Endure. Alex Hutchinson

traits like economy and “fractional utilization” along with VO₂max—the equivalent of considering a car’s fuel economy and the size of its gas tank in addition to its raw horsepower.

      It was in this context that Michael Joyner proposed his now-famous 1991 thought experiment on the fastest possible marathon. As a restless undergraduate in the late 1970s, Joyner had been on the verge of dropping out of the University of Arizona—at six-foot-five, and with physical endurance that eventually enabled him to run a 2:25 marathon, he figured he might make a pretty good firefighter—when he was outkicked at the end of a 10K race by a grad student from the school’s Exercise and Sport Science Laboratory. After the race, the student convinced Joyner to volunteer as a guinea pig in one of the lab’s ongoing experiments, a classic study that ended up demonstrating that lactate threshold, the fastest speed you can maintain without triggering a dramatic rise in blood lactate levels, is a remarkably accurate predictor of marathon time. The seed was planted and Joyner was soon volunteering at the lab and embarking on the first stages of an unexpected new career trajectory that eventually led to a position as physician-researcher at the Mayo Clinic, where he is now one of the world’s mostly widely cited experts on the limits of human performance.

      That first study on lactate threshold offered Joyner a glimpse of physiology’s predictive power. The fact that such an arcane lab test could pick the winner, or at least the general gist of finishing order, among a group of endurance athletes was a tantalizing prospect. And when, a decade later, Joyner finally pushed this train of thought to its logical extreme, he arrived at a very specific number: 1:57:58. It was a ridiculous, laughable number—a provocation. Either the genetics needed to produce such a performance were exceedingly rare, he wrote in the paper’s conclusions, “or our level of knowledge about the determinants of human performance is inadequate.”

      By Day 56, the relentless physical demands of Henry Worsley’s solo trans-Antarctic trek were taking a toll. He woke that morning feeling weaker than he’d felt at any point in the expedition, his strength sapped by a restless night repeatedly interrupted by a “bad stomach.” He set off as usual, but gave up after an hour and slept for the rest of the day. “You have to listen to your body sometimes,” he admitted in his audio diary.

      Still, he was more than 200 miles from his destination and already behind his planned schedule. So he roused himself that night, packed up his tent, and set off again at ten minutes after midnight under the unblinking polar sun. He was approaching the high point of the journey, slogging up a massive ice ridge known as the Titan Dome, more than 10,000 feet above sea level. The thin air forced him to take frequent breaks to catch his breath, and a stretch of sandy, blowing snow bogged his sled down and slowed his progress for several hours. By 4 P.M., having covered 16 miles in 16 hours, he was once again utterly spent. He had hoped to cross from the 89th degree of southern latitude—the one closest to the South Pole—into the 88th, but he was forced to stop one mile short of his goal. “There was nothing left in the tank,” he reported. “I had completely run empty.”

      The next day was January 9, the day that Shackleton had famously turned back from his South Pole quest in 1909. “A live donkey is better than a dead lion, isn’t it?” Shackleton had said to his wife when he returned to England. Worsley was camped just 34 miles from Shackleton’s turnaround latitude, and he marked the anniversary with a small cigar—which he chomped with a gap-toothed grin, having lost a front tooth to a frozen energy bar a few days earlier—and a dram of Dewar’s Royal Brackla Scotch whiskey, a bottle of which he had hauled across the continent.

      Of the many advantages Worsley had over Shackleton, perhaps the most powerful was the Iridium satellite phone he carried in his pack, with which he could choose at any moment to call for an air evacuation. But this blessing was also a curse. In calculating his limits, Shackleton had been forced to leave a margin of error due to the impossibility of predicting how the return journey would go. Worsley’s access to near-instantaneous help, on the other hand, allowed him to push much closer to the margins—to empty his tank day after day, after struggling through the snow for 12, 14, or 16 hours; to ignore his increasing weakness and 50-pound weight loss; to fight on even as the odds tilted further against him.

      Eventually, it became clear that he wouldn’t make it to his scheduled pickup. He’d been trying to log 16-hour days to get back on schedule, but soft snow and whiteouts combined with his continuing physical deterioration to derail him. He contemplated a shorter goal of reaching the Shackleton glacier, but even that proved out of reach. On January 21, his seventieth day of travel, he made the call. “When my hero Ernest Shackleton stood 97 [nautical] miles from the South Pole on the morning on January 9, 1909, he said he’d shot his bolt,” Worsley reported in his audio diary. “Well today, I have to inform you with some sadness that I too have shot my bolt. My journey is at an end. I have run out of time, physical endurance, and the simple sheer ability to slide one ski in front of the other.”

      The next day, he was picked up for the six-hour flight back to Union Glacier, where logistical support for Antarctic expeditions is based, and then airlifted to the hospital in Punta Arenas, Chile, to be treated for exhaustion and dehydration. It was a disappointing end to the expedition, but Worsley appeared to have successfully followed Shackleton’s advice to remain a “live donkey.” In the hospital, though, the situation took an unexpected turn: Worsley was diagnosed with bacterial peritonitis, an infection of the abdominal lining, and rushed into surgery. On January 24, at the age of fifty-five, Henry Worsley died of widespread organ failure, leaving behind a wife and two children.

      When avalanches claim a skier, or sharks attack a surfer, or a puff of unexpected wind dooms a wingsuit flier, it’s always news. Like these other “extreme” deaths, Worsley’s tragic end was reported and discussed around the world. There was a difference, though. There had been no avalanche, no large, hungry predator, no high-speed impact. He didn’t freeze to death, he wasn’t lost, and he still had plenty of food to eat. Though it may never be clear exactly what pushed him over the edge, he seemed, in essence, to have voluntarily driven himself to oblivion—a rarity that added a grim fascination to his demise. “In exploring the outer limits of endurance,” Britain’s Guardian newspaper asked, “did Worsley not realize he’d surpassed his own?”

      In a sense, Worsley’s death seemed a vindication of the mathematical view of human limits. “The machinery of the body is all of a chemical or physical kind. It will all be expressed some day in physical and chemical terms,” Hill had predicted in 1927. And every machine, no matter how great, has a maximum capacity. Worsley, in trying to cross Antarctica on his own, had embarked on a mission that exceeded his body’s capacity, and no amount of mental strength and tenacity could change that calculation.

      But if that’s true, then why is death by endurance so rare? Why don’t Olympic marathoners and Channel swimmers and Appalachian Trail hikers keel over on a regular basis? That’s the riddle a young South African doctor named Tim Noakes posed to himself as he was preparing to deliver the most important talk of his life, a prestigious honorary lecture at the annual meeting of the American College of Sports Medicine, in 1996: “I said, now hold on. What is really interesting about exercise is not that people die of, say, heatstroke; or when people are climbing Everest, it’s not that one or two die,” he later recalled. “The fact is, the majority don’t die—and that is much more interesting.”

CHAPTER 3

       The Central Governor

      To catch the ferry, Diane Van Deren needed to cover 36 miles in just over 8 hours. That would normally be no problem for the veteran ultra-runner—except, in this case, for the unforgiving terrain, the torrential rain and sumo-force winds left in the wake of Tropical Storm Beryl, and the fatigue and horrendous blisters accrued over the first 19 days and 900 miles of the Mountains-to-Sea Trail across North Carolina. Worse, Van Deren was startled to hear a “savage and malicious” roar from the darkness to her right. “What is that?” she yelled to her trail guide, Chuck Millsaps, the owner of a local outfitting company. It was just an airplane,