On a frigid Saturday night in the university town of Sherbrooke, Quebec, in February 1996, I was pondering—yet again—one of the great enigmas of endurance: John Landy. The stocky Australian is one of the most famous bridesmaids in sport, the second man in history to run a sub-four-minute mile. In the spring of 1954, after years of concerted effort, centuries of timed races, millennia of evolution, Roger Bannister beat him to it by just 46 days. The enduring image of Landy, immortalized in countless posters and a larger-than-life bronze statue in Vancouver, British Columbia, comes from later that summer, at the Empire Games, when the world’s only four-minute milers clashed head-to-head for the first and only time. Having led the entire race, Landy glanced over his left shoulder as he entered the final straightaway—just as Bannister edged past on his right. That split-second tableau of defeat confirmed him as, in the words of a British newspaper headline, the quintessential “nearly man.”

But Landy’s enigma isn’t that he wasn’t quite good enough. It’s that he clearly was. In pursuit of the record, he had run 4:02 on six different occasions and eventually declared, “Frankly, I think the four-minute mile is beyond my capabilities. Two seconds may not sound much, but to me it’s like trying to break through a brick wall.” Then, less than two months after Bannister blazed the trail, Landy ran 3:57.9 (his official mark in the record books is 3:58.0, since times were rounded to the nearest fifth of a second in that era), cleaving almost four seconds off his previous best and finishing 15 yards ahead of a four-minute pace—a puzzlingly rapid, and bittersweet, transformation.

Like many milers before me and since, I was a Bannister disciple, with a creased and nearly memorized copy of his autobiography in permanent residence on my bedside table, but in that winter of 1996, I was seeing more and more Landy when I looked in the mirror. Since the age of 15, I’d been pursuing my own, lesser four-minute barrier—for 1,500 meters, a race that’s about 17 seconds shorter than a mile. I ran 4:02 in high school, and then, like Landy, hit a wall, running similar times again and again over the next four years. Now, as a 20-year-old junior at McGill University, I was starting to face the possibility that I’d squeezed out every second my body had to offer. During the long bus ride from Montreal to Sherbrooke, where my teammates and I were headed for a meaningless early season race on one of the slowest tracks in Canada, I remember staring out the window into the swirling snow and wondering if my long-sought moment of Landyesque transformation would ever arrive.

To break four minutes, I would need to execute a perfectly calibrated run, pacing each lap just two-tenths of a second faster than my best time of 4:01.7. Sherbrooke, with its amusement-park track and an absence of good competition, was not the place for this supreme effort, I decided. Instead, I would run as easily as possible and save my energy for the following week. Then, in the race before mine, I watched my teammate Tambra Dunn sprint fearlessly to an enormous early lead in the women’s 1,500, click off lap after metronomic lap all alone, and finish with a scorching personal best time that qualified her for the national collegiate championships. Suddenly my obsessive calculating and endless strategizing seemed ridiculous and overwrought. I was here to run a race; why not just run as hard as I could?

Reaching the “limits of endurance” is a concept that seems yawningly obvious until you actually try to explain it. Had you asked me in 1996 what was holding me back from sub-four, I would have mumbled something about maximal heart rate, lung capacity, slow-twitch muscle fibers, lactic acid accumulation, and various other buzzwords I’d picked up from the running magazines I devoured. On closer examination, though, none of those explanations hold up. You can hit the wall with a heart rate well below max, modest lactate levels, and muscles that still twitch on demand. To their frustration, physiologists have found that the will to endure can’t be reliably tied to any single physiological variable.

Part of the challenge is that endurance is a conceptual Swiss Army knife. It’s what you need to finish a marathon; it’s also what enables you to keep your sanity during a cross-country flight crammed into the economy cabin with a flock of angry toddlers. The use of the word endurance in the latter case may seem metaphorical, but the distinction between physical and psychological endurance is actually less clear-cut than it appears. Think of Ernest Shackleton’s ill-fated Antarctic expedition and the crew’s two-year struggle for survival after their ship, the Endurance, was crushed in the ice in 1915. Was it the toddlers-on-a-plane type of endurance that enabled them to persevere, or straightforward physical fortitude? Can you have one without the other?

A suitably versatile definition that I like, borrowing from researcher Samuele Marcora, is that endurance is “the struggle to continue against a mounting desire to stop.” That’s actually Marcora’s description of “effort” rather than endurance, but it captures both the physical and mental aspects of endurance. What’s crucial is the need to override what your instincts are telling you to do (slow down, back off, give up) and the sense of elapsed time. Taking a punch without flinching requires self-control, but endurance implies something more sustained: holding your finger in the flame long enough to feel the heat; filling the unforgiving minute with 60 seconds’ worth of distance run.

This is why endurance athletes are obsessed with their splits. As John L. Parker Jr. wrote in his cult running classic, Once a Runner, “A runner is a miser, spending the pennies of his energy with great stinginess, constantly wanting to know how much he has spent and how much longer he will be expected to pay. He wants to be broke at precisely the moment he no longer needs his coin.” In my race in Sherbrooke, I knew I needed to run each 200-meter lap in just under 32 seconds in order to break four minutes, and I had spent countless training hours learning the feel of this exact pace. So it was a shock, an eye-widening physical jolt to my system, to hear the timekeeper call out, as I completed my first circuit of the track, “Twenty-seven!”

