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

In his keynote lecture at the 1996 ACSM conference, Noakes had argued that A. V. Hill’s concept of VO

max was fundamentally flawed: that physical exhaustion isn’t a consequence of the heart’s inability to pump enough oxygen to the muscles. Otherwise, he reasoned, the heart itself, and perhaps the brain, would also be starved of oxygen, with catastrophic results. He liked to point out a famous picture of South African marathoner Josia Thugwane, moments after winning the 1996 Olympic marathon, jogging around the track with silver medalist Lee Bong-Ju, whom he had outsprinted by just three seconds. “Do you notice he’s not dead?” he’d say, pointing at Lee. “What does that tell you? It means he could have run faster.”

But if Hill’s ideas about oxygen were wrong, what was the alternative? Noakes felt the brain had to be involved, and in a 1998 paper he coined the term “central governor,” (#litres_trial_promo) borrowing terminology that A. V. Hill himself had used seventy years earlier. But the details remained unclear. Over the next decade, working with collaborators such as Alan St. Clair Gibson (#litres_trial_promo), then at the University of Cape Town, Frank Marino (#litres_trial_promo), of Charles Sturt University in Australia, and a succession of other students and postdoctoral researchers in his own lab, he began to assemble a coherent picture with two key planks. First, the limits we encounter during exercise aren’t a consequence of failing muscles; they’re imposed in advance by the brain to ensure that we never reach true failure. And second, the brain imposes these limits by controlling how much muscle is recruited at a given effort level (an idea we’ll explore in detail in Chapter 6).

The first point—the concept of “anticipatory regulation,” as Noakes and his colleagues refer to it—is subtle, so it’s worth pausing to unpack it. Long before Noakes, researchers had theorized that the brain might sense distress signals from elsewhere in the body and shut things down when the warnings exceeded a critical level. Exercise in the heat is a classic example: if you run to exhaustion on a treadmill in a hot room, your brain will stop driving your muscles when your core temperature hits a critical threshold of about 40 degrees Celsius (#litres_trial_promo). But Noakes takes this idea a step further, arguing that in real-world situations like running a 10K on a hot day, the brain gets involved long before you reach that critical temperature. You don’t hit 40 and keel over; you slow down and run at a pace that keeps you below 40.

The most controversial claim is that this pacing instinct isn’t entirely voluntary: your brain forces you to slow down, long before you’re in real physiological distress. In experiments led by Noakes’s student Ross Tucker, cyclists started at a slower pace (#litres_trial_promo) right from the outset when the temperature was high—and crucially, the amount of muscle recruited by the brain was also lower within the first few minutes. At a conscious level, the cyclists were trying just as hard (as their reported level of effort indicated), but fewer muscle fibers in their legs were contracting thanks to their central governor’s inbuilt caution. The difference between the traditional and revised views of the brain’s role, Tucker explained during my visit in Cape Town, is that “they’re really looking at the off switch, whereas we’re looking at the dimmer control.”

It’s easy to get lost in the weeds of this debate. Over the course of my visit, I spent hours with various students, postdocs, and colleagues of Noakes, learning about the various tentacles of evidence that buttressed their brain-centered view of endurance. There were long-standing historical anomalies, like the puzzlingly low lactate levels (#litres_trial_promo) observed when people exercise to exhaustion at high altitudes, contrary to what Hill’s model would predict. And there was a steady stream of new observations: an instant performance boost when you swish a carbohydrate drink (#litres_trial_promo) in your mouth and then trick your brain by spitting it out; marathon runners setting world records despite supposedly crippling levels of dehydration (#litres_trial_promo); brain-altering drugs like Tylenol (#litres_trial_promo) that boost endurance without any effect on the muscles or heart.

But when I asked Noakes for the single most convincing piece of evidence in favor of his theory, he said, without hesitation, “the end spurt.” How could the runners at Comrades, after pushing themselves through 56 miles of hell, summon a finishing sprint to beat the 12-hour limit? Conventional physiology suggests that you get progressively more fatigued over the course of a run, as muscle fibers fail and fuel stores are emptied. But then, when the end is in sight, you speed up. Clearly your muscles were capable of going faster in the preceding miles; so why didn’t they? “That shows that our understanding of fatigue is totally wrong,” Noakes said. It must be the brain that holds you back during long efforts, and then releases the final reserves when you’re nearly finished and the danger is past.

