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Growing evidence suggests that our physical limits are actually governed by the brain – and we have the power to control it.Getty Images/iStockphoto

If you exercised outdoors this summer, there's a good chance you cursed the record-breaking heat – and in doing so, you made it feel worse.

According to a new study from environmental physiologists at Brock University, the internal monologue running through your head as you struggle through a workout in hot conditions has measurable effects on how well your mind and muscles function. The results add to growing evidence that seemingly immutable physical limits are actually governed by the brain –and that, with some simple changes, we can alter those limits.

The study, led by Dr. Stephen Cheung and his student Phillip Wallace, explored the use of "motivational self-talk." A group of 18 trained cyclists performed a series of tests that included a timed bike ride to exhaustion while maintaining a constant pedalling power, and a battery of cognitive tests in 35 C heat. Half of the group then received two weeks of self-talk training, and then they repeated the same series of physical and cognitive tests.

The self-talk training, based on well-established sports psychology techniques, involved identifying negative thoughts that occurred to the cyclists during the first set of tests, such as "It's so hot in here" or "I'm boiling," and learning to replace them with motivational statements such as "Keep pushing, you're doing well." Each volunteer identified a set of statements that felt effective to them, with specific statements chosen for different parts of the test.

The results, which were published online in the journal Medicine & Science in Sports & Exercise last month, showed a clear difference. The self-talk group increased their cycling endurance from eight minutes to over 11 minutes, and also improved their speed and accuracy on a cognitive test that involved figuring out and remembering a route through a maze. The control group saw no change in either test.

How is this possible? Some researchers believe that exhaustion in the heat occurs when your core temperature reaches a critical threshold, typically around 40 C. But during his doctoral studies in the 1990s, Cheung showed that trained athletes are able to push their core temperatures higher than untrained volunteers, suggesting that this supposed threshold isn't carved in stone.

Sure enough, in the new study, the cyclists reached a final core temperature of 38.8 C after self-talk training, compared to 38.5 C before – another blow to the idea of a critical core temperature.

"We're now pretty sure it's not just a physical thing," Cheung says. "There seems to be a strong mental-psychological component to it."

That helps to explain some puzzling results from the past. For example, a 2012 study by British researchers used rigged thermometers to trick volunteers into thinking the room was cooler than it really was – and believing it was 26 C led them to cycle 4 per cent faster than when they were shown the real temperature of 32 C.

And the conclusions remain applicable even now that summer is ending. A 2013 study by Dr. Samuele Marcora, the head of the University of Kent's Endurance Research Group, tested a similar self-talk program in cooler conditions, and found a similarly large boost of 18 per cent in time to exhaustion.

(It's worth noting that time-to-exhaustion tests are highly sensitive to changes, which is why researchers like them. An improvement of 18 per cent in a time-to-exhaustion test typically corresponds to a 1- to 2-per-cent improvement in a race – still significant, but not quite as eye-popping.)

Marcora argues that such findings indicate that the limits of endurance are dictated solely by the balance between your motivation and the effort you feel, which can be altered by anything from self-talk to caffeine to (as he showed in a 2014 study) subliminal images of smiling or frowning faces.

Cheung's results showed that after self-talk training, the cyclists were indeed willing to tolerate high levels of perceived effort for longer before giving up. He's still not sure exactly how this happens, and is currently planning follow-up studies that would simultaneously measure levels of various neurotransmitters, "so that we can break into that black box of the brain, and see what's happening during exercise."

For now, perhaps the key takeaway from the study is simply that it works –that the seemingly inconsequential words you whisper to yourself act as self-fulfilling prophesies. After the study ended, Cheung says, the cyclists lucky enough to be assigned to the self-talk group happily kept using their new skill through the scorching summer.

"Even if you're already fit," he says, "you can still improve your perception of heat and how you perform in it."

Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience

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