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Germany's Tobias Angerer (front) and Canada's Devon Kershaw cross the finish line at the men's 50 km mass start classic cross-country final at the Vancouver 2010 Winter Olympics in Whistler, British Columbia, February 28, 2010.STEFAN WERMUTH/Reuters

The third in a four-part series on physiological extremes encountered by Olympic athletes.

After five days and almost 250 kilometres of running through the shifting sands and hair-dryer heat of the Sahara Desert, Simon Donato had reached the limits of his physical endurance. The 37-year-old from Canmore, Alta., who travels the world competing in the most gruelling ultra-endurance races as co-host of the TV adventure series Boundless, knew he had to cover another 30 km to reach the finish line, but he could barely put one foot in front of the other.

Then, as he staggered on, a strange thing happened: He began to speed up. He gained strength as he felt the finish line draw nearer, he says, "and the final five kilometres were probably my fastest of the entire day."

Donato's experience is hardly unusual. We've all experienced the surge of energy that comes with realizing we're almost finished a difficult task. And the same phenomenon will be on display in Sochi, where events such as the 50-km cross-country ski race will last for over two hours but still be decided by fractions of a second in a furious final sprint. (Just ask Devon Kershaw, the Canadian who finished fifth at the last Games, 1.6 seconds behind the winner.)

But it's also puzzling. If tired muscles and depleted energy stores are what slow you down during prolonged exercise, then how are you able to speed up at the end, when you should be most tired and depleted? Over the past decade, this puzzle has forced physiologists to reconsider what limits endurance. Their conclusion is that, to a much greater degree than suspected, the limits are in your head.

"Top-level athletes get closer and closer to their real physiological maximum," says Dr. Samuele Marcora, a fatigue researcher at the University of Kent in Britain. But they never quite reach it: The brain applies the brakes before the heart, lungs or muscles fail.

Marcora points to a classic study by French researcher Dr. Michel Cabanac, published in 1986. He asked volunteers to "sit" against a wall with their knees bent but no chair for support, and offered them a monetary reward, ranging from a few cents to a few dollars, for every 20-second interval they endured. The more money he offered, the longer the subjects lasted – again, an obvious result. But it illustrated that the timing of their collapse depended on factors in their brain, not their muscles.

Marcora believes our limits are dictated by an ever-changing balance between perceived effort and motivation. Fatigued muscles affect how hard your effort feels, but so too do many other seemingly unrelated factors, like how mentally fatigued you are or how confident you are. He and his colleagues published a study last fall showing that two training sessions in "motivational self-talk" reduced the perception of effort during a cycling test and allowed subjects to last 18-per-cent longer before reaching exhaustion.

So does this mean the secret to physical success is simply mind over muscle? Not quite. "The sense of effort is generated by the brain, but it's still your reality," Marcora says. "You can't just ignore it."

That reality can be tweaked, though, by researchers who deceive their volunteers into achieving otherwise impossible feats. One study showed that cyclists lasted longer in a ride to exhaustion when the clock was secretly adjusted to run 10-per-cent slower than normal; another found that cyclists performed better in hot conditions when the thermometers were rigged to display artificially low temperatures.

Knowing how much longer you have to go also affects how hard your effort feels, an effect known as teleoanticipation. That's why athletes like Donato are able to speed up as they approach the end of a race: Even though their bodies are more tired than ever, their brains interpret the signals differently. In the lab, tricking subjects by making them continue for shorter or longer times than expected produces sudden changes in their perception of how tired they are.

All these studies demonstrate the power of the mind, but they don't offer much useful insight on how to push harder, since it's difficult to deceive yourself. For practical applications, one promising approach comes from neuroscientists at the University of California, San Diego, who have been studying the brain patterns of elite adventure racers like Donato in fMRI scanners.

The experiments involve completing cognitive tests while enduring the unpleasant sensation of having the oxygen flow through their breathing mask temporarily restricted. Compared to non-athlete controls, the adventure racers have a heightened anticipatory response in their insular cortex, which monitors incoming signals from the rest of their body; they're then able to remain calm and perform well during the period of restricted breathing.

The difference, explains Dr. Martin Paulus, the lead researcher, is that the athletes have thousands of hours of training that has taught them to accept discomfort without panicking. "Through repeated exposure, you learn to tolerate it," he says.

Paulus and his colleagues then put a group of Navy Seal recruits through an eight-week mindfulness-training program, teaching them techniques rooted in Buddhist meditation. They saw changes in brain patterns after the mindfulness training that mimicked patterns seen in the elite adventure racers – an indication that it may be possible to directly train the brain to better handle adversity and physical discomfort.

This approach is hardly a simple shortcut, though. Whatever the form of the training, it takes time and effort to push the cautious limits imposed by your brain. And even experienced athletes like Donato still have to constantly remind themselves to believe in their ability to go just a bit farther.

"The races we choose for Boundless are some of the toughest out there, and regularly push me to redefine my personal limits," Donato says. "This is a mental redefinition, though, not a physical one, because it's the mind that needs to overcome the obstacles. Once this happens, the body will usually follow orders."

Alex Hutchinson blogs about exercise research at

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