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Who says your job leaves you no time to hit the gym? A detailed new study of U.S. physical activity patterns shows that men who work full-time — whether their jobs are active or sedentary— end up getting more exercise than healthy working-age men without a job.

The new study comes from researchers at the National Institutes of Health (NIH) and is published this week in the American Journal of Preventive Medicine. As part of the National Health and Nutrition Examination Survey, some 1,800 working-aged adults were asked a battery of questions about their lifestyle and work habits, and, crucially, they then agreed to wear an accelerometer — a device to measure their physical activity — over the course of several days in 2003-04.

Those data from accelerometers provide a rare opportunity to nail down how much activity the typical American actually does.

They show that men or women who work in active jobs do more physical activity on weekdays than men or women working in sedentary jobs. That's perhaps not surprising, but the NIH researchers suggest that it still matters because of an ongoing shift in the economy toward sedentary work.

The more surprising finding is the one that compares full-time workers to people who don't work. The study shows that men with full-time jobs do more physical activity than healthy men without jobs. ("Healthy men," in this case, were those men who said their primary reason for being out of work was something other than health or disability.) In fact, even sedentary full-time workers performed more weekday physical activity overall than the healthy non-workers.

The results looked very different for women. Women in sedentary jobs did less physical activity on weekdays than their healthy non-working peers.

So what drives the gender difference? The study looks at the patterns, and can't unfortunately provide too much detail about their causes. There could be many possible answers, including, perhaps, different abilities to pay for leisure time activities, or different norms and attitudes about work and physical activity. It could also be that more non-working women than men are choosing to be at home running around full-time after the kids.

But the NIH researchers do find evidence, they write, to suggest that, whatever causes the difference, healthy non-working women "are replacing work with active pursuits whereas" — for some reason — "[non-working] men generally are not."

• Laura Blue