I always try to evaluate scientific theories dispassionately, based on evidence rather than anecdote. But in this case, my head was nodding involuntarily as Noakes spoke. This phenomenon wasn’t just familiar to me—it was, in some ways, my nemesis. In my mid-twenties, after a few injury-plagued years, I’d moved up from 1,500 to 5,000 meters. But every time I raced the longer distance, my pace would gradually tail off in the later stages of the race—and then I’d launch a sizzling last lap, leaving everyone (including myself) puzzled about why I had slowed down so much in the previous laps. At first I chalked it up to inexperience, and then to lack of concentration. And there may be some truth to both those explanations, but it felt like something deeper.

By the time I ran what would turn out to be my fastest 5,000, on a perfect evening in Palo Alto, California, in 2003, I’d decided I needed a new mental strategy: I would pretend I was only running 4,000 meters, and simply not worry if I had to jog the last kilometer. I wanted to run 2:45 per kilometer, and my first three kilometers were 2:45, 2:45, 2:47. The moment of truth: I knuckled down and vowed to run the fourth kilometer as hard as I could—but little by little, I drifted back from the pack I was running with. My next split was a disappointing 2:53. That was as fast as I could move my legs, and my pace slowed even more as I entered the final kilometer. I’d bitten off more than I could chew and was paying the price.

At most track races, officials mark the start of your final 400-meter lap by ringing a cowbell in your ear. It’s a handy Pavlovian cue that tells you that your suffering is almost over. And on that night on the Stanford track, I once again felt the curious and familiar transformation in my legs as the bell rang for me. I passed ten runners while running the last lap in around 57 seconds, a full 10 seconds faster than my average pace for the race. My last kilometer, at 2:42, was my fastest even though I only started sprinting with a lap to go. And—I can’t emphasize this enough—I was trying as hard as I could right up to the penultimate lap. A friend who’d come to watch asked if I was trying to impress her by slowing down late in the race so I could finish with a flourish. No, I said, I just … But I didn’t have an explanation. I didn’t understand it myself.

As it turns out, it’s not just me. Noakes showed me a study that he, Tucker, and Michael Lambert had published in 2006, analyzing the pacing patterns of almost every world record (#litres_trial_promo) set in the modern era in the men’s 800 meter, mile, 5,000, and 10,000 meter races. For the three longer races, the pattern was startlingly consistent: after a quick start, the record breakers would settle into a steady pace until the final stages of the race. Then, even though they were running faster than they’d ever run before, and their oxygen-starved muscles were presumably awash in a sea of fatigue-inducing metabolites, they accelerated. Of the 66 world records in the 5,000 and 10,000 meters dating back to the early 1920s, the last kilometer was either the fastest of the race or the second fastest (behind the opening kilometer) in all but one. I was willing to attribute my own uneven pacing to incompetence—but these were the finest runners in history on the best day of their lives, which suggests that the pattern is more deeply ingrained than a mere pacing error.

In fact, there’s good reason to think that pacing is driven as much by instinct as by choice, according to Dominic Micklewright, a researcher at the University of Essex. Micklewright followed an unorthodox route to academia, going straight from high school to the Royal Navy, where he served as a diver on nuclear submarines for seven years, and then spending nine years as a police officer in London before studying sport and exercise psychology. His interest in pacing dates back to his training as a military diver, when he and the other trainees had to swim submerged to the other end of a 1,200-meter saltwater lake on Horsea Island, on Britain’s south coast, without using up their supply of air. “If they caught you breaching, (#litres_trial_promo) you would get clobbered over the back of the head with an oar, or they’d throw in one of those underwater scare charges,” he recalls. With that incentive, you inevitably thought very carefully about the challenge of spending your energy—and oxygen—as frugally as possible.

