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

by Alex Hutchinson

In a wide variety of human activity, achievement is not possible without discomfort.

There are many things Garmin cannot tell you. And luckily, for those many things, we have Alex Hutchinson.

The limits of endurance running, according to physiologists, could be quantified with three parameters: aerobic capacity, also known as VO2max, which is analogous to the size of a car’s engine; running economy, which is an efficiency measure like gas mileage; and lactate threshold, which dictates how much of your engine’s power you can sustain for long periods of time.

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.

This is why the psychology and physiology of endurance are inextricably linked: any task lasting longer than a dozen or so seconds requires decisions, whether conscious or unconscious, on how hard to push and when. Even in repeated all-out weightlifting efforts—brief five-second pulls that you’d think would be a pure measure of muscular force—studies have found that we can’t avoid pacing ourselves: your “maximum” force depends on how many reps you think you have left.

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.”

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 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.

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.

The experiments on Hill and his colleagues involved running in tight circles around an 85-meter grass loop in Hill’s garden (a standard track, in comparison, is 400 meters long) with an air bag strapped to their backs connected to a breathing apparatus to measure their oxygen consumption.19 The faster they ran, the more oxygen they consumed—up to a point. Eventually, they reported, oxygen intake “reaches a maximum beyond which no effort can drive it.”20 Crucially, they could still accelerate to faster speeds; however, their oxygen intake no longer followed. This plateau is your VO2max, a pure and objective measure of endurance capacity that is, in theory, independent of motivation, weather, phase of the moon, or any other possible excuse.

Hill surmised that VO2max reflected the ultimate limits of the heart and circulatory system—a measurable constant that seemed to reveal the size of the “engine” an athlete was blessed with.

Your VO2max reflects your aerobic limits. At higher speeds, your legs demand energy at a rate that aerobic processes can’t match, so you have to draw on fast-burning anaerobic (“without oxygen”) energy sources. The problem, as Hopkins and Fletcher had shown in 1907, is that muscles contracting without oxygen generate lactic acid. Your muscles’ ability to tolerate high levels of lactic acid—what we would now call anaerobic capacity—is the other key determinant of endurance, Hill concluded, particularly in events lasting less than about ten minutes.

The exhaustive tests in his back garden showed that his VO2max was 4.0 liters of oxygen per minute, and his lactic acid tolerance would allow him to accumulate a further “oxygen debt” of about 10 liters. Using these numbers, along with measurements of his running efficiency, he could plot a graph that predicted his best race times with surprising accuracy.

But there was a mystery at the longest distances. Hill’s calculations suggested that if the speed was slow enough, your heart and lungs should be able to deliver enough oxygen to your muscles to keep them fully aerobic. There should be a pace, in other words, that you could sustain pretty much indefinitely.

In 1981, he reported the case of Eleanor Sadler, a forty-six-year-old woman who collapsed during the Comrades Marathon, and diagnosed her problem as hyponatremia, a result of drinking too much, rather than the more common problem of drinking too little.4 It took another two decades—and a handful of deaths—before the scientific community fully acknowledged the dangers of overdrinking during exercise.5

Then Winter offered a first-hand demonstration of the optimal running face. “Like that,” he said, flicking his tension-free lower lip with his fingers. “It’s got to be loose.”15 In fact, smiles and other facial expressions can have even more subtle effects, as one of Marcora’s most remarkable experiments showed.

Just like a smile or frown, the words in your head have the power to influence the very feelings they’re supposed to reflect.

Closely linked to the sustained attention required by adventure motorcyclists and soldiers is another cognitive process called “response inhibition”—the ability to consciously override your impulses.

Think of it this way: If you stick your finger in a candle flame, your natural response will be to yank it out as soon as you start feeling heat. The essence of pushing to your limits in endurance sports is learning to override that instinct so that you can hold your finger a little closer to the flame—and keep it there, not for seconds but for minutes or even hours.

There are two ways to explain these findings. One is that the pros were born with superior response inhibition and resistance to mental fatigue, and that’s one of the reasons they’ve ended up as elite athletes. The other is that long years of training help the mind adapt to resist mental fatigue, just as the body adapts to resist physical fatigue. Which is it? I suspect a bit of both, and the smattering of evidence that exists supports the idea that these traits are partly inherited but also can be improved with training.

