after sitting and demonstrated that resting shear rate and reactive hyperaemia responses remained low up to 1 h post-sitting (P < 0.001), whereas FMD was unchanged throughout (P = 0.99). Additionally, time control experiments were performed with participants in an immobile supine position, which demonstrated no change in macrovascular function (P = 0.94) but, unexpectedly, a reduction in microvascular function (P = 0.008). Importantly, when calf muscle contractions were performed during supine rest, reactive hyperaemia responses were maintained (P = 0.76), along with FMD (P = 0.88). These findings suggest that the leg microcirculation might be more vulnerable to short periods of inactivity, whereas conduit artery vasodilatation appears well maintained. Moreover, intermittent skeletal muscle contractions are beneficial for microvascular function.

Subjective and objective assessment of sedentary behavior among college employees

BACKGROUND

High levels of sedentary behavior are linked to increased mortality. In the United States, individuals spend 55-70% of their waking day being sedentary. Since most individuals spend large portions of their daily lives at work, quantifying the time engaged in sedentary behavior at work is emerging as an important health determinant. Studies profiling academic institutions, where a variety of personnel with diverse job descriptions are employed, are limited. Available studies focus mostly on subjective methods, with few using objective approaches. Therefore, the purpose of the current study was to assess sedentary behavior among all occupational groups of a college in the Northeastern United States utilizing both a subjective and an objective method.

METHODS

College employees (n = 367) completed the Occupational Sitting and Physical Activity Questionnaire (OSPAQ). A sub-sample of these employees (n = 127) subsequently wore an activPAL3 accelerometer 24 h per day for seven consecutive days. Outcome variables were time spent sitting, standing, stepping, and total number of steps. To assess fragmentation of sedentary behavior, the average duration of a sitting bout and sitting bouts/sitting hour were calculated. Differences between administrators, faculty, and staff, were analyzed using multivariate and univariate analyses of variance.

RESULTS

The OSPAQ results indicated that administrators spent more of their working day sedentary (73.2 ± 17.7%) than faculty members (58.5 ± 19.6%, p < 0.05). For the objective phase of the study, complete data were analyzed from 86 participants. During a waking day, administrators (64.0 ± 8.1%) were more sedentary than faculty (56.0 ± 7.9%, p < 0.05) and fragmented their sitting less than staff (3.7 ± 0.7 and 4.5 ± 7.9 bouts of sitting/sitting hour, respectively; p < 0.05). This pattern was also seen during working hours, with administrators (4.9 ± 2.1) taking fewer breaks per hour than staff (6.9 ± 3.0, p < 0.05).

CONCLUSIONS

Administrators are the most sedentary members of the campus community. However, overall, the level of sedentary behavior among employees was high. This study highlights the need for sedentary behavior interventions in the college/university environment.

Prolonged leg bending impairs endothelial function in the popliteal artery

Uninterrupted sitting blunts vascular endothelial function in the lower extremities; however, the factors contributing to this impairment remain largely unknown. Herein, we tested the hypothesis that prolonged flexion of the hip and knee joints, as it occurs during sitting, and associated low shear stress and disturbed (i.e., turbulent) blood flow caused by arterial bending, impairs endothelial function at the popliteal artery. Bilateral measurements of popliteal artery flow‐mediated dilation (FMD) were performed in 12 healthy subjects before and after a 3‐h lying‐down period during which one leg was bent (i.e., 90‐degree angles at the hip and knee) and the contralateral leg remained straight, serving as internal control. During the 3‐h lying down period, the bent leg displayed a profound and sustained reduction in popliteal artery blood flow and mean shear rate; whereas a slight but steady decline that only became significant at 3 h was noted in the straight leg. Notably, 3 h of lying down markedly impaired popliteal artery FMD in the bent leg (pre: 6.3 ± 1.2% vs. post: 2.8 ± 0.91%; P < 0.01) but not in the straight leg (pre: 5.6 ± 1.1% vs. post: 7.1 ± 1.2%; P = 0.24). Collectively, this study provides evidence that prolonged bending of the leg causes endothelial dysfunction in the popliteal artery. This effect is likely secondary to vascular exposure to low and disturbed blood flow resulting from arterial angulation. We conclude that spending excessive time with legs bent and immobile, irrespective of whether this is in the setting of sitting or lying‐down, may be disadvantageous for leg vascular health.

