There are a bunch of different theories about what makes prolonged sitting so bad, Alex Hutchinson notes, but one of them relates to the associated reduction in blood flow in your legs:
Your blood vessels sense the frictional drag of blood rushing past the vessel walls, and respond by producing molecules such as nitric oxide that help keep the vessels supple and responsive. If you spend too much time sitting, this signal is reduced, and you end up with blood vessels that are stiffer and less capable of dilating and contracting in response to changes in blood flow. Over time, that leaves you more likely to develop atherosclerosis, a hardening and narrowing of the arteries, and ultimately heart disease.
You can test how responsive your blood vessels are with a technique called flow-mediated dilation. Basically, you temporarily restrict blood flow with an inflatable cuff like the ones doctors use to measure your blood pressure, then release the cuff and see how much the vessels dilate in response. If you take this measurement before and after a three-hour bout of sitting, you find that the amount of dilation is dramatically reduced after sitting—a bad sign for the health of your arteries.
That’s the protocol used in the new study, which compared 10 male cyclists from the university’s racing team with matched controls who didn’t do any regular endurance training. The graph below shows the percentage increase in blood flow through the lower leg’s popliteal artery when the cuff is released. On the left, you can see that even before sitting, the trained cyclists (black) have a somewhat bigger response than the control group (white), which is expected since endurance training enhances baseline levels of nitric oxide. But the starkest difference, on the right, emerges after three hours of sitting.
The bout of sitting almost wipes out the flow-mediated dilation response in the control group, but it barely changes in the cyclists. Hooray! I can leave my desk in the sitting position for another hour!