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The key to building bigger muscles – and to staying happy, healthy and young, if you believe the conventional wisdom – is to unleash a torrent of hormones. Design your exercise program to maximize the release of testosterone, growth hormone and other related molecules, and you’ll be ripped in no time – or so we’re told.

But a new study from researchers at McMaster University undercuts this picture, finding that the hormone surge triggered by exercise has no impact on how much muscle you gain over time. Instead, what matters is how sensitive your muscles are to hormonal signals, a finding that may explain why some people put on muscle much more easily than others.

Hormones such as testosterone, growth hormone and insulin-like growth factor play a key role during development and puberty, and have long been linked to muscle growth. Since studies have found that levels of these hormones can increase tenfold after a session of strength training, researchers have long assumed that they play a role in the growth of new muscle. That conclusion is bolstered by the clear muscle-building effects of injecting testosterone and other steroids.

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The subjects in the study who gained the most muscle didn’t have higher postworkout hormone levels, but they did have significantly more receptors in their muscles than those who gained the least muscle.UberImages/istockphoto

Over the past decade, though, McMaster kinesiology professor Stuart Phillips and his colleagues have conducted a series of studies in which postworkout hormone levels showed no correlation with muscle gains after weeks or months of training – a finding that has sparked considerable controversy.

The latest study, published this month in Frontiers in Physiology, involved 49 volunteers who did a full-body strength training workout four times a week for 12 weeks. Careful measurements of nine different hormones, both circulating in the blood and in the muscles themselves, found no link between any individual hormone or combination of hormones and the amount of muscle gained.

“We think this paper will end the debate,” says Robert Morton, the doctoral student who led the new study, “because it offers an alternative explanation of how hormones do affect muscle growth.”

That alternative explanation focuses on “androgen receptors,” a class of proteins found inside muscle cells that detect the presence of hormones such as testosterone and respond by switching muscle-building genes on or off. The subjects in the study who gained the most muscle didn’t have higher postworkout hormone levels, but they did have significantly more of these receptors in their muscles than those who gained the least muscle.

“[Hormones like testosterone] can be thought of simply as keys, and their receptors as locks,” Morton explains. “Healthy young men have plenty of keys, so the number of locks matters more.”

From a practical perspective, that’s mildly disappointing news. Workout gurus have long advocated structuring your workout to maximize hormone production, for example by starting with full-body exercises that recruit large amounts of muscle and focusing on heavy lifts rather than more repetitions. But the resulting hormone boost seems to be irrelevant – and there are no comparable tactics to boost your receptor levels, which are likely set by genetics, Morton says.

That doesn’t mean hormone levels are totally irrelevant. After all, injected steroids do work. That’s partly a function of timing: While a workout might boost your hormone levels for half an hour, a steroid regimen will elevate them to an even higher level night and day for weeks at a time.

And even if muscle growth is unaffected, hormone levels may affect other parts of the body, points out Keith Baar, a muscle physiologist at the University of California, Davis, who was one of the study’s independent reviewers. For example, experiments in Baar’s lab suggest that postworkout hormones may influence the strength of ligaments.

Still, the overall message is that testosterone simply isn’t as miraculous as once thought. Lower-than-normal levels in older men can have negative consequences such as loss of sex drive and reduced bone density – but taking supplemental testosterone also carries risks, such as an increase in stiffened coronary arteries.

Fortunately, there is one therapy that covers both bases, enabling older men to reverse the age-related decline in hormone levels and in androgen receptors. It’s clinically proven, largely free of side effects and not banned by any international sporting bodies. It’s called strength training.

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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