Your leg muscles are getting lazier as you sit and read this sentence.
In fact, according to one of the first studies to precisely measure leg muscle activity during movement and at rest, our legs become almost 100 percent lazier if we sit for too long.
The study used specially designed workout shorts embedded with sensors that can read electrical activity in people’s leg muscles. Electrical activity signals muscular contractions.
Minute by minute, the sensors tracked how rapidly people’s leg muscles became inert when they sat.
Laziness set in quickly, the high-tech shorts showed. Within nine minutes of sitting still, people’s leg-muscle activity had typically declined by about 90 percent, compared to walking. If they stayed seated, it slowed even more.
“We documented that, on average, human physiology gets more muscle-lazy, the longer we sit,” said Christian Brakenridge, a postdoctoral research fellow at the Swinburne University of Technology in Melbourne, Australia, who led the study.
The health consequences of this muscular sloth can be worrying, including rising blood sugar, worsening vascular function and, over time, increased risks for other metabolic problems, including heart disease and some cancers, even among people who consider themselves active and fit.
The good news is that the study also found that movement — in the right amounts and at the right times — staves off muscle laziness and might make sitting a little less unhealthy.
Why sitting for too long can be bad for your health
“Prolonged sitting is ubiquitous,” the authors of the new study wrote, pointing out that almost all of us spend far too much time plopped in a desk chair or on the couch. By most estimates, American adults sit for at least eight and often 11 or 12 hours a day, rising and moving around only sparingly.
This isn’t good for us. When our muscles contract, especially our leg muscles, the body’s largest, they draw in blood sugar from our bloodstreams for fuel, stabilizing blood sugar levels and helping to maintain healthy insulin responses. The contractions also squeeze deep veins and other blood vessels, improving blood flow and keeping vessels healthy.
When we sit and our muscles grow limp, none of that happens.
Unsurprisingly, many past epidemiological studies have linked prolonged sitting with multiple health problems.
But those studies assumed muscles stop contracting when we sit. They didn’t directly measure contractions or establish how rapidly and completely muscles slacken while we sit. They also couldn’t precisely determine whether and when moving around might alter the process.
So, for the new study, the scientists turned to what are, in effect, contraction-tracking underwear.
Special shorts that can ‘read’ muscles
More technically, the researchers turned to “textile electromyography,” in the form of thigh-length tights, like bicycle shorts. Threaded throughout with sensitive electrodes, the shorts can pick up even slight electrical activity in the muscle cells of the quadriceps, hamstrings and glutes.
The researchers asked 84 men and women of varying ages and lifestyles to wear the shorts under their normal clothing all day for at least three days, while continuing with their everyday lives. The scientists also gave the volunteers activity trackers, as a separate measure of movement.
The volunteers went to work or lounged in front of their TVs or did whatever else they’d usually do, while the shorts kept constant track of their leg muscles’ activity.
Then the scientists analyzed the readouts, including from the activity trackers, to determine when people had been sitting, standing, walking or otherwise moving around and what had been going on inside their leg muscles at the time.
How fast do muscles get ‘lazy’?
The researchers found, as expected, that muscular activity fell rapidly during sitting. After nine minutes of sitting still, leg-muscle activity was about 10 percent of what it was when people were up and moving. Four minutes later, it was at 5 percent, and within 25 minutes, muscular activity was barely detectable, falling below 1 percent of the levels seen when people got up and moved.
But those numbers changed when people did get up and move, especially if they didn’t just stand up but walked around briskly for more than three minutes at a time, at least every 25 minutes.
In that case, their leg muscles maintained lingering activity for quite a bit longer the next time they sat.
Over a full day, the researchers calculated, breaking up prolonged sitting every 25 minutes with a few minutes of sustained walking could increase leg muscle contractions during subsequent sitting by about 1.5 hours, reducing leg muscles’ total daily laziness by about 10 percent.
Get up every 25 minutes and move around
“This study provides some really important motivation to break up prolonged sitting with standing or movement breaks,” said David Raichlen, a professor of human and evolutionary biology at the University of Southern California in Los Angeles. He’s studied activity and inactivity patterns among hunter gatherers and urban Americans but wasn’t involved with the new study.
“In fact,” he continued, “moving around before sitting was linked with higher muscle activity when sedentary, giving some extra incentive to get up off the chair or couch.”
The study has limits. It was relatively small and short-term, and the researchers didn’t check people’s blood sugar or other measures of health to see how movement breaks influenced them. It focused only on muscles.
But even with those caveats, the study’s takeaway seems simple and feasible. “The more activity we can do, the better,” Brakenridge said. “But that’s not always possible. So we should instead probably limit the worst, most muscularly inactive sitting we do,” by getting up at least every 25 minutes and moving around.
For his own part, Brakenridge has installed “sit-to-stand desks at both of my work settings,” he said, but sometimes becomes too engrossed in work to readjust them.
“Thanks,” he said, “for the reminder to stand up.”
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