Physiologists spent most of the twentieth century on an epic quest to understand how our bodies fatigue. They cut the hind legs off frogs and jolted the severed muscles with electricity until they stopped twitching; lugged cumbersome lab equipment on expeditions to remote Andean peaks; and pushed thousands of volunteers to exhaustion on treadmills, in heat chambers, and on every drug you can think of. What emerged was a mechanistic—almost mathematical—view of human limits: like a car with a brick on its gas pedal, you go until the tank runs out of gas or the radiator boils over, then you stop.
But that’s not the whole picture. With the rise of sophisticated techniques to measure and manipulate the brain, researchers are finally getting a glimpse of what’s happening in our neurons and synapses when we’re pushed to our limits. It turns out that, whether it’s heat or cold, hunger or thirst, or muscles screaming with the supposed poison of “lactic acid,” what matters in many cases is how the brain interprets these distress signals. With new understanding of the brain’s role come new—and sometimes worrisome—opportunities. At its Santa Monica, California, headquarters, Red Bull has experimented with transcranial direct-current stimulation, applying a jolt of electricity through electrodes to the brains of elite triathletes and cyclists, seeking a competitive edge. The British military has funded studies of computer-based brain training protocols to enhance the endurance of its troops, with startling results. And even subliminal messages can help or hurt your endurance: a picture of a smiling face, flashed in 16-millisecond bursts, boosts cycling performance by 12 percent compared to frowning faces.
Over the past decade, I’ve traveled to labs in Europe, South Africa, Australia, and across North America, and spoken to hundreds of scientists, coaches, and athletes who share my obsession with decoding the mysteries of endurance. I started out with the hunch that the brain would play a bigger role than generally acknowledged. That turned out to be true, but not in the simple it’s-all-in-your-head manner of self-help books. Instead, brain and body are fundamentally intertwined, and to understand what defines your limits under any particular set of circumstances, you have to consider them both together. That’s what the scientists described in the following pages have been doing, and the surprising results of their research suggest to me that, when it comes to pushing our limits, we’re just getting started.
After fifty-six days of hard skiing, Henry Worsley glanced down at the digital display of his GPS and stopped. “That’s it,” he announced with a grin, driving a ski pole into the wind-packed snow. “We’ve made it!” It was early evening on January 9, 2009, one hundred years to the day since British explorer Ernest Shackleton had planted a Union Jack in the name of King Edward VII at this precise location on the Antarctic plateau: 88 degrees and 23 minutes south, 162 degrees east. In 1909, it was the farthest south any human had ever traveled, just 112 miles from the South Pole. Worsley, a gruff veteran of the British Special Air Service who had long idolized Shackleton, cried “small tears of relief and joy” behind his goggles, for the first time since he was ten years old. (“My poor physical state accentuated my vulnerability,” he later explained.) Then he and his companions, Will Gow and Henry Adams, unfurled their tent and fired up the kettle. It was −35 degrees Celsius.
For Shackleton, 88°23' south was a bitter disappointment. Six years earlier, as a member of Robert Falcon Scott’s Discovery expedition, he’d been part of a three-man team that set a farthest-south record of 82°17'. But he had been sent home in disgrace after Scott claimed that his physical weakness had held the others back. Shackleton returned for the 1908–09 expedition eager to vindicate himself by beating his former mentor to the pole, but his own four-man inland push was a struggle from the start. By the time Socks, the team’s fourth and final Manchurian pony, disappeared into a crevasse on the Beardmore glacier six weeks into the march, they were already on reduced rations and increasingly unlikely to reach their goal. Still, Shackleton decided to push onward as far as possible. Finally, on January 9, he acknowledged the inevitable: “We have shot our bolt,” he wrote in his diary. “Homeward bound at last. Whatever regrets may be, we have done our best.”
To Worsley, a century later, that moment epitomized Shackleton’s worth as a leader: “The decision to turn back,” he argued, “must be one of the greatest decisions taken in the whole annals of exploration.” Worsley was a descendant of the skipper of Shackleton’s ship in the Endurance expedition; Gow was Shackleton’s great-nephew by marriage; and Adams was the great-grandson of Shackleton’s second in command on the 1909 trek. The three of them had decided to honor their forebears by retracing the 820-mile route without any outside help. They would then take care of unfinished ancestral business by continuing the last 112 miles to the South Pole, where they would be picked up by a Twin Otter and flown home. Shackleton, in contrast, had to turn around and walk the 820 miles back to his base camp—a return journey that, like most in the great age of exploration, turned into a desperate race against death.
What were the limits that stalked Shackleton? It wasn’t just beard-freezingly cold; he and his men also climbed more than 10,000 feet above sea level, meaning that each icy breath provided only two-thirds as much oxygen as their bodies expected. With the early demise of their ponies, they were man-hauling sleds that had initially weighed as much as 500 pounds, putting continuous strain on their muscles. Studies of modern polar travelers suggest they were burning somewhere between 6,000 and 10,000 calories per day—and doing it on half rations. By the end of their journey, they would have consumed close to a million calories over the course of four relentless months, similar to the totals of the subsequent Scott expedition of 1911–12. South African scientist Tim Noakes argues these two expeditions were “the greatest human performances of sustained physical endurance of all time.”
Shackelton’s understanding of these various factors was limited. He knew that he and his men needed to eat, of course, but beyond that the inner workings of the human body remained shrouded in mystery. That was about to change, though. A few months before Shackleton’s ship, the Nimrod, sailed toward Antarctica from the Isle of Wight in August 1907, researchers at the University of Cambridge published an account of their research on lactic acid, an apparent enemy of muscular endurance that would become intimately familiar to generations of athletes. While the modern view of lactic acid has changed dramatically in the century since then (for starters, what’s found inside the body is actually lactate, a negatively charged ion, rather than lactic acid), the paper marked the beginning of a new era of investigation into human endurance—because if you understand how a machine works, you can calculate its ultimate limits.
The nineteenth-century Swedish chemist Jöns Jacob Berzelius is now best remembered for devising the modern system of chemical notation—H2O and CO2 and so on—but he was also the first, in 1807, to draw the connection between muscle fatigue and a recently discovered substance found in soured milk. Berzelius noticed that the muscles of hunted stags seemed to contain high levels of this “lactic” acid, and that the amount of acid depended on how close to exhaustion the