In the late 1990s, South African researcher Tim Noakes proposed that a “central governor” in the brain prevents us from getting too dangerously close to the absolute limits of our bodies. Physiologists have been arguing ever since about the brain’s role in determining truly “maximal” effort, but the bottom line is clear: “We know there’s something in the brain that regulates performance,” says MacRae. “Now we want to see if we can manipulate it.”

To do so, they’re using a technique called transcranial direct current stimulation, or tDCS, which has experienced a wild surge in popularity among researchers over the last few years. There are studies on pain, depression, memory and learning, and enhancing the motor rehab of Parkinson’s and stroke. Then, last year, Brazilian researchers published a study in the British Journal of Sports Medicine showing that trained cyclists produced 4 percent more power and had lower heart rate and perceived effort during an incremental test after a 10-minute bout of tDCS—and suddenly, the sports world was interested.

“It’s about the nature of fatigue,” explains MacRae, a trim, straight-backed figure with a faint South African accent. “Why do we slow down? Why do we make that decision to slow down?” If the answer seems obvious, think again. It’s true that if you take an isolated piece of muscle in a Petri dish and jolt it with electricity over and over again, it will eventually stop twitching. That’s how we usually think of fatigue—as a purely corporeal phenomenon, a mechanical breakdown. But that’s not what happens in a race. You cross the line and you’re still moving. Your muscles still work, and your heart’s still beating. So why didn’t you go faster?

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