In the office, on transport, at home…Sitting down is ingrained in most peoples’ days. But, due in part to vascular dysfunction, staying sedentary for too long can increase the risk of serious health conditions like cardiovascular disease and type 2 diabetes.
In 1953, epidemiologist Jeremy Morris found that London bus drivers were more than twice as likely as bus conductors to develop coronary heart disease.
Demographically (in age, sex and income range) the two groups of workers were the same, so why was there such a significant difference?
Morris’ answer: bus conductors were required to be on their feet and regularly climb the steps of London’s iconic double-decker buses as they sold tickets to passengers, whereas the drivers remained seated for long stretches of time. His landmark study laid the groundwork for research on the links between physical activity and coronary health.
Whilst London bus conductors may now be a thing of the past, Morris’ results are more relevant than ever. Since the Covid-19 pandemic, there has been a huge shift towards working from home, which is likely to increase our collective sitting time. Without the trips to the water cooler and trot from meeting room to meeting room, it’s easy to spend hours sat behind a desk without getting up. (Office culture, however, had already changed our working lives so much by the 1980s that some researchers joked that our species had become Homo sedens – the “seated man” – rather than Homo sapiens.)
Prolonged sitting is a form of sedentary behaviour, characterised by a highly reduced energy expenditure in a seated or reclined position. Typical sedentary behaviours including television-watching, gaming, driving and desk-bound work. Sedentary behaviours are associated with a higher risk of cardiovascular disease, type 2 diabetes and premature mortality, and the time we spend sitting down in particular has been identified as an independent risk factor in a range of health conditions. In 2020, the World Health Organization suggested measures to reduce sedentary behaviour.
Since 2010, researchers have been keen to point out that sedentary behaviour is distinct from a lack of physical activity. You can get sufficient exercise per day, and still sit for too long. However, the risks of sedentary behaviour are heightened for those who do not exercise enough.
Why does sedentary behaviour increase risk of cardiovascular disease? The primary hypothesis is increased vascular dysfunction, particularly in the legs. The vascular system is responsible for keeping blood and lymph fluid – which forms part of the immune system – moving through the blood vessels.
David Dunstan, a physiologist at Deakin University’s Institute for Physical Activity and Nutrition, in Melbourne, Australia, has extensively researched the effects of prolonged sitting and possible interventions.
“What characterises sitting is a reduction in muscular activity,” says Dunstan. “If I’m on a chair, the chair is taking all the responsibility there.”
The combined effect of reduced muscular activity, lower metabolic demand and gravitational forces decreases peripheral blood flow to the leg muscles, which can lead to blood pooling in the calves. The biomechanics of sitting, with the legs usually bent, can also reduce blood flow.
Researchers suggest that 120-180 minutes of prolonged sitting is the threshold of too long spent seated
Reduced muscular activity of the leg muscles reduces their metabolic demand. Metabolic demand is the primary determinant of blood flow, so blood flow in the legs is also reduced. The biomechanics of sitting, with the legs usually bent can cause to blood pool in the calves – one study of 21 young healthy volunteers saw their calves increase in circumference by nearly 1cm (0.4in) over the course of two hours. This can also reduce blood flow.
Normal blood flow provides friction, known as arterial shear stress, against the endothelial cells which line the walls of blood vessels. The endothelium responds to this force and secretes vasodilators, like adenosine, prostacyclin and nitric oxide, which keep the vessels sufficiently dilated and maintain the ability of the vascular system to regulate itself – known as homeostasis.
Reduced blood flow, however, reduces shear stress, and the endothelium produces vasoconstrictors like endothelin-1 which cause the blood vessels to narrow. In a vicious cycle, vasoconstriction further reduces blood flow, and blood pressure rises to keep the blood moving. High blood pressure, or hypertension, is one of the predominant risk factors for cardiovascular disease.
“That’s [vascular dysfunction] one of the potential mechanisms,” says Dunstan. “But the truth is that we haven’t been able to pinpoint the exact mechanisms, and there’s likely to be multiple.
Whilst the underlying mechanisms are hypothetical, recent studies support the theory. A study with 16 young, healthy men found that sitting for three-hour periods increased blood pooling in the legs, peripheral vascular resistance, diastolic blood pressure and leg circumference. Another study finds that blood pressure increases with time spent sitting uninterrupted. Researchers generally agree that 120-180 minutes of uninterrupted sitting is probably the threshold at which you have probably spent too long in a seated position, but vascular dysfunction generally increases with time spent sitting.
Sitting for a prolonged period after a high-fat meal is thought to be particularly harmful.
The muscular skeletal system is also likely to be affected. Prolonged sitting contributes to reduced muscle strength, lower bone density and increased total and visceral fat in adipose tissue. Additionally, prolonged sitting is associated with physical discomfort, stress at work and higher depression, and can even lead to pressure ulcers.
Dunstan, who specialises in researching type 2 diabetes, also notes that sedentary behaviour increases post-meal, or post-prandial, rises in blood glucose and insulin. Impaired insulin sensitivity and impaired vascular function, both contribute to a higher risk of cardiovascular disease and type 2 diabetes.
People are becoming more sedentary because it’s what society has encouraged. As things get more efficient, we don’t have to move around so much – Benjamin Gardner
Given all these well-known potential consequences, why is it that we sit for so long – and can we break the habit?
“I think people are becoming more sedentary because it’s what society has encouraged,” says
Benjamin Gardner, a social psychologist specialising in habitual behaviour at the University of Surrey, who has been researching why people sit for so long. “It’s not that anyone’s deliberately pushing it. It’s just as things get more efficient, we don’t have to move around so much.”
In 2018, Gardner and colleagues found that encouraging standing in meetings presented unique social obstacles.
“We encouraged people to try this [standing up] in three different meetings, and we interviewed them after each one to find out how they got on, and the findings were fascinating,” says Gardner. “In a formal meeting, it was felt it was not appropriate to be standing.”
Other interventions include height-adjustable workstations, sit/stand chairs, treadmill workstations, and fidgeting the legs which enhances blood flow. (Learn more about why fidgeting is good for you.)
Just getting up every-so-often and going for a light walk or climbing some stairs has also been shown to be beneficial.
Wearable technology may also help nudge us into action. In a promising new study, wearable devices called accelerometers provided 24-hour data on individual behaviour patterns including sitting, standing, sleeping and exercising. As Dunstan pointed out, this potentially allows for tailored optimal sitting and standing times, with devices then sending automatic reminders whenever we sit for too long. However, the use of technology is not without its drawbacks, as some may become frustrated by or desensitised to its prompts.
Above all, Gardner and colleagues encourage moving between sitting and standing positions more frequently. The premise of breaking up sedentary time by just standing up is simple, but has significant health benefits, particularly for low-activity individuals. For wheelchair users or others with mobility constraints, specific, adapted exercises can be beneficial.
For many, sedentary behaviour can seem like an unavoidable consequence of modern life and work. But even small changes to your routine – be it stretching more, fidgeting or standing up to make a cup of tea – can help break your sitting habit.