The second big factor is drafting. In my 2014 analysis, I had argued that the cost of overcoming air resistance, even on a perfectly still day, might amount to 100 seconds over the course of a two-hour marathon.3 That might seem far-fetched—until you remember that the runners will be sustaining a pace of about 4:35 per mile, which for most of us is essentially a sprint. Studies dating back to the 1970s have suggested that running directly behind another runner can eliminate most of this extra effort, but in practice it’s difficult to draft that closely behind someone else.4 And to pace a two-hour marathon all the way to the finish, you’d need someone—or preferably several people—who can run a two-hour marathon themselves, since world-record rules forbid having fresh pacers jump in partway through the race. Nike’s solution: give up on the idea of setting an official world record, so that they can deploy a large team of pacemakers who will rotate in and out of the race in order to pace the chosen ones all the way to the finish.

They considered the athletes’ swagger, their response to challenges, and other elements of attitude and outlook that might make or break the mission.

Kipchoge, whose English is fluent, is different. Though he’s so soft-spoken that you have to lean forward and squint to hear him, his words—and his demeanor, and the aura that David and I later agree he exudes—reveal a serene and imperturbable confidence. Is this what winning an Olympic gold medal does for you, I wonder? Or is it what you need to get there in the first place?

All three men, like the vast majority of the world’s best distance runners these days, were born, grew up, and train in the East African highlands along the Great Rift Valley, at elevations of at least 6,000 feet above sea level. The thin, oxygen-poor air at these heights makes running harder and triggers adaptations like an increase in the number of red blood cells available to shuttle oxygen from the lungs to working muscles. In fact, anyone born into this environment carries oxygen-sparing traits like enhanced lung volume with them for the rest of their lives. Shalane Flanagan, the second-fastest women’s marathoner in U.S. history, was born in Boulder (elevation 5,430 feet); Ryan Hall, the fastest American-born men’s marathoner, grew up in Big Bear Lake (elevation 6,752).

While great riders are often distinguished by the extremes of their physiology or their grace in the saddle, Voigt’s singular characteristic during an eighteen-year professional career was his appetite for suffering. His “open acknowledgment of pain as a state of mind to be combated, repressed and ultimately overcome,” Cycling Weekly opined, “is perhaps part of the reason he is revered by cycling fans as the hardman of the peloton.”

In the popular imagination (and the thesaurus), endurance and suffering are inextricably linked. “No pain, no gain” is a motto across most sports, but in skill sports this relationship is more negotiable, says Wolfgang Freund, a researcher at University Hospitals Ulm in Germany who studies pain in athletes. The incomparable Argentine soccer star Diego Maradona, for example, “at least had the illusion that a brilliant soccer player didn’t need to suffer,” he says. For cyclists and other endurance athletes, though, pain is unavoidable, and how you handle it is intimately tied to how well you perform.

In 2013, Freund published a telling study on the pain tolerance of ultra-endurance runners competing in the TransEurope Footrace, an epic pain-fest in which participants covered 2,789 miles over 64 days with no rest days. He asked eleven of the competitors to dunk their hands in ice water for three minutes; by the end, they rated the pain as about 6 out of 10 on average. In contrast, the nonathlete control group gave up after an average of just 96 seconds when their pain maxed out at 10; only three of them even completed the test.4 Such findings reinforce the idea that, all else being equal, the gold medal goes to whoever is willing to suffer a bit more than everyone else.

the link between what’s happening in your muscles and what you feel in your head turns out to be much more indirect than you might assume. “Pain is more than one thing,” says Dr. Jeffrey Mogil, the head of the Pain Genetics Lab at McGill University. It’s a sensation, like vision or touch; it’s an emotion, like anger or sadness; and it’s also a “drive state” that compels action, like hunger. For athletes, the role of pain depends on how these different effects mingle together in their specific situation. Sometimes pain slows them to a halt; other times it drives them to even greater heights.

This shows that simply getting fitter doesn’t magically increase your pain tolerance; how you get fit matters: you have to suffer.

Without pain, in other words, they’re incapable of pacing themselves.

If you whack your shin against a chair, your first instinct will be to rub your bruised shin with your hand. Why? Because the nonpainful sensation of rubbing competes with the pain of the bruise for the same neural signaling pathways that report back to your brain. The more you rub, the less bandwidth is left for pain signals.

