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A construction worker at a job site in downtown Toronto.Fred Lum/The Globe and Mail

It’s been a little more than a decade since Danish occupational health researcher Andreas Holtermann first identified what he dubbed the “physical activity paradox.” Exercise, he observed, seems to be good for your health if you do it on your own time, but bad if you do it at work.

It’s a puzzling observation. Why should it matter whether you get your 10,000 steps in a park or on your delivery route? But a paradox, as G. K. Chesterton once wrote, is just truth standing on its head to gain attention – and a series of recent studies suggests that Holtermann’s paradox is now getting the attention it deserves.

The latest such research, published in the British Journal of Sports Medicine by a team led by Norwegian epidemiologist Edvard Sagelv, tracks the health of nearly 30,000 adults in the city of Tromsø between 1986 and 2016. The subjects filled out periodic surveys that included information on their levels of leisure and work-related physical activity, and data on those who died and their cause of death was taken from national health records.

The results confirm Holtermann’s observation – but only to a point. His original study, published in 2012, looked for correlations between exercise habits and long-term sick leave in a relatively small sample of 7,000 Danish workers. The more leisure-time exercise they accumulated, the less likely they were to take sick leave. But the pattern was reversed for physical activity at work: more exercise was associated with more sick leave.

Since then, a series of progressively larger studies have produced conflicting results, and scientists have proposed a number of explanations for the apparent paradox.

The most eyebrow-raising suggestion is that the effects of exercise depend on your mindset. For example, a famous 2007 study of hotel cleaners by Harvard University psychologist Ellen Langer found that if they were told that their daily labour counted as exercise, it led to measurable drops in blood pressure and weight after a month, while those who didn’t receive any information didn’t see any health gains.

More prosaic explanations include lack of autonomy, since workers, unlike leisure-time exercisers, often can’t choose when they need a break. Some physical work may be too easy to trigger health gains; other jobs may be so hard and have such long hours that it’s impossible to recover from one day to the next. Heavy lifting, in particular, can raise blood pressure.

In Sagelv’s new data, there was a dose effect. Men with jobs that required only walking saw no longevity benefit compared to those with sedentary jobs. Those with harder jobs involving walking and lifting saw a 32-per-cent drop in their likelihood of dying of heart disease during the study, consistent with the conventional wisdom that exercise is good for your heart.

But those benefits were reversed in those performing heavy manual labour, who were no better off than those with desk jobs. There was a sweet spot, in other words: physical activity at work was good, but only up to a point.

Surprisingly, occupational exercise had neither positive nor negative effects for women, a non-finding that has turned up in previous studies. It’s not clear whether that’s because male-dominated and female-dominated fields have different demands, or because of physiological differences in how the sexes respond to exercise.

The most interesting detail in Sagelv’s data is that, among the men performing heavy manual labour at work, the healthiest and longest-lived were those who were also vigorously active in their leisure time. This runs counter to the idea that it’s the amount of physical activity that’s the problem. Instead, it’s consistent with observations that an increasing proportion of workers aren’t fit enough to handle the physical demands of their job – meaning that more leisure-time exercise, rather than less, might be beneficial.

So is there a physical activity paradox? The new study weighs in with a firm maybe. What’s clear is that, for better or worse, physical activity at work and outside of work both contribute to your overall health. To get the most out of your workouts, you need to get the balance right.

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

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