Reducing sitting time versus adding exercise: differential effects on biomarkers of endothelial dysfunction and metabolic risk

Recent studies suggest that substituting sitting with light physical activity has beneficial metabolic effects, but it is unclear if this is associated with parallel changes in endothelial function. Data from three randomized cross-over studies were analyzed, in which 61 subjects (with normal weight, overweight and type 2 diabetes) followed different activity regimens (Sit, SitLess and/or Exercise) of four days each. Subjects were instructed to sit 14 h/day (‘Sit’), to substitute 1 h/day of sitting with moderate-to-vigorous cycling (‘Exercise’) or to substitute 5–6 h/day sitting with light-intensity walking and standing (‘SitLess’). Physical activity was assessed 24 h/day by accelerometry (ActivPAL) and diet was standardized. Fasted circulating biomarkers of endothelial dysfunction, lipids and insulin sensitivity were assessed the morning after each activity regimen. The endothelial dysfunction score (ED-score) was computed by averaging the Z-scores of the circulating biomarkers of endothelial dysfunction. Compared to Sit, Exercise resulted in lower ED-score, sICAM1 and sE-selectin (p < 0.05), while no significant changes were observed after SitLess. The ED-score, sVCAM1 and sE-selectin were lower after Exercise compared to SitLess (p < 0.05). In contrast, compared to Sit, insulin sensitivity (HOMA2-IR) and plasma lipids (HDL-cholesterol, non-HDL-cholesterol, total cholesterol and Apo B) did not change significantly after Exercise but were improved after SitLess (p < 0.05). In conclusion, light physical activity and moderate-to-vigorous physical activity had a differential effect on risk markers of cardio-metabolic health and suggest the need of both performing structured exercise as well as reducing sitting time on a daily basis.

Effects of heat and cold on health, with special reference to Finnish sauna bathing

Environmental stress such as extremely warm or cold temperature is often considered a challenge to human health and body homeostasis. However, the human body can adapt relatively well to heat and cold environments, and recent studies have also elucidated that particularly heat stress might be even highly beneficial for human health. Consequently, the aim of the present brief review is first to discuss general cardiovascular and other responses to acute heat stress, followed by a review of beneficial effects of Finnish sauna bathing on general and cardiovascular health and mortality as well as dementia and Alzheimer's disease risk. Plausible mechanisms included are improved endothelial and microvascular function, reduced blood pressure and arterial stiffness, and possibly increased angiogenesis in humans, which are likely to mediate the health benefits of sauna bathing. In addition to heat exposure with physiological adaptations, cold stress-induced physiological responses and brown fat activation on health are also discussed. This is important to take into consideration, as sauna bathing is frequently associated with cooling periods in cold(er) environments, but their combination remains poorly investigated. We finally propose, therefore, that possible additive effects of heat- and cold-stress-induced adaptations and effects on health would be worthy of further investigation.

Microvascular Vasodilator Plasticity After Acute Exercise

Endothelium-dependent vasodilation is reduced after acute exercise or after high intraluminal pressure in isolated arterioles from sedentary adults but not in arterioles from regular exercisers. The preserved vasodilation in arterioles from exercisers is hydrogen peroxide (H2O2) dependent, whereas resting dilation is nitric oxide (NO) dependent. We hypothesize chronic exercise elicits adaptations allowing for maintained vasodilation when NO bioavailability is reduced.

NK cell recruitment and exercise: Potential immunotherapeutic role of shear stress and endothelial health

Positive cancer patient outcomes, including increased time to recurrent events, have been associated with increased counts and function of natural killer (NK) cells. NK cell counts and function are elevated following acute exercise, and the generally accepted mechanism of increased recruitment suggests that binding of epinephrine releases NK cells from endothelial tissue via decreases in adhesion molecules following. I propose that blood flow-induced shear stress may also play a role in NK cell recruitment from the endothelium. Additionally, shear stress may play a role in improving NK cell function by decreasing oxidative stress. The relationship between shear stress and NK cell count and function can be tested by utilizing exercise and local heating with cuff inflation. If shear stress does play an important role, NK cell count and function will be improved in the non-cuffed exercise group, but not the cuffed limb. This paper will explore the mechanisms potentially explaining exercise-induced improvements in NK cell count and function, and propose a model for investigating these mechanisms. This mechanistic insight could aid in providing a novel, safe, relatively inexpensive, and non-invasive target for immunotherapy in cancer patients.