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World records in long-distance races are run with a strikingly consistent pattern that includes an acceleration in the final stages, according to a 2006 analysis in the International Journal of Sports Physiology and Performance. This finishing kick is notably absent in shorter 800-meter races, for reasons we’ll discuss in Chapter 6. The intermediate splits above are every 400 meters for the two shorter races, and every 1,000 meters for the two longer ones. (#ulink_058c2626-1f51-5a71-b606-77d3a01c3769)

In 2012, Micklewright had more than a hundred schoolchildren (#litres_trial_promo) ranging in age from five to fourteen complete a battery of tests to assess their cognitive development, in order to slot them into one of the four developmental stages proposed by Swiss psychologist Jean Piaget; then the kids ran a race lasting about four minutes. The younger kids in the two lower Piaget stages opted for the unfettered sprint-and-then-hang-on-for-dear-life approach, starting fast then steadily fading. In contrast, the kids in the two higher Piaget stages had already adopted the familiar U-shaped pacing profile that world-record holders use: a fast start, gradual slowing, then a fast finish. Sometime around the age of eleven or twelve, in other words, our brains have already learned to anticipate our future energy needs and hold back something in reserve—a relic, Micklewright speculated, of the delicate balance between searching for food and conserving energy deep in our evolutionary past.

Not everyone buys Noakes’s argument that pacing patterns like the end spurt reveal the workings of a central governor. For example, it could be that you speed up at the end of a race because you finally tap into your precious but limited reserves of anaerobic energy, the high-octane fuel source that powers you in short races lasting less than a minute. But there are other hints that the finishing kick isn’t just physiological.

In 2014, a group of economists from the University of Southern California; the University of California, Berkeley; and the University of Chicago mined a massive dataset containing the finish times of more than nine million marathoners (#litres_trial_promo) from races around the world spanning four decades. The distribution of finishing times looks a bit like the classic bell-shaped curve, but with a set of spikes superimposed. Around every significant time barrier—three hours, four hours, five hours—there are far more finishers than you’d expect just below the barrier, and fewer than you’d expect just above. Similar but smaller spikes show up at half-hour intervals, and there are barely perceptible ripples even at ten-minute increments. The cruel metabolic demands of the marathon, which inevitably depletes your stores of readily available fuel, mean that most people are slowing in the final miles. But with the right incentive, some are able to speed up—and it’s only the brain that can respond to abstract incentives like breaking four hours for an arbitrary distance like 26.2 miles.

A further curious detail from this dataset: the faster the runners were, the less likely they were able to summon a finishing sprint. Of the runners finishing near the three-hour barrier, about 30 percent were able to speed up in the final 1.4 miles of the race; 35 percent of those trying to break four hours sped up; and more than 40 percent of those trying to break five hours managed it. One possible interpretation is that, over the course of their long hours of training, the more committed runners had gradually readjusted the settings on their central governors, learning to leave as little as possible in reserve. Perhaps that’s another, slower way of achieving the run-in-the-present-moment strength that allows Diane Van Deren to race so close to her limits. I tried to trick myself into forgetting the last kilometer of my 5,000-meter races; Van Deren’s bittersweet gift is that she can forget without even trying.

Right from the start, the central governor proposal was highly controversial. After his 1996 speech, Noakes recalls, “people got very, very angry.” There were rebuttals and surrebuttals in a cycle that is still continuing, more than two decades later. In a 2008 article in the British Journal of Sports Medicine, Noakes argued that physiologists’ focus on VO

max had “produced a brainless model of human exercise performance.” (#litres_trial_promo) Roy Shephard, an influential professor emeritus at the University of Toronto, shot back with an article in the journal Sports Medicine in 2009 titled “Is It Time to Retire the ‘Central Governor’?” Following a further exchange, Shephard concluded, “In the parlance of my North American colleagues, (#litres_trial_promo) the time may now be ripe for proponents of the hypothesis to ‘Put up or shut up.’”

If anything, the controversies swirling around Noakes have increased since his retirement from the University of Cape Town in 2014. His book on hydration, Waterlogged, accused most of the world’s leading hydration researchers, including former colleagues and collaborators, of selling out to the commercial interests of sports-drink makers. He is now a vocal proponent of low-carb, high-fat diets for both health and athletic performance, leading him to disown the chapters he wrote on nutrition and carbohydrate loading in Lore of Running and earning him a disciplinary hearing (#litres_trial_promo) that threatened to revoke his medical license after he tweeted advice to a breastfeeding mother about weaning babies onto a low-carb, high-fat diet.

