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Before and after the 1901 Boston Marathon, a Harvard doctor named Ralph Larrabee took blood samples from four of the runners. Their white blood cell counts, a key measure of immune function, were way out of whack, indicating that “the exertion had gone far beyond physiological limits.”

Scientists have been debating the link between hard exercise and the risk of catching passing infections ever since and the question has never been more salient than now. For some, self-isolation is severely curtailing their usual exercise habits; for others, a sudden and unwanted excess of free time is allowing them to train harder than ever before. Neither approach, it turns out, is ideal.

Exercise immunologists usually describe the relationship between infection risk and exercise dose (which is a combination of duration and intensity) as a J-curve. Doing regular moderate exercise lowers your risk compared to doing nothing; studies typically find that near-daily moderate exercisers report about half the typical number of upper-respiratory tract infections. That’s an important message for anyone who’s tempted to slack off their fitness routine until life returns to normal.

But according to the J-curve theory, if you ramp the dose up too high, your risk climbs steadily until you’re more vulnerable than if you’d done nothing at all. For that reason, Oregon-based elite track coach Jonathan Marcus recently argued on Twitter that athletes should avoid the type of gut-busting workouts that might put them at higher risk. “To train hard now is irresponsible,” he wrote.

Here's how to start working out again after a break

Figuring out what counts as “too hard” is tricky, though. Neil Walsh, an exercise immunologist at Liverpool John Moores University in Britain, compared the effects of two hours of low-intensity running with 30 minutes of high-intensity running. The longer bout disrupted immune response more than the shorter one, suggesting that duration is a bigger risk factor than intensity.

That may be because prolonged exercise depletes the fuel stores that your immune cells rely on – an effect that seems to kick in after about an hour and get even worse after 90 minutes, according to research from Appalachian State University immunologist David Nieman.

Not everyone agrees with this take, though. In a 2018 paper in Frontiers in Immunology, University of Bath researchers John Campbell and James Turner questioned the idea that hard exercise, on its own, suppresses immune function, even in an extreme challenge such as running a marathon.

“If people do get an infection, it’s probably due to their attendance at a mass-participation event, where lots of people, and their bugs, are gathered,” Campbell argued.

Travel and big crowds aren’t an issue for self-isolating solo exercisers. But other external factors such as stress and disrupted sleep – not exactly rare in the midst of a pandemic – also wreak havoc on immune function, says Walsh. He and his colleagues found that anxiety and psychological stress, as measured by a simple questionnaire, had just as much influence on the immune system’s response to exercise as the length and intensity of the workout.

All of this makes it difficult to issue a one-size-fits-all prescription for how to work out. Nieman offers a simple rule of thumb: stick to 60 minutes or less at an average heart rate of 60 per cent of your maximum. Inserting some more intense surges is fine; it’s sustained intensity that seems to tax the body the most.

The details, though, will differ depending on your fitness, your usual habits and perhaps also your psychological needs. Running for an hour might be unfamiliar, stressful and physically exhausting for someone who’s looking for an alternative to their cancelled weekly hockey game. But it might be relatively easy for a habitual runner, and an important source of stress relief, which, in turn, will have positive immune benefits.

The bottom line? Find a way to keep exercising and don’t be afraid to push hard now and then. But if you’re planning to run a hard solo marathon around and around your block, make sure to rest up and stay well away from other people for a few days afterward – you know, like you’re supposed to be doing anyway.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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