Effects of prior aerobic exercise on sitting-induced vascular dysfunction in healthy men

INTRODUCTION

Acute aerobic exercise prevents sitting-induced impairment of flow-mediated dilation (FMD). Further, evidence suggests that sitting-induced impairment of FMD occurs via an oxidative stress-dependent mechanism that disrupts endothelial function.

PURPOSE

We hypothesized that acute aerobic exercise would prevent impairment of femoral artery FMD by limiting oxidative stress responses that increase endothelin-1 (ET-1) levels and disrupt nitric oxide (NO) status.

METHODS

In a randomized, cross-over study, healthy men (n = 11; 21.2 ± 1.9 years) completed two 3 h sitting trials that were preceded by 45 min of either quiet rest (REST) or a single bout of continuous treadmill exercise (65% maximal oxygen consumption) (EX). Superficial femoral artery FMD, plasma glucose, malondialdehyde (MDA), ET-1, arginine (ARG) and its related metabolites [homoarginine (HA), asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA)] were assessed at baseline, 1 h following EX (or REST) (0 h), and at 1 h intervals during 3 h of uninterrupted sitting. Data were analyzed using repeated measures ANOVA.

RESULTS

During REST, femoral artery FMD declined from baseline (2.6 ± 1.8%) at 1, 2, and 3 h of sitting and resting shear rate decreased at 3 h. In contrast, when sitting was preceded by EX, femoral artery FMD (2.7 ± 2.0%) and resting shear rate responses were unaffected. No between trial differences were detected for plasma glucose, MDA, ET-1, ARG, HA, ADMA, or SDMA.

CONCLUSION

Prior aerobic exercise prevented the decline in femoral artery FMD that is otherwise induced by prolonged sitting independent of changes in oxidative stress, ET-1, and NO status.

Sedentary Behavior and Cardiovascular Disease Risk: Mediating Mechanisms

Sedentary behavior has a strong association with cardiovascular disease (CVD) risk, which may be independent of physical activity. To date, the mechanism(s) that mediate this relationship are poorly understood. We hypothesize that sedentary behavior modifies key hemodynamic, inflammatory, and metabolic processes resulting in impaired arterial health. Subsequently, these vascular impairments directly and indirectly contribute to the development of CVD.

Targeting Reductions in Sitting Time to Increase Physical Activity and Improve Health

: New evidence suggests that reductions in sedentary behavior may increase physical activity and improve health. These findings point to new behavioral targets for intervention and new ways to think about intervening to increase overall physical activity in the population. This report provides a knowledge update reflecting the rapid accumulation of new evidence related to sedentary behavior and health among adults. Recent observational studies suggest that leveraging the time-inverse relationship between sedentary and active behaviors by replacing sitting with standing, light- or moderate-intensity activity can have important health benefits, particularly among less active adults. Clinical studies are providing evidence of the probable physiologic mechanisms underlying these associations, as well as insights into the cardiometabolic impact of breaking up and reducing sedentary behavior. In contrast to the well-established behavioral theories that guide the development and dissemination of evidence-based interventions to increase moderate- to vigorous-intensity physical activity, much less is known about how to reduce sedentary time to increase daily activities. It has become clear that the environmental, social, and individual level determinants for sedentary time are distinct from those linked to the adoption and maintenance of moderate- to vigorous-intensity physical activity. As a result, novel intervention strategies that focus on sitting and lower-intensity activities by leveraging the surrounding environment (e.g., workplace, school, and home) as well as individual-level cues and habits of sedentary behavior are being tested to increase the potency of interventions designed to increase overall physical activity. Herein we summarize the solutions-oriented research across the behavioral research framework, with a focus on highlighting areas of synergy across disciplines and identifying gaps for future research.