If you’re looking for the midpoint between the muscle’s role in hoisting a car and the brain’s role in running an ultra, this is as good a definition as any: that agonizing point, about 600 meters into an 800-meter race, where you’re holding nothing back but can feel yourself slowing anyway. Runners have a phrase for that feeling, though it doesn’t show up in dictionaries: to rig, as in “I thought I was going to win the race, but I started rigging on the final turn.” It’s derived from rigor mortis, the stiffening of the body after death, and it’s one of those words that perfectly capture an otherwise baffling sensation.

immersion in water had triggered a set of automatic responses, including a dramatic slowing of the heartbeat, that conserved oxygen.12 These responses are now collectively known as the “mammalian dive reflex,” or in the more poetic formulation of Swedish-American researcher Per Scholander, the “Master Switch of Life.”

While it’s hard to draw definitive conclusions from two small studies, the researchers suggested that being born at altitude and having very active childhoods ensured that the Kenyans were better equipped to maintain the brain’s oxygen supply: they had more blood vessels to the brain, with thicker walls that were harder to squeeze shut.

The heat generated by your car’s engine can be pretty useful on a cold day: it’s what blasts through your heating vents to warm up the interior. The same is true for human heat production, which is why even extreme cold is rarely a limiting factor for endurance athletes, whose furnaces burn far hotter than most.

Tests by Australian sports scientists showed that a crushed ice slurry sweetened to the same degree as a sports drink could lower core temperatures by one degree Fahrenheit and, in consequence, boost endurance in the heat.

Heat doesn’t act like a light switch that flicks your muscles off; in most real-world situations, as Tucker explained to me, it’s a dimmer switch, controlled by the brain for your own protection.

In the 1980s, a biochemist (and enthusiastic marathon runner) at the University of Oxford, Eric Newsholme, proposed that fatigue during endurance exercise might result in part from changes in the concentration of neurotransmitters in the brain.30 That hypothesis didn’t pan out, but it led to string of studies testing the effects of various brain-altering drugs on endurance: Paxil, Prozac, Celexa, Effexor, Wellbutrin, Ritalin, and others. Under normal conditions, the drugs had minimal effects; but in hot conditions, drugs that increased concentrations of the neurotransmitter dopamine in the brain had a dramatic effect.

The human body is about 50 to 70 percent water, and it needs pretty much all of it.2 You’re constantly losing water, not just from sweat but also from urine and more subtle leaks like the moisture in your breath. And, under normal circumstances, you’re constantly replacing it by eating and drinking. Your fluid balance fluctuates a bit throughout the day thanks to meal and activity patterns, but from one day to the next it’s regulated with remarkable precision.

A 150-pound person typically carries around about forty liters of water, and that total is fixed to within less than a liter (one exception is the fluctuations that occur throughout a woman’s menstrual cycle, which can add and then subtract more than two liters of retained water). When you fail to replace lost fluids, you start craving a drink, and your kidneys begin reabsorbing fluid that would otherwise become urine. If that’s not enough to restore your internal balance, fluid will start draining out of your cells and into your veins and arteries to maintain the necessary volume of blood pumping through your body. These adjustments will buy you some time, but eventually your blood will get so concentrated that your brain will start shrinking as fluid is sucked out by osmosis, tearing delicate cerebral veins and ultimately killing you. According to calculations by U.S. Army researchers in a wilderness medicine textbook, you might last, in theory, for about seven days without water under ideal indoor conditions before reaching this critical point. If you’re lost in a hot desert and travel only by night, your expected survival plummets to twenty-three hours; if you also travel during the day, it’s sixteen hours.

Then came hyponatremia. The death of twenty-eight-year-old Cynthia Lucero, who collapsed four miles from the finish line of the 2002 Boston Marathon, focused worldwide attention on a problem that had first been identified more than two decades earlier.9 Though Lucero complained of feeling “dehydrated and rubber-legged” before she collapsed, hospital tests revealed the opposite problem: following the prevailing advice to athletes, she had drunk as much as she could stomach during her run, causing the levels of sodium in her blood to become diluted (that’s what “hyponatremia,” sometimes referred to as “water intoxication,” means).

This seemingly contradictory pattern—heatstroke without dehydration, dehydration without heatstroke—is no fluke, it turns out. Dehydration is a greater concern in longer races, because you have more time to sweat; heatstroke, in contrast, is most common in shorter races.