As these other battles rage, the central governor controversy has to some extent faded into the background. With their own retirements on the horizon, it’s clear that the older generation of physiologists—Noakes’s peers—will never be convinced. On the other hand, says American Society of Exercise Physiologists cofounder Robert Robergs of Noakes’s influence, “most of the younger breed of exercise physiologists, in which I would group myself, recognize that, boy, some of his challenges are correct.” Whether the brain plays a role in defining the limits of endurance is no longer in doubt; the debate now is how.

One way to answer that question would be to peer inside the brain during strenuous exercise—a task that, until recently, was completely impossible. With advances in brain imaging, it’s now just very, very difficult. Functional magnetic resonance imaging, or fMRI, allows researchers to observe changes in blood flow to different regions of the brain with great spatial precision, but can’t capture changes that occur in less than a second or two. You also have to remain perfectly still inside the bore of a powerful magnet—a restriction that presents serious challenges for exercise studies. During my visit to Cape Town, Noakes showed me video of a Rube Goldberg–esque contraption (#litres_trial_promo), developed by collaborators in Brazil, that allows subjects to pedal an externally mounted bike (you can’t have metal parts in the same room as the MRI magnet) via a 10-foot-long driveshaft, while lying supine inside the cylindrical bore of the magnet, with cushions jammed around their heads to keep them still. But the initial results, published in 2015, didn’t manage to push subjects to exhaustion and produced unclear patterns of brain activity.

Other researchers have tried electroencephalography (#litres_trial_promo), or EEG, which uses a web of electrodes mounted on the head to measure the brain’s electrical activity. The advantage of EEG is that it can truly measure changes in real time; the disadvantage is that it’s highly sensitive to body or head motion—just blinking or letting your gaze wander garbles the results. Such studies are already yielding insights about the brain areas involved in fatigue, and (as we’ll see in Chapter 12) even being used to identify promising regions for electrical stimulation in an attempt to enhance endurance.

But these approaches are unlikely to ever truly pinpoint the central governor. “One of the big issues with the central governor is that it was initially portrayed to be a specific point, as if there was going to be one structure that did all this,” Tucker told me. “And people were like, show me the structure.” But endurance isn’t simply a dial in the brain; it’s a complex behavior that will involve nearly every brain region, Tucker suspects, which makes proving its existence (or nonexistence) a dauntingly abstract challenge.

Ultimately, the most convincing route to proving the central governor’s existence might also be the first and most obvious question that pops into people’s minds when they first hear about the theory, which is: Can you change its settings? Can you gain access to at least some of the emergency reserve of energy that your brain protects? There’s no doubt that some athletes are able to wring more out of their bodies than others, and those who finish with the most in reserve would dearly love to be able to reduce that margin of safety. But is this really a consequence of the brain’s subconscious decision to throttle back muscle recruitment—or is it, as a rival brain-centered theory of endurance posits, simply a matter of how badly you want it?

CHAPTER 4 (#ulink_9fc3f68b-d69e-5f7b-a8a2-b83d01a45b34)

The Conscious Quitter (#ulink_9fc3f68b-d69e-5f7b-a8a2-b83d01a45b34)

Since the days of Marco Polo, no trip along the Silk Road has ever been straightforward—and Samuele Marcora’s 13,000-mile motorcycle ride (#litres_trial_promo) from London overland to Beijing in 2013 was no exception. Unlike Polo, Marcora didn’t encounter any dragons or men with dogs’ faces along the route, but he and his trip-mates did spend seventeen hours crossing the Caspian Sea on a rusty Soviet-era freighter; navigate the crumbling roads and stifling bureaucracy of Turkmenistan, Uzbekistan, Tajikistan, and Kyrgyzstan (the ’Stans, as he refers to them affectionately); skid along endless soft sand and mud trails in the thin air of the Tibetan plateau, up to 16,700 feet above sea level, for two weeks; and splash through monsoon-drenched roads on the final leg of their journey through China. Oh, and he also broke his ankle in Uzbekistan and shattered a rib on the road from Everest Base Camp, making the bone-rattling corrugated roads of Central Asia even more painful than normal.