Prolonged sitting leg vasculopathy: contributing factors and clinical implications

Atherosclerotic peripheral artery disease primarily manifests in the medium- to large-sized conduit arteries of the lower extremities. However, the factors underlying this increased vulnerability of leg macrovasculature to disease are largely unidentified. On the basis of recent studies, we propose that excessive time spent in the sitting position and the ensuing reduction in leg blood flow-induced shear stress cause endothelial cell dysfunction, a key predisposing factor to peripheral artery disease. In particular, this review summarizes the findings from laboratory-based sitting studies revealing acute leg vascular dysfunction with prolonged sitting in young healthy subjects, discusses the primary physiological mechanisms and the potential long-term implications of such leg vasculopathy with repeated exposure to prolonged sitting, as well as identifies strategies that may be effective at evading it.

Substantial Inter-Subject Variability in Blood Pressure Responses to Glucose in a Healthy, Non-obese Population

Aim: A large inter-subject variability in the blood pressure (BP) response to glucose drinks has been reported. However, the underlying factors remain elusive and we hypothesized that accompanying changes in glucose metabolism affect these BP responses.

Methods: Cardiovascular and glycemic changes in response to a standard 75 g oral-glucose-tolerance-test were investigated in 30 healthy, non-obese males. Continuous cardiovascular monitoring, including beat-to-beat BP, electrocardiographically deduced heart rate and impedance cardiography, was performed during a 30 min baseline and continued up to 120 min after glucose ingestion. Blood samples were taken at baseline, 30, 60, 90, and 120 min for the assessment of glucose, insulin and c-peptide. Additionally, we evaluated body composition by using validated bioelectrical impedance techniques.

Results: Individual overall changes (i.e., averages over 120 min) for systolic BP ranged from −4.9 to +4.7 mmHg, where increases and decreases were equally distributed (50%). Peak changes (i.e., peak averages over 10 min intervals) for systolic BP ranged from −1.3 to +9.5 mmHg, where 93% of subjects increased systolic BP above baseline values (similar for diastolic BP) whilst 63% of subjects increased peak systolic BP by more than 4 mmHg. Changes in peak systolic BP were negatively associated with the calculated Matsuda-index of insulin sensitivity (r = −0.39, p = 0.04) but with no other evaluated parameter including body composition. Moreover, besides a trend toward an association between overall changes in systolic BP and total fat mass percentage (r = +0.32, p = 0.09), no association was found between other body composition parameters and overall BP changes.

Conclusion: Substantial inter-subject variability in BP changes was observed in a healthy, non-obese subpopulation in response to an oral glucose load. In 63% of subjects, peak systolic BP increased by more than a clinically relevant 4 mmHg. Peak systolic BP changes, but not overall BP changes, correlated with insulin sensitivity, with little influence of body composition.

Prior exercise and standing as strategies to circumvent sitting-induced leg endothelial dysfunction

We have previously shown that local heating or leg fidgeting can prevent prolonged sitting-induced leg endothelial dysfunction. However, whether physical activity prevents subsequent sitting-induced leg endothelial dysfunction remains unknown. Herein, we tested the hypothesis that sitting-induced leg endothelial dysfunction would be prevented by prior exercise. We also examined if, in the absence of exercise, standing is an effective alternative strategy to sitting for conserving leg endothelial function. Fifteen young healthy subjects completed three randomized experimental trials: (1) sitting without prior exercise; (2) sitting with prior exercise; and (3) standing without prior exercise. Following baseline popliteal artery flow-mediated dilation (FMD) measurements, subjects maintained a supine position for 45 min in the sitting and standing trials, without prior exercise, or performed 45 min of leg cycling before sitting (i.e. sitting with prior exercise trial). Thereafter, subjects were positioned into a seated or standing position, according to the trial, for 3 h. Popliteal artery FMD measures were then repeated. Three hours of sitting without prior exercise caused a significant impairment in popliteal artery FMD (baseline: 3.8±0.5%, post-sitting: 1.5±0.5%, P<0.05), which was prevented when sitting was preceded by a bout of cycling exercise (baseline: 3.8±0.5%, post-sitting: 3.6±0.7%, P>0.05). Three hours of standing did not significantly alter popliteal artery FMD (baseline: 4.1±0.4%, post-standing: 4.3±0.4%, P>0.05). In conclusion, prolonged sitting-induced leg endothelial dysfunction can be prevented by prior aerobic exercise. In addition, in the absence of exercise, standing represents an effective substitute to sitting for preserving leg conduit artery endothelial function.

