Endure: Mind, Body and the Curiously Elastic Limits of Human Performance. Alex Hutchinson

and diagnosed her problem as hyponatremia, a result of drinking too much, rather than the more common problem of drinking too little. It took another two decades—and a handful of deaths—before the scientific community fully acknowledged the dangers of overdrinking during exercise.

      That same year, Noakes cofounded a dedicated sports science unit in the basement of the University of Cape Town’s physiology department, with a single stationary bicycle and a nearly obsolete treadmill. He and his colleagues began bringing athletes in and testing their maximal oxygen consumption—“because,” he says, “in 1981, to be a sports scientist, you had to have a VO₂max machine, to measure VO₂max.” But it didn’t take long for Noakes to grow dissatisfied with the insights provided by A. V. Hill’s signature measurement. One day in the lab’s early years, he tested track star Ricky Robinson and Comrades champion Isavel Roche-Kelly, less than an hour apart—and despite their vastly different racing speeds, they both recorded the same VO₂max. Noakes’s conclusion: “Clearly the VO₂max was totally useless, because here we had a sub-four-minute miler and it couldn’t say he was any better than the lady who could run a five-minute mile.”

      Over the next decade, Noakes began searching for better ways of predicting and measuring endurance, and other ways of explaining the apparent limits runners like Robinson and Roche-Kelly encountered when they finally had to step off the treadmill at the end of a test to exhaustion. Hill and his successors had focused on oxygen: at your limits, your heart was incapable of pumping any more oxygen to your muscles, or your muscles were incapable of extracting any more oxygen from your bloodstream. Noakes’s first idea for an alternative to VO₂max, in the late 1980s, was that the limits might reside in the contractility of the muscle fibers themselves, but that theory fizzled.

      By the 1990s, Noakes had become an internationally renowned running guru, thanks to the enduring pop-sci classic Lore of Running, a 944-page doorstopper that first appeared in 1985. In 1996, he received one of the highest honors in the field of exercise physiology: an invitation to deliver the J. B. Wolffe Memorial Lecture at the annual meeting of the American College of Sports Medicine. True to his reputation, he decided to harangue his eminent audience about their stubborn adherence to the “ugly and creaking edifices” of old theories that were unsupported by “empirical science.” It was in preparing for this talk that he had his crucial epiphany about the rarity of deaths from exhaustion, like Henry Worsley’s. Whatever our limits are, something must prevent us from exceeding them by too much. And that something, he reasoned, must be the brain.

      The history of brain research is, in some ways, a tale of unfortunate injuries and illnesses. Phineas Gage, for example, was a twenty-five-year-old construction foreman working on a new railway route in 1848 when a misfired explosive blast sent a 43-inch-long tamping iron rocketing up through his cheek and out the top of his skull. His survival was remarkable enough, but even more surprising were the alterations in his personality. A polite, competent man was suddenly transformed, through damage to his frontal lobes, to a profane and unreliable one: to his friends, the doctor who treated him reported, Gage was “no longer Gage.” Since then, we’ve learned a great deal about how the brain works by observing the distinctive changes that follow damage to different parts of the brain—an assortment of strange and mostly sad transformations of the type chronicled with tenderness and humanity by the late neurologist Oliver Sacks.

      For Diane Van Deren, the first warning signs came when she was just sixteen months old, when a prolonged seizure sent her to the hospital where she lay, packed in ice, convulsing for nearly an hour. There were no apparent aftereffects, and Van Deren grew up to be a star tennis player, got married, and had children. Then, when she was twenty-nine and pregnant with her third child, the seizures returned, and over the next few years they got progressively worse. Working with neurologists at the University of Colorado, she eventually decided to have a partial right temporal lobectomy, to remove the portion of the brain where the seizures were originating. The surgery went well, and the seizures stopped—but not without a cost.

      Even before the surgery, Van Deren had found running therapeutic. When she felt an “aura”—the odd out-of-body sensation that, for her, signaled an impending seizure—she was often able to ward off the seizure by heading out the door and running, sometimes for hours. After the surgery, she kept running, and began venturing farther and farther into the trails near her home south of Denver. Soon she was covering distances that would have daunted even the fittest runners, and in 2002 she entered her first ultramarathon, a 50-mile trail run with only one other entrant. The 50-miler turned out to be just a stepping-stone to 100-milers, which in turn led to multi-day races like the Yukon Arctic Ultra and, eventually, the three-week assault on the Mountains-to-Sea Trail in North Carolina in 2012.

      In the final days of the record attempt, Van Deren’s feet were so beat up that she had to start each day by crawling along the trail until, thanks to the familiar numbing of endorphins, she could stand up and start putting weight on them. Then she would resume clicking off the miles, one by one. By 12:20 P.M. on the twentieth day of the run, she and Millsaps were still four miles away from the crucial 1 P.M. Okracoke ferry—so they accelerated. They caught the ferry with just minutes to spare, and the ferry operator solved the mystery of the “airplane” that had buzzed them earlier: “You must have just come through those tornadoes back there,” he marveled. Two days later, Van Deren climbed an 85-foot sand dune in Jockey’s Ridge State Park to complete the trail in a new record of 22 days, 5 hours, and 3 minutes. “That,” she told a small crowd of supporters, “is the hardest thing I have ever done.”

      In the Runner’s World profile, neuropsychologist Don Gerber, who worked with Van Deren at Craig Hospital in Denver, speculated that brain surgery might have made her a better runner. Thanks to the region of her brain that was damaged, he said, “Diane’s brain interprets pain differently than yours or mine does.”

      Van Deren, for her part, rejects this suggestion. “They all think, ‘Oh, great, you don’t feel pain,’” she argued in a subsequent profile. “Well, shit—I don’t feel pain? I feel pain. I just push through it.” And indeed, her suffering during the run in North Carolina was evident.

      Still, it’s hard to escape the sense that how Van Deren experiences a prolonged endurance challenge is inescapably different from how it is for most people. Unable to read maps or keep track of where she is on a course, she doesn’t focus on the challenge ahead of her. Hampered by poor short-term memory, she doesn’t dwell on the effort already expended, either. “I could be out running for two weeks, but if someone told me it was day one of a race,” she once joked, “I’d be like, ‘Great, let’s get started!’” Instead, she has no choice but to focus on the immediate task of forward motion, taking one more step, and then another. Semi-oblivious to the passage of time, she is also free of the cognitive challenge—the shackles, perhaps—of pacing herself. She is all hare and no tortoise—which, Aesopian morality aside, has its advantages.

      To get a visceral feel for the struggle between mind and muscle, there’s no better place to stand than at the finish line of the Comrades Marathon, the largest, oldest, and most prestigious ultra-race (that is, any race longer than the standard marathon distance of 26.2 miles) in the world, as the clock ticks down toward its rigid 12-hour cutoff. By the time the runners enter the cricket stadium in the coastal city of Durban, they’ve covered 56 miles of relentlessly undulating terrain, the downhills shredding their quads as mercilessly as the uphills burn their lungs, under the fierce South African sun. (In odd-numbered years, the course runs in the opposite direction, finishing in the inland city of Pietermaritzburg.)

      In 2010, I joined thousands of other spectators in the stadium counting down the final seconds as the race director assumed his position