With 2,000 meters left in the weekend’s World Cross Country Championship, 20-year-old Ugandan Joshua Cheptegei was leading the most important race of his young life, in front of the home crowd. The glory was his; he had almost 50 meters on pre-race favorite and cross country legend Geoffrey Kamworor. The crowd in Kampala was going wild.

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Cheptegei was sailing. Smooth, untroubled, strong—you could point to him and confidently say, “That’s what running is supposed to look like.” Knees high, chest forward, the look of a champion. Sure, he’d already run a blistering 8000 meters, at a pace the very experienced Kamworor would later called “suicidal,” but see—he was clear, he was going to keep it up. With only five-ish minutes of running remaining, he was invincible, focused, glorious.

At least, that’s what it looked as an outside observer, unable to see Cheptegei’s brain or heart. “Here’s what happens normally,” sports scientist Ross Tucker explained via email. “The brain monitors the body’s state of health—how hot it is, how much energy is in the muscles, in the blood, the osmolality, how much oxygen is available primarily to the brain, blood pressure.

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The brain then utilizes a complex algorithm to allow the body to adjust his pace and outputs to get himself across the finish line as quickly as possible. But sometimes a runner goes too hard, too fast, and crosses into the danger zone. “Body temperature is perhaps the most obvious: you go too fast, you produce too much heat, and if you can’t lose it [it was about 82 degrees at race time], your body temperature rises,” wrote Tucker. “And which organ is under threat? The brain, because it doesn’t do well at all once it hits temperatures around 40C (104F). So basically, the judgment of pace is a balance between how much muscle can be activated before the potential for physical harm becomes too great.”

It was glorious, too, because Cheptegei was a hometown hero, a Ugandan beating perennial powerhouses Kenya and Ethiopia on Ugandan soil, even leaving his mentor—countryman Stephen Kiprotich—in the dust. The last of five circuits of Kololo Independence Grounds would be a victory lap, a coming-out ceremony for running’s next big thing.

“But that decision about how fast to go, or how much muscle can be activated by the brain, is further modulated by emotional and tactical factors,” Tucker continued. “Cheptegei was running beyond his physiological means because of the magnitude of the moment. This is a safe assumption, because the manner with which he created that 10- to 15-second lead suggests he was running at a level that he would not normally have been comfortable at, or perhaps even dared to try. Remember, Cheptegei put 10 to 15 seconds into Kamworor, but a lot more into other runners who would, on balance, fancy themselves better than him.”

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“You can imagine an internal conversation, and part of his brain is saying ‘SLOW DOWN, this is crazy and we’re heading for disaster,’ while the other half is saying ‘GO GO GO, you have the crowd, the momentum, think of the glory. Only 2km. Only 5 minutes.’”

With just 2000 meters to go, you can probably guess what happened next. The camera panned away from Cheptegei, and when it came back he was a terribly different man. The champion had drained away, and the shell that remained could barely lift his knees. Though he was still leading, gone was the unified mission of forward movement; individual body parts made disjointed efforts to regain the smooth coordinated movement that seconds earlier had seemed so easy and natural. Cheptegei’s face too registered surprise, then despair.

About 600 meters from the finish line, the hard-charging Geoffrey Kamworor swept past Cheptegei, who subsequently disintegrated.

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Head back and held stiffly to the side, Cheptegei staggered, lurched, tottered, wavered, tiptoed. He slid his hand along the tape that marked the winding path to the finish. The last 600 meters took him an excruciating four minutes. Miraculously he crossed the finish line without assistance or flat out collapsing, as the crowd watched, horrified.

“Unfortunately, emotion only goes so far, because the consequence of that huge effort is some kind of physiological failure,” Tucker explained. “The analogy I always use is that you can’t commit suicide by holding your breath, because eventually the brain will have its way. A short circuit of sorts is reached where the hot brain says ENOUGH, and it basically stops the recruitment of muscle entirely. That’s why he resembled a drunk staggering home from the pub—total failure of the motor cortex of the brain.”

Cheptegei’s meltdown seemed sudden, but probably wasn’t on the inside. “There was a progressive increase in temperature and a progressive rise in Cheptegei’s perception of effort,” wrote Tucker. “He had to have been hitting maximum effort at around 8K, regardless of how smooth he looked. It would be fascinating to know how he was feeling at, say, 6K and 7K.”

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“Head back, leaning backwards is a classic hyperthermia pose, as is the loss of muscle coordination,” he continued. “He actually holds it together remarkably well, despite the obvious brain-related disaster going on. That’s another indication that the [motor cortex] failure happened before the point of total catastrophe [collapse, unable to even remain standing].”

“In the end, it’s so dramatic because his courage and mental effort have overridden normal regulation to the point where his body has to use its final ace to protect him,” Tucker said.

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Amazingly, Cheptegei’s heroic finish made him the fourth scorer for team Uganda, helping his country to a bronze medal. Despite the implosion, he crossed 30th out of 146 runners, in 30:08. To put his blistering pace through 9,400 meters in perspective, Cheptegei tottered home just four seconds back of the U.S.’s fourth runner, Stanley Kebenei, who was in full control of his physical being.

As quick and utter complete as his collapse was, Cheptegei’s recovery was equally remarkable—minutes later he was accepting the team bronze medal and chatting with Uganda’s president.

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Tucker doubted Cheptegei had done himself irreparable harm: “If the athlete goes right away to heat stroke where their body temp hits 41 C, and they need hospitalization, then there may be long-term effects. But I think he pulled up and failed early enough. The fact that he managed to ‘run’ all the way to the finish tells you he hadn’t crossed into ‘deep red’ territory.”

Just 20 years old, Cheptegei has won junior titles, and even competed at the Rio Olympics. He’ll be competing in elite races for a long time, and the next time his brain starts yelling at him to hit the brakes, he’ll probably be more likely to listen. But since he’s so young, his already exceptional physiology may have improved to a point such that he’s already crossed the finish line, in first, when that happens.