Something is definitely better than nothing: simple strategies to prevent vascular dysfunction

Understanding the negative health consequences of a physical inactivity has been the topic of much investigation as an alarming number of adults have adopted a sedentary lifestyle. With the rise in sedentarism the field of inactivity physiology has emerged. The goal of inactivity physiology is to identify the impact in inactivity on health and develop strategies that effectively minimize the risk of a sedentary lifestyle. Arising from this field is the finding that excessive sitting is linked to increased cardiovascular and metabolic disease and all-cause mortality. Most importantly, these relationships exist even in individuals that are physically active. Clearly, excessive sitting is an occupational hazard with significant health consequences. Through a series of investigations, including research published this issue of Clinical Science, Padilla and colleagues have identified that prolonged sitting evokes vascular dysfunction and that this dysfunction is caused by reduced shear stress. This commentary highlights this series of investigations and culminates with an overview of how prior exercise and standing are effective strategies to circumvent vascular dysfunction that is caused by excessive sitting.

Impaired popliteal artery flow-mediated dilation caused by reduced daily physical activity is prevented by increased shear stress

We recently showed that 5 days of reduced daily physical activity impair popliteal artery, but not brachial artery, flow-mediated dilation (FMD). However, the mechanisms by which physical inactivity causes leg vascular dysfunction are unclear. We reason that a reduction in leg blood flow-induced shear stress is a primary underlying mechanism by which reduced daily physical activity impairs popliteal artery FMD. Thus the purpose of this study was to determine whether increased leg blood flow and shear stress during inactivity prevent the reduction in popliteal artery FMD. Bilateral popliteal artery FMD measures were performed at baseline and after 5 days of a transition from high (>10,000 steps/day) to low levels (<5,000 steps/day) of physical activity in 13 healthy and physically active men [20 ± 2 (SD) yr]. During the inactive period, one foot was submerged in ~42°C water (i.e., heated leg) three times a day for 30 min each period, to increase blood flow and thus shear stress, whereas the contralateral leg remained dry and served as internal control (i.e., nonheated leg). During heating, popliteal artery mean shear rate was increased in the heated leg (change of 119.3 ± 26.4%, P < 0.01) but slightly decreased in the nonheated leg (change of -21.8 ± 7.5%, P = 0.03). Popliteal artery FMD was impaired after 5 days of reduced daily physical activity in the control nonheated leg (P < 0.01) but was unchanged in the heated leg (P = 0.34). These results support the hypothesis that reduced leg blood flow-induced shear stress during physical inactivity is a key underlying mechanism mediating leg vascular dysfunction.NEW & NOTEWORTHY We found that the impairment in popliteal artery flow-mediated dilation caused by physical inactivity can be prevented by increased shear stress. These findings indicate that reduced leg blood flow-induced shear stress during physical inactivity may be a key underlying mechanism mediating the detrimental leg vascular effects of physical inactivity. Heating the foot area may be used as a nonpharmacological therapy to combat inactivity-induced leg vascular dysfunction, especially in people who are unable or unwilling to be active.

Influence of sex on microvascular and macrovascular responses to prolonged sitting

Increased daily sitting time is associated with greater cardiovascular risk, and, on average, women are more sedentary than men. Recent reports have demonstrated that prolonged sitting reduces lower leg microvascular (reactive hyperemia) and macrovascular [flow-mediated dilation (FMD)] vasodilator function. However, these studies have predominately included men, and the effects of sitting in young women are largely unexplored. This becomes important given known sex differences in vascular function. Thus, herein, we assessed popliteal artery reactive hyperemia and FMD before and after a 3-h sitting period in healthy young women (n = 12) and men (n = 8). In addition, resting popliteal artery hemodynamics (duplex Doppler ultrasound) and calf circumference were measured before, during, and after sitting. Resting popliteal artery shear rate was reduced to a similar extent in both groups during the sitting period (women: -48.5 ± 8.4 s^-1 and men: -52.9 ± 12.3 s^-1, P = 0.45). This was accompanied by comparable increases in calf circumference in men and women (P = 0.37). After the sitting period, popliteal artery FMD was significantly reduced in men (PreSit: 5.5 ± 0.9% and PostSit: 1.6 ± 0.4%, P < 0.001) but not women (PreSit: 4.4 ± 0.6% and PostSit: 3.6 ± 0.6%, P = 0.29). In contrast, both groups demonstrated similar reductions in hyperemic blood flow area under the curve (women: -28,860 ± 5,742 arbitrary units and men: -28,691 ± 9,685 arbitrary units, P = 0.99), indicating impaired microvascular reactivity after sitting. These findings indicate that despite comparable reductions in shear rate during 3 h of uninterrupted sitting, macrovascular function appears protected in some young women but the response was variable, whereas men exhibited more consistent reductions in FMD. In contrast, the leg microvasculature is susceptible to similar sitting-induced impairments in men and women.NEW & NOTEWORTHY We demonstrate that leg macrovascular function was consistently reduced in young men but not young women after prolonged sitting. In contrast, both men and women exhibited similar reductions in leg microvascular reactivity after sitting. These data demonstrate, for the first time, sex differences in vascular responses to prolonged sitting.

