The surest way to stay safe from COVID-19 is straightforward: just stay home. That carries a price, however, in less active lives. A medical team from the University of California, San Francisco recently looked at step count measurements from mobile phones around the world and found that, in the month after the pandemic began, physical activity dropped precipitously—as much as 50% in places that declared lockdowns and social distancing.

The dangers of a sedentary life have become much clearer over the past few decades. A study from the U.S. Department of Health and Human Services laid out the stark evidence in a 2018 report: Being inactive for long periods can lead to a higher risk for cardiovascular disease, type 2 diabetes and endometrial, colon and lung cancers, as well as death from heart disease and other causes. But many questions remain about how much inactivity—increasingly a fact of life during lockdowns and work-from-home arrangements—triggers the worst effects.

“Considering the evidence that we have, 9 to 10 hours of sitting a day is likely to be too much for most people. Twelve hours is definitely not a good idea for any of us,” says Neville Owen, a public health researcher and head of the Behavioural Epidemiology Laboratory at the Baker Heart and Diabetes Institute in Melbourne, Australia. “Where a lot more work needs to be done is to quantify exactly where the sitting-risk curve starts to bend upwards for different groups—especially older folks and those living with type 2 diabetes,” he says.

Owen’s team is studying those thresholds as well as how people can fight the worst effects of a life spent sitting. According to Owen, meeting current physical activity recommendations alone won’t be enough, and those who mostly sit will likely need to significantly exceed current exercise recommendations to counteract the negative effects.

One study in support of that idea followed 149,077 people over nearly nine years. Even those who met the recommended physical activity benchmarks of 150 to 300 minutes of weekly exercise, including brisk walking, but reported very high amounts of sitting time (greater or equal to eight hours per day) had an increased risk of death. For those who got more than 300 minutes of moderate or vigorous physical activity, however, that risk disappeared.

The study’s lead author, Emmanuel Stamatakis, professor of Physical Activity Lifestyle and Population Health at the Charles Perkins Centre of the University of Sydney, notes that mapping this correlation—how much exercise can work as an antidote to long periods of sitting—will be a critical addition to national and global guidelines on physical activity. “What is clear is that the effects of sitting are conditional on the intensity of and how much physical activity people do,” he says.

One possible recommendation: physical activity “sprints.” The team of Edward Coyle, director of the Human Performance Laboratory at the University of Texas at Austin, has been looking at short bursts of activity as a way to offset the harms of very sedentary lives. They’ve recently found that doing five four-second sprints on an exercise bike every hour over the course of eight hours can significantly affect some of the physical benchmarks associated with inactivity. The “sprints” lowered triglyceride levels by a third and increased fat burning by almost half the next day compared with those same measurements taken a day after spending eight hours sitting. “Get up every hour and move vigorously. This might offset as much sitting as you want,” Coyle says.

A similar study from the Baker Institute in adults with type 2 diabetes found that interrupting long periods of sitting with three-minute bouts of either low-intensity walking or simple resistance exercises every half-hour were linked to other improved metabolic markers—such as glucose, insulin and triglyceride levels—in the short term. Evidence is now needed on whether these benefits will be long-term.

Another unanswered question is whether a fixed number of daily steps might offset long sedentary stretches. Coyle’s group has recently finished a study, not yet published, that seems to indicate a threshold for better health outcomes between 5,000 and 8,500 steps per day. While one measure of activity won’t address every harm, “counting steps isn’t a bad place to start,” Coyle says, “because it’s the most accessible activity.” 

In June, an international team of authors proposed an ambitious research agenda for studying physical inactivity during COVID-19. They would measure how much or little people are exercising, the effects that sedentary lives might have on health and, most important, what can be learned for the next stay-at-home crisis. “There is an urgent need for research,” write the authors, “that can inform more effective physical activity policies and practices in the short term and prepare global public health for better responses to future crises.” The natural experiment of COVID-19, they hope, can help us better understand the role of exercise and build a roadmap for which public interventions keep everyone moving.