The records for longest survival without food are both grim and confusing, depending on the precise circumstances and the trustworthiness of the witnesses. A frequently cited benchmark is Kieran Doherty, an Irish Republican Army prisoner at the infamous Maze Prison near Belfast, who refused food for 73 days in 1981 before dying.4 If you bend the rules a bit to allow vitamins in addition to water, then you’ll be able to continue accessing your body’s fat stores for much longer. A 1973 journal article by a Scottish doctor reports the case of A.B., a twenty-seven-year-old man who weighed 456 pounds before undertaking a medically supervised fast lasting 382 days, during which he lost a remarkable 276 pounds.

The cliché of the pasta-fueled marathoner can be traced back to the work of Swedish scientists Jonas Bergström and Eric Hultman in 1960s. Bergström pioneered the use of needle biopsies, a technique that allowed researchers to slice out small pieces of muscle from their long-suffering research volunteers—or, as was the habit in Scandinavian labs at the time, from their own muscles.11 In one notable study, Bergström and Hultman sat on opposite sides of a stationary bike, each pedaling with one leg while the other leg rested, until they were both too exhausted to continue. Self-inflicted muscle biopsies before and after cycling showed that levels of glycogen, the form in which carbohydrate is stored in muscles, had dropped to zero in the exercised leg. Running out of this specific muscle fuel, in other words, seemed to coincide with exhaustion. Over the next three days, the two men ate a high-carbohydrate diet and performed regular biopsies. Glycogen levels stayed roughly constant in their rested legs, but in the exercised legs, levels shot up to twice their initial value—a supercompensation effect that launched the idea of “carbohydrate loading” before long-distance races.

Subsequent biopsy studies confirmed that the amount of glycogen you can stuff into your muscles is a pretty good predictor of how long you’ll last on a treadmill or stationary bike test to exhaustion. There are other sources of carbohydrate in the body; your liver, for example, can store 400 or 500 calories of glycogen for use throughout the body, compared to about 2,000 for fully loaded leg muscles. (That’s why it’s useful to eat a small breakfast a few hours before a morning marathon: while your muscles remain fully stocked, your liver glycogen gets depleted because it fuels your energy-hungry brain while you sleep.)12 Your muscles can also dip into the glucose circulating in your blood, though the total amount of glucose in circulation at any given moment is very small. Overall, the picture that emerged from these studies is relatively simple, harking back to A. V. Hill’s “human body as a machine” view: you can store a finite amount of carbohydrate fuel in your body, and when you use it up, you bonk.

If that’s the case, then it makes sense for endurance athletes to stock up on carbohydrates as much as possible. And that, more or less, is what sports nutritionists have been advocating since the 1970s. Keep your glycogen levels high by consuming a diet that gets 60 to 65 percent of its calories from carbohydrate; top up your stores by carbo-loading in the final few days before a competition; and in events lasting longer than about ninety minutes, eat or drink some easily digested carbohydrates to supplement your stored glycogen, which will otherwise run out. (Modern sports nutrition guidelines, Hawley points out, actually recommend a daily target amount of carbohydrate per pound of total body weight based on the type of training y...

In the last chapter, I described the hydration plan that Haile Gebrselassie used when he set a world record of 2:04:26 at the Berlin Marathon in 2007, which involved drinking about two liters of fluid during the race. In practice, his plan was as much focused on fueling as on hydration. Of the two liters of fluid he planned to consume during the race, 1.25 L was sports drink (the rest was water), and he also took five sports gels, providing a total of between 60 and 80 grams of carbohydrate per hour. That number is significant, because scientists have traditionally figured that 60 grams an hour (about 250 calories) is pretty much the maximum amount you can absorb during exercise. The rate-limiting step is the absorption of carbohydrate from the intestine into the bloodstream.

Other studies have found similar patterns for swallowing sports drinks in events shorter than about 90 minutes: it only helps if your body is low on fuel to start with. In practice, these findings mean that the benefits of sports drinks and other mid-race carbohydrates for short bouts of exercise are irrelevant as long as you don’t start out with an empty stomach and depleted fuel stores. (Pro tip: you shouldn’t.)

Michael Del Monte, who spent months in the heart of Kenyan running culture while filming the documentary Transcend about the rise of marathoner-turned-politician Wesley Korir, it comes down to belief. Even the humblest Kenyan runner, he noticed, wakes up every morning with the firm conviction that today, finally, will be his or her day. They run with the leaders because they think they can beat them, and if harsh reality proves that they can’t, they regroup and try again the next day. And that belief, fostered by the longstanding international dominance of generations of Kenyan runners, becomes a self-fulfilling prophecy.

On the simplest level, taking the equivalent of a sugar pill, if you really believe it will help you race faster, will often work.