Blue-collar jobs that involve heavy lifting really do make men more manly, a study suggests.

Those with physically demanding jobs had nearly 50 percent higher sperm concentration and total sperm count than white-collar workers.

They also had higher levels of testosterone — the sex hormone that gives men large testicles and aids in sperm production.

Lifting heavy objects – and exercise more generally – has been shown to lower inflammation in the body, which is linked to better sexual health.

Men’s sperm rates have more than halved since the 1970s, which has been blamed on sedentary lifestyles and bad diets.

The researchers did not speculate on why the correlation might have occurred, but previous studies suggest that exercise such as heavy lifting promotes healthy hormone levels that create a better environment in the testes for sperm production

Men who said they lift often had almost double the sperm concentration of those who said they never do heavy lifting at work

The study, published in Human Reproduction, looked at 377 men in couples who were seeking treatment at a fertility center.

The men – who had a median age of 36 and were mainly White – filled out a questionnaire on how often they lifted or moved heavy objects, their typical shift pattern and their level of physical exertion at work.

Samples of their semen were analyzed to analyze their sperm and concentration of different reproductive hormones.

Of the men who filled out the survey, 12 percent indicated that they often lifted or moved heavy objects at work. 

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Those men had a 46 percent higher sperm concentration and 44 percent greater total sperm count compared with men who reported never shifting heavy objects at work.

Sperm concentration is the number of sperm per unit volume (milliliter) of semen and total sperm count is the total number of sperm in the ejaculate.

The researchers did not speculate on why the correlation might have occurred, but previous studies suggest that exercise such as heavy lifting promotes healthy hormone levels that create a better environment in the testes for sperm production. 

Experts have said that even moderate exercise curbs oxidative stress, which damages sperm.

Men who said their job was heavily or moderately physically demanding were found to have higher testosterone concentrations too, compared to those involved in light-level exertion.

Those working evening or rotating shifts had 24 percent higher testosterone than men on the day shifts.

Men who often took part in lots of physical labor at work also had higher levels of estrogen, the female sex hormone.

The researchers hypothesized that this is due to excess testosterone being converted into estrogen by the body to ensure a balance of the two.

Because the participants were men from couples seeking fertility treatment, the results might not hold true for men from the general population.

However, other studies have confirmed that men with a sedentary lifestyle are worse off in multiple semen parameters, including volume, viability and motility.

Other research published in the journal Human Reproduction Update illustrated men’s plummeting sperm rates.  

It included data from 57,000 men across 53 countries — the largest study ever conducted.

The paper showed that sperm counts – or the amount of sperm in an ejaculation – fell from 101million to 49million sperm per millimeter of semen between 1973 and 2018.