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Exhausted woman sitting on a sofa after working out in her living-room

When you push yourself to the limits of exhaustion, what stops you?

If you're lifting a heavy weight, you might blame the muscle fibres in your arms; if you're running, you might point an accusatory finger at your legs, lungs or heart. And until recently, most physiologists would have agreed with you.

But newer studies have offered hints that the ultimate arbiter of fatigue is situated between the ears – that the brain makes the decision to back off or stop before the muscles have reached their ultimate limits. Now researchers in Brazil have gone one step further: They've figured out how to get competitive cyclists to race faster by zapping a key part of the brain with electricity. It's a remarkable scientific result – and, perhaps, a new ethical dilemma for the sporting world.

Until a few years ago, most of the evidence about the brain's role in exercise was indirect. Scientists would trick their subjects by rigging thermometers to display incorrect values, or by secretly speeding up the virtual competitors they were racing against – and sure enough, the onset of exhaustion would be altered by these mind games, showing that fatigue wasn't a purely physical phenomenon.

Advances in brain imaging and measurement tools are now starting to permit researchers to peer into the skull during exercise. In 2011, a Swiss team at the University of Zurich, led by neuropsychologist Dr. Kai Lutz, made a key breakthrough by outfitting cyclists in caps with 128 electrodes to measure the electrical activity in their brains as they cycled to exhaustion.

Just before the cyclists gave up, Lutz saw a steady increase in communication between two regions of the brain: the motor cortex, which plans and controls movement, and the insular cortex, which receives and process signals from elsewhere in the body for a variety of purposes.

"It's not just muscle signals," Lutz explains. "The insular cortex is also involved in the emotional response of hearing your heart pound and so on."

Combined with previous studies that used spinal-blocking drugs to interrupt signals between the brain and muscles, Lutz's results suggest that your insular cortex monitors distress signals from throughout the body and then commands the motor cortex to apply the brakes. So is it possible to tweak the insular cortex to release the brakes?

That, effectively, is what Dr. Alexandre Okano of Brazil's Federal University of Rio Grande do Norte and his colleagues set out to do, in a study recenlty posted online in the British Journal of Sports Medicine. They used a non-invasive form of brain stimulation called transcranial direct-current stimulation (tDCS) to apply a small electrical current to a targeted portion of their subjects' brains, causing temporary changes in how the affected neurons communicate with each other.

Ten competitive cyclists participated in the study; each completed two all-out cycling tests, after receiving 20 minutes of either real or faked brain stimulation. The electrodes were placed over the left temporal cortex of the brain, which is directly above the insular cortex, so that both regions were affected. The results: After brain stimulation, the cyclists had lower heart rates, slower increase in perceived exertion and produced 4 per cent more power in the cycling test.

That's a very big boost.

Explaining exactly where this boost comes from is far from simple. In addition to the role of the insular cortex, Okano notes that the temporal cortex helps control heart rate and blood pressure, and the left side in particular is associated with "pleasant feelings as occurs, for example, when subjects either see or make a smile, or listen to happy voices, or hear pleasant music." The right side, in contrast, is associated with pain and exertion.

In other words, the brain stimulation triggers a complex mishmash of physiological and psychological effects, which have the net effect of allowing cyclists to push harder for a given level of effort. It will take much more research to isolate exactly how each part of the brain contributes to exercise and fatigue – but there's no longer any doubt that it plays a crucial role.

As for the obvious question about the potential use and abuse of this technology by athletes, Okano has no illusions.

"Strategies that modulate neuronal activity during training or during sport competition will lead to benefits comparable to those of using drugs," he said in an e-mail. "In addition, there is no known way to detect reliably whether or not a person has recently experienced brain stimulation."

The health risks of using this brain stimulation technique are thought to be low, he adds. Still, the prospect of future athletes zapping their brains in pursuit of victory, and perhaps overriding the brain's evolutionary safety mechanisms, is worrying. But it's a topic that sports officials, scientists, and ethicists will have to deal with soon – because when it comes to unlocking the brain's potential for extending our physical limits, we're just getting started.

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