Reaching the “limits of endurance” is a concept that seems yawningly obvious until you actually try to explain it.

The science of how we pace ourselves turns out to be surprisingly complex. You judge what’s sustainable based not only on how you feel, but on how that feeling compares to how you expected to feel at that point in the race. As I started my second lap, I had to reconcile two conflicting inputs: the intellectual knowledge that I had set off at a recklessly fast pace, and the subjective sense that I felt surprisingly, exhilaratingly good. I fought off the panicked urge to slow down and came through the second lap in 57 seconds—and still felt good. Now I knew for sure that something special was happening.

As the race proceeded, I stopped paying attention to the split times. They were so far ahead of the four-minute schedule I’d memorized that they no longer conveyed any useful information. I simply ran, hoping to reach the finish before the gravitational pull of reality reasserted its grip on my legs. I crossed the line in 3:52.7, a personal best by a full nine seconds. In that one race, I’d improved more than my cumulative improvement since my first season of running, five years earlier. Poring through my training logs—as I did that night and have many times since—revealed no hint of the breakthrough to come. My workouts suggested, at most, incremental gains compared to previous years.

After the race, I debriefed with a teammate who had timed my lap splits for me. His watch told a very different story of the race. My first lap had taken 30 seconds, not 27; my second lap was 60, not 57. Perhaps the lap counter calling the splits at the finish had started his watch three seconds late; or perhaps his effort to translate on the fly from French to English for my benefit had resulted in a delay of a few seconds. Either way, he’d misled me into believing that I was running faster than I really was, while feeling unaccountably good. As a result, I’d unshackled myself from my prerace expectations and run a race nobody could have predicted.

After Roger Bannister came the deluge—at least, that’s how the story is often told. Typical of the genre is The Winning Mind Set, a 2006 self-help book by Jim Brault and Kevin Seaman that uses Bannister’s four-minute mile as a parable about the importance of self-belief. “[W]ithin one year, 37 others did the same thing,” they write. “In the year after that, over 300 runners ran a mile in less four minutes.” Similar larger-than-life (that is, utterly fictitious) claims are a staple in motivational seminars and across the web: once Bannister showed the way, others suddenly brushed away their mental barriers and unlocked their true potential. But to draw any meaningful conclusions, it’s important to get the facts right. For one thing, Landy was the only other person to join the sub-four club within a year of Bannister’s run, and just four others followed the next year. It wasn’t until 1979, more than 20 years later, that Spanish star José Luis González became the 300th man to break the barrier.

And there’s more to Landy’s sudden breakthrough, after being stuck for so many races, than simple mind over muscle. His six near-misses all came at low-key meets in Australia where competition was sparse and weather often unfavorable. He finally embarked on the long voyage to Europe, where tracks were fast and competition plentiful, in the spring of 1954—only to discover, just three days after he arrived, that Bannister had already beaten him to the goal. In Helsinki, he had a pacer for the first time, a local runner who led the first lap and a half at a brisk pace. And more important, he had real competition: Chris Chataway, one of the two men who had paced Bannister’s sub-four run, was nipping at Landy’s heels until partway through the final lap. It’s not hard to believe that Landy would have broken four that day even if Roger Bannister had never existed.

Still, I can’t entirely dismiss the mind’s role—in no small part because of what happened in the wake of my own breakthrough. In my next attempt at the distance after Sherbrooke, I ran 3:49. In the race after that, I crossed the line, as confused as I was exhilarated, in 3:44, qualifying me for that summer’s Olympic Trials. In the space of three races, I’d somehow been transformed. The TV coverage of the 1996 trials is on YouTube, and as the camera lingers on me before the start of the 1,500 final (I’m lined up next to Graham Hood, the Canadian record-holder at the time), you can see that I’m still not quite sure how I got there. My eyes keep darting around in panic, as if I expect to glance down and discover that I’m still in my pajamas.

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.

I spent a lot of time over the next decade chasing further breakthroughs, with decidedly mixed results. Knowing (or believing) that your ultimate limits are all in your head doesn’t make them any less real in the heat of a race. And it doesn’t mean you can simply decide to change them. If anything, my head held me back as often as it pushed me forward during those years, to my frustration and befuddlement. “It should be mathematical,” is how U.S. Olympic runner Ian Dobson described the struggle to understand the ups and downs of his own performances, “but it’s not.” I, too, kept searching for the formula—the one that would allow me to calculate, once and for all, my limits. If I knew that I had run as fast as my body was capable of, I reasoned, I’d be able to walk away from the sport with no regrets.

At 28, after an ill-timed stress fracture in my sacrum three months before the 2004 Olympic Trials, I finally decided to move on. I returned to school for a journalism degree, and then started out as a general assignment reporter with a newspaper in Ottawa. But I found myself drawn back to the same lingering questions. Why wasn’t it mathematical? What held me back from breaking four for so long, and what changed when I did? I left the newspaper and started writing as a freelancer about endurance sports—not so much about who won and who lost, but about why. I dug into the scientific literature and discovered that there was a vigorous (and sometimes rancorous) ongoing debate about those very questions.

Physiologists spent most of the 20th 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 scientists around the world 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.

Adapted from the book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Copyright ©2018 by Alex Hutchinson. Reprinted by permission of William Morrow, an imprint of HarperCollins Publishers.