My sweaty search to discover how much exercise is too much

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As I pedalled, the grainy image of my pulsating heart kept flickering in and out of focus. The exercise bike was tilted upward, its front end propped a couple of feet in the air on blocks, and my knee kept knocking into the ultrasound technician's elbow.

"Hold on," said Dr. Jack Goodman, the head of the University of Toronto's Exercise and Cardiac Health Lab. "Let's make some adjustments."

I had come to Goodman's lab to get a first-hand look at his research on the long-term effects of endurance exercise on the heart. Over the past few years, several controversial studies have suggested that "too much" exercise might damage the heart. But how much is too much, and how serious are the risks? To get accurate answers, Goodman says, we need to use cutting-edge cardiac imaging technology to look at the hearts of long-time hard-core athletes, and see how they differ from the rest of us.

That's easier said than done, though. In the test I tried, the bike's backward tilt was chosen to make the heart, which dangles loosely from the aorta, flop into a better position for a clear ultrasound image. Finding an angle where I wasn't kneeing the ultrasonographer was another hurdle. And interpreting images from hearts that are pulsing 130 times a minute will be yet another challenge.

The intricacies of cardiac imaging help explain why the "too much exercise" debate has so far focused mostly on epidemiological studies that track large groups of people and look for patterns linking their exercise habits to long-term outcomes.

A 2014 study of 55,000 patients at the Cooper Clinic in Texas, for example, found that runners had a 45-per-cent lower risk of death from heart disease than non-runners. Surprisingly, those benefits accrued even to people who ran as little as five to 10 minutes a day, while those who ran more didn't get any further benefits, and in some cases even seemed to lose some of the benefits – a hint, the authors suggested, that excessive running was damaging their hearts.

In contrast, the long-running National Runners' and Walkers' Health Study at Lawrence Berkeley National Laboratory in California, with 150,000 subjects, has found that those running more than 64 kilometres a week are 26 per cent less likely to develop heart disease than those running 21 kilometres a week.

To help reconcile those conflicting findings, Goodman is studying 200 people between the ages of 45 and 65, comparing masters runners and cyclists with decades of training under their belts with recreationally active controls. The subjects are undergoing maximal exercise tests, cardiac MRIs, ultrasounds, ECGs, 24-hour heart rhythm monitoring, and in some cases cardiac catheterization, which involves sliding a tube through a blood vessel into the heart itself.

Under such close scrutiny, Goodman cautions, it's not easy to figure out what's significant and what isn't. "It's like these fancy medical clinics that executives pay lots of money to belong to: the more you look, the more things you find."

It's well known that the heart adapts to exercise, getting bigger and stronger in order to pump more blood with each beat. But Goodman plans to look closely at other subtle changes in athletes' hearts, both at rest and during exercise, to understand how these changes might contribute to the risk of conditions, such as atrial fibrillation, a type of irregular heart beat that some studies suggest is more common in endurance athletes.

In the general population, atrial fibrillation combined with other factors, such as high blood pressure, raises your risk of having a stroke. But "athletes are different," Goodman points out; for them, there's no evidence that atrial fibrillation is associated with other problems.

One source of confusion in the debate about exercise levels, Goodman adds, is that the risk of relatively benign complications, as with atrial fibrillation, is sometimes mixed up with the risk of far more serious outcomes, such as premature death – "but there's no evidence that the two are linked."

Still, such subtleties make it difficult to issue guidance about the "right" level of exercise. Moreover, the appropriate levels almost certainly depend on who you are – not just your current fitness, but your genetics. If Goodman and others can identify cardiac changes that predict who will eventually develop atrial fibrillation, they may be able to link those changes to certain genes, allowing more personalized exercise advice.

Ultimately, no matter what the results show, our exercise choices, as with many other decisions in life, from highway driving to hamburger eating, will continue to involve weighing the odds.

And Goodman, for one, thinks those odds are pretty clear.

"To me, the tradeoffs are worthwhile," he says. "We're adapted to run. We're not adapted to be inactive."

Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com. His latest book is Which Comes First, Cardio or Weights?