Physical activity is linked to better health and a longer life — except if you're a man in a strenuous job. Then you're at higher risk of early death, according to a newly-released study.

A research paper published in the British Journal of Sports Medicine took data from 17 previous studies and found what the authors are calling a "physical activity paradox".

So, while exercise in leisure time is proven to be good for you, the exercise you get as part of a physically demanding job may not be.

This study found men in physically demanding jobs were at an 18 per cent increased risk of early death, compared to men in work with low levels of physical activity.

What's more, the researchers only found this link between strenuous work and early death in men, not women.

Is the risk in active jobs or other social factors?

Some of the earliest studies demonstrating the health benefits of physical activity were actually based on people who were active as part of their jobs.

A study of transport workers in the 50s found conductors were healthier than the more sedentary drivers. ( Flickr: Paul Townsend )

Landmark research in the 1950s looked at the difference between London Transport Authority conductors, whose jobs kept them active, and drivers, whose jobs kept them relatively sedentary. The drivers were found to be at higher risk of coronary heart disease.

But there's a big difference between the kind of physical activity a transport conductor does, and other types of manual labour — for example road work, and construction jobs.

Since the 1950s, many physically strenuous jobs have been made superfluous by technological advances, suggested Professor Jo Salmon from Deakin University's Institute for Physical Activity and Nutrition, who was not involved in the study.

"Times have changed and we know there's not that many highly active occupations left," she said.

Professor Salmon also pointed out that physically strenuous jobs today are often held by people from lower socioeconomic backgrounds.

And there's a lot of research pointing to the fact that people from poorer backgrounds have poorer health outcomes generally.

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"While some of the studies adjusted for things that we call confounders — like smoking, alcohol intake, dietary intake, which are all independent risk factors for premature death — we do know that people who are living in disadvantage are more likely to have these clustered risk behaviours. This is all bundled up together," Professor Salmon said.

Curtin University's Professor Leon Straker, who co-authored the paper, agreed socioeconomic status was difficult to extricate from the physical nature of the jobs in the study, and added that these jobs often had additional risk factors, such as chemical exposure or increased sun exposure.

But he thinks the link is still there.

Some of the studies that were analysed as part of the research published on Tuesday tried to control for socioeconomic status.

However, Professor Straker said there was still evidence for physical activity playing a role in risk of death.

"We think that there's a reasonable likelihood that physical activity is at least one of the mechanisms that's giving the poorer health and life outcomes for these people," he said.

Professor Straker said the study showed health guidelines needed to differentiate between the exercise people do as part of their work and what they do in their leisure time.

"Our hypothesis is that the physical activity that people do in work is different to the physical activity people do in leisure, and it's different in ways that's really important for how a body responds and becomes fitter, stronger and healthier," he said.

"If you think about a lot of physically active jobs, they're often active for really long periods of time, eight hours as a typical workday or longer, and the intensity that they're working at isn't necessarily as high as somebody who is going out for a 30-minute run.

"So the intensity's lower, but it's over a much longer period of time and you don't have the same recovery options at work as you do in leisure."

Professor Straker speculated it could be something about the difference in intensity or the repetitive movements of occupational exercise that might mean men don't get the same health benefits as they do from leisure activity.

Men versus women

The study's link between higher physically demanding jobs and risk of death was only found in men, not women — who got the same benefit from workplace physical activity as they'd get from leisure activity.

Women with strenuous jobs had similar health benefits to what was expected from leisure activity. ( Flickr: Pam Loves Pie )

Professor Straker thought there were two reasons why this might be the case.

"One is that the sorts of physically demanding, active jobs that women do may be different from the physically demanding jobs that men do," he said.

"The second thing is that women have been shown to respond differently to different types of exercise than men.

"Our best guess is that it's probably a combination of the two."

I'm in a strenuous job. How do I lower my risk?

Both experts agreed that exercise as a leisure activity had health benefits — no matter how physically active you are for work.

They also both acknowledged that was the last thing people shattered from physically demanding jobs usually wanted to hear.

"It seems like a contradiction," Professor Salmon said.

But she said workers with physical jobs who took the time to exercise too, would likely be fitter and better able to cope with the demands of the job — as well as having the longer-term benefit of reducing the risk of premature death.

"Even if you are feeling exhausted from your job — it is worth investing in your leisure-time physical activity," Professor Salmon said.

"Because probably, what will happen is you will manage your job easier and find the job less stressful."

But she recommended choosing a different type of activity to what you do in the workplace — something enjoyable and with a social element.