A sharp bang, like a rifle shot, echoes off the walls of the converted warehouse. There's a brief silence, then everyone heads for their bikes, checking to see whose tire has blown. I'm more worried about the guy slumped in a dentist's chair at the far end of the room, dripping sweat and dangling wires, who's getting zapped by a brain stimulator that looks like a ping-pong paddle with two heads. Did we just blow out Tim Johnson's brain?

I'm at Red Bull HQ in Santa Monica for the second iteration of Project Endurance, a boundary-pushing five-day training camp-slash-science experiment. Five world-class cyclists and triathletes will be prodded, zapped, and repeatedly pushed to their physical limits by a multinational swarm of several dozen researchers who will measure their every twitch and palpitation. The big question they're hunting: What role does the brain play in setting our physical limits? And can we change those limits—break through to another level—by trickling a small electric current through the brain's motor cortex?

To find out, Red Bull enlisted Dylan Edwards and David Putrino, a pair of Australian neuroscientists at the Burke Rehabilitation Center and Weill Cornell Medical College in New York, to devise a five-day testing protocol—three days at Red Bull HQ in Santa Monica, two at the StubHub velodrome 20 miles down the 405 in Carson—using electric and magnetic brain stimulation, peripheral nerve stimulation, EMG, EEG, and an array of other measurement tools to tease apart the effects of central (in the brain) and peripheral (in the muscles) fatigue as the athletes are pushed to the breaking point again and again.

"I think of my brain as a tool," Johnson, a six-time national cyclocross champion, had been explaining to me a few minutes before the bang. Fortunately, it turns out that his tool is fine. If anything, it's the other way around: Johnson's brain has somehow blown a circuit in one of the brain stim machines. Testing halts for a few hours while a replacement machine is rushed into place, and I seize the opportunity to quiz Holden MacRae, a sports medicine professor at Pepperdine University who also serves as Red Bull's chief physiologist, about the project's ultimate goals.

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?

When I arrived in Santa Monica (Red Bull flew me and several other science journalists in to watch the fun), the second of three days of testing in the controlled environment of Red Bull HQ was just getting started. Along with Johnson, the athletes included mountain biker Rebecca Rusch, and BMXer Mike Day, and triathletes Jesse Thomas and Sarah Piampiano, though Piampiano had to drop out of testing due to an injury she'd suffered two days earlier in the closing miles of Ironman Texas (it turned out she'd fractured her femur). Each day followed an identical program of alternating brain stimulation and cycling tests. The only difference was who received real brain stimulation and who receive sham stimulation: the subtle ants-on-your-scalp tickle that accompanies the first jolt of tDCS current fades so quickly that it's impossible to tell whether the machine is on or off after the first minute or so.

tDCS is disarmingly—almost disturbingly—simple: you connect a voltage source (a 9-volt battery will do) to two electrodes placed on opposite sides of your head. The precise placement of the electrodes determines which regions of your brain the current flows through. As it passes, the current changes the excitability of the neurons in the affected region, making them slightly easier to trigger (or harder, depending on which direction the current flows). Edwards and Putrino's primary interest in tDCS is to help patients recover from brain and spinal cord injuries—but "rehab and high-intensity training are not as different as people believe," Putrino says. "Whether you're a high-end athlete or a patient fighting locked-in syndrome, you're dealing with the same limitations of muscle fatigue."

Rusch was the first athlete on the bike. "My first thought was 'How is this different from the electroshock therapy they did in the 50s,'" she admitted as a crowd of scientists clustered around her affixing wires, sensors, and electrodes to her body. "I was like, they're going to do what to my head?" (The key difference is magnitude: electroconvulsive therapy delivers 500 to 1,000 times more current through the brain, enough to trigger seizures.) But she'd come around to the idea, lured by the promise of learning more about the hidden reserves she relies on to win races. "If you're being chased by a lion, or a car falls on a baby, you find something extra," she said. "I think we're just touching the iceberg of 'How do we train that?'"

Edwards fitted Rusch with a neoprene cap embedded with eight electrodes: one to send current, one to receive it, and the rest to monitor her brain activity. Then she took a seat in a comfortable leather recliner, closed her eyes, and let the electrons flow for the next 20 minutes. The active electrodes were positioned just above her forehead and just behind the crown of her head, sending the current through the chunk of primary motor cortex that sends signals to the legs—and, in theory, tweaking the hair-trigger response of those neurons to make sure signals would keep flowing through mounting fatigue.

That's one picture of how tDCS works, but the truth is that nobody is entirely sure how it produces all the effects observed in studies. In addition to keeping the brain's output signals high, it may alter how information from the rest of the body is received and interpreted. "We can say that (tDCS) somehow amplifies signals, but also decreases pain induced by muscle fatigue," says Alberto Priori, a researcher at the University of Milan whose seminal 2007 study showed that tDCS to the motor cortex could boost muscular endurance even in healthy subjects. It may turn out that tDCS can act in many different ways, depending on where you zap.

What will it mean if the experiment works—if brain stimulation really does make the Red Bull athletes faster? One obvious specter is brain doping, a possibility that Brazilian researcher Alexander Okano acknowledged when I spoke to him about his research last year. The technique will lead to "benefits comparable to using drugs," he said. And "there is no known way to detect reliably whether or not a person has recently experienced brain stimulation." The safety risks of tDCS are thought to be minimal (though some researchers point out the lack of long-term studies, especially on the developing brains of young people), but the ethics of brain boosting will nonetheless require plenty of debate.

But there's a more subtle benefit from discovering that a jolt to your brain allows your muscles to go faster—because tDCS doesn't create that extra power in your muscles. It just (in theory) unlocks what was always there. And once you know it's there, you have a better chance of accessing it next time. That's what 1968 Boston Marathon champion Amby Burfoot was getting at a few years ago when he described the benefits of a "deception" workout where the coach makes you tackle an extra repeat after you think you're finished. "From this workout," he wrote, "you'll learn forever that you're capable of much more than you think. It's the most powerful lesson in running."

That lesson—and the many ways we search for it—was on my mind as I watched the cyclists tear around the velodrome on the fourth day of testing. Away from the controlled environment of the lab and its futile stationary bikes, it was easier to connect the dry clinical discussions of "maximal voluntary contraction" and "task failure" with the messy reality of no-holds-barred competition. On the first 4-kilometer time trial of the day, Johnson notched the fastest time with a 5:20, two seconds quicker than Thomas. A few hours later, after another round brain stimulation, Thomas managed to drop his time to 5:10, then stood on the sidelines cheering as Johnson, wheels tracing a perfectly level contour around the steep curves, tried to reclaim the throne.

Stopwatches clicked as Johnson whizzed past the finish in 5:17. "Did I get him?" he panted as he circled past the finish line again a moment later. Thomas laughed. "That's the first thing I asked when I finished too. It's the same mindset." He glanced around at the hundreds of thousands of dollars worth of machinery arrayed on the infield, the laptops and transmitters, the sensors and wires poking out of his bike shorts. "You can do all this shit, but it all comes down to two guys on a bike, trying to beat each other."

That's worth remembering before we get carried away with neuro-hype. The next morning, before heading to the airport, I cornered one of Edwards's colleagues and asked for a peek at the randomization protocol from the previous day, when Johnson and Thomas had been battling back and forth. I wanted to see who got the real tDCS and who got the sham. For the first trial, Thomas got zapped and Johnson didn't. For the second trial, it was the other way around.