Intermittent hypoxia and arterial blood pressure control in humans: role of the peripheral vasculature and carotid baroreflex

Intermittent hypoxia (IH) occurs in association with obstructive sleep apnea and likely contributes to the pathogenesis of hypertension. The purpose of this study was to examine the putative early adaptations at the level of the peripheral vasculature and carotid baroreflex (CBR) that may promote the development of hypertension. Ten healthy male participants (26 ± 1 yr, BMI = 24 ± 1 kg/m(2)) were exposed to 6 h of IH (1-min cycles of normoxia and hypoxia) and SHAM in a single-blinded, counterbalanced crossover study design. Ambulatory blood pressure was measured during each condition and the following night. Vascular strain of the carotid and femoral artery, a measure of localized arterial stiffness, and hemodynamic shear patterns in the brachial and femoral arteries were measured during each condition. Brachial artery reactive hyperemia flow-mediated vasodilation was assessed before and after each condition as a measure of endothelial function. CBR function and its control over leg vascular conductance (LVC) were measured after each condition with a variable-pressure neck chamber. Intermittent hypoxia 1) increased nighttime pulse pressure by 3.2 ± 1.3 mmHg, 2) altered femoral but not brachial artery hemodynamics, 3) did not affect brachial artery endothelial function, 4) reduced vascular strain in the carotid and possibly femoral artery, and 5) shifted CBR mean arterial pressure (MAP) to higher MAP while blunting LVC responses to CBR loading. These results suggest limb-specific vascular impairments, reduced vascular strain, and CBR resetting combined with blunted LVC responses are factors in the early pathogenesis of IH-induced development of hypertension.

Prolonged sitting-induced leg endothelial dysfunction is prevented by fidgeting

Prolonged sitting impairs endothelial function in the leg vasculature, and this impairment is thought to be largely mediated by a sustained reduction in blood flow-induced shear stress. Indeed, preventing the marked reduction of shear stress during sitting with local heating abolishes the impairment in popliteal artery endothelial function. Herein, we tested the hypothesis that sitting-induced reductions in shear stress and ensuing endothelial dysfunction would be prevented by periodic leg movement, or "fidgeting." In 11 young, healthy subjects, bilateral measurements of popliteal artery flow-mediated dilation (FMD) were performed before and after a 3-h sitting period during which one leg was subjected to intermittent fidgeting (1 min on/4 min off) while the contralateral leg remained still throughout and served as an internal control. Fidgeting produced a pronounced increase in popliteal artery blood flow and shear rate (prefidgeting, 33.7 ± 2.6 s(-1) to immediately postfidgeting, 222.7 ± 28.3 s(-1); mean ± SE; P < 0.001) that tapered off during the following 60 s. Fidgeting did not alter popliteal artery blood flow and shear rate of the contralateral leg, which was subjected to a reduction in blood flow and shear rate throughout the sitting period (presit, 71.7 ± 8.0 s(-1) to 3-h sit, 20.2 ± 2.9 s(-1); P < 0.001). Popliteal artery FMD was impaired after 3 h of sitting in the control leg (presit, 4.5 ± 0.3% to postsit: 1.6 ± 1.1%; P = 0.039) but improved in the fidgeting leg (presit, 3.7 ± 0.6% to postsit, 6.6 ± 1.2%; P = 0.014). Collectively, the present study provides evidence that prolonged sitting-induced leg endothelial dysfunction is preventable with small amounts of leg movement while sitting, likely through the intermittent increases in vascular shear stress.