In a sense, all of these stressors were part of the plan. Their inevitability was the reason Marcora, an exercise scientist in the University of Kent’s Endurance Research Group, joined the eighty-day expedition, which was organized by adventure motorcycling outfitter GlobeBusters. Packed on the back of Marcora’s BMW R1200GS Triple Black was his “lab in a pannier,” crammed with portable scientific equipment to perform daily measurements of the trip’s mounting mental and physical toll, with himself and his thirteen fellow riders as lab rats: swallowable thermometer pills to record core temperature, “bioharness” straps to record heart rhythms and breathing rate, a finger-mounted oximeter to measure oxygen saturation in the blood, a grip-strength tester to measure muscular fatigue, a portable reaction-time device to assess cognitive fatigue, and more.

Marcora’s interest in adventure motorcycling dates back to his teens. His first long trip, as a fourteen-year-old growing up in northern Italy, was a solo ride of more than 100 miles from his hometown outside Milan to Lake Maggiore, near the Swiss border, to visit his girlfriend. He taped a map to the gas tank of his 50cc Fantic Caballero dirt bike and navigated on back roads, to avoid the highways he wasn’t yet allowed to drive on. But he also nurtured an interest in bikes of the nonpowered variety—and, more broadly, in the enduring riddle of endurance. He trained as an exercise physiologist, and early in his career served as a consultant for Mapei Sport Service, a research center charged with providing a scientific edge for one of the top road cycling teams in the world in the 1990s and early 2000s, publishing research on mountain biking and soccer. His focus, as for thousands of other physiologists around the world, was on figuring out how to extend the limits of the human body by a percent here and a fraction of a percent there.

It was his mother—a very important figure in any Italian man’s life, he says, only half-jokingly—who gave his career trajectory a crucial nudge in a new direction. In 2001 she was diagnosed with thrombotic thrombocytopenic purpura, a rare autoimmune disorder that causes tiny blood clots to form in small blood vessels throughout the body. After one attack, she was left with kidney damage that necessitated seven years of dialysis and, eventually, a transplant. What puzzled her son was the seemingly subjective nature of the extreme fatigue that she and other patients with similar conditions endured, which fluctuated rapidly and couldn’t be clearly linked to any single physical root cause—a disconnect reminiscent of other enigmatic conditions like chronic fatigue syndrome. The feeling of fatigue was debilitating, but from the usual below-the-neck perspective of an exercise physiologist, there was seemingly nothing to fix.

This riddle led Marcora to the brain—and to tackle it, he decided he needed to learn more about what brain experts already knew. In 2006, he took a sabbatical from his teaching position at the University of Bangor, in Wales, to take courses in the university’s psychology department. Over the next few years, he formulated a new “psychobiological” model of endurance, integrating exercise physiology, motivational psychology, and cognitive neuroscience. In his view, the decision to speed up, slow down, or quit is always voluntary, not forced on you by the failure of your muscles. Fatigue, in other words, ultimately resides in the brain—an insight as relevant to motorcyclists as to marathoners. As Marcora rolled along the Silk Road collecting data on the mental and physical performance of his fellow adventure riders, he was gathering support for his contention that mind and muscle are inextricably linked—a brain-centered view of endurance, like Tim Noakes’s central governor, but with several key differences.

In 2011, I drove 120 miles through Australia’s Blue Mountains (#litres_trial_promo) from Sydney, where I was living at the time, to an old gold-rush town in the country’s sparsely populated interior called Bathurst. The local campus of Charles Sturt University was hosting an international conference called “The Future of Fatigue: Defining the Problem”—a title that reflected the continuing controversy and confusion surrounding even the most basic concepts in endurance research. “Every time I say the word ‘fatigue’ I have to put it in quotes,” joked one of the hosts, “because I’m not even sure what it means.” Scientists from around the world had gathered to present their ideas and try to hash out their differences. One of the featured speakers, and the main reason I’d decided to make the trip, was Samuele Marcora.

Marcora had made his first big splash two years earlier, not just among researchers but among the New York Times–reading public (#litres_trial_promo), with a provocative study of mental fatigue. He’d asked sixteen volunteers to complete a pair of time-to-exhaustion tests on a stationary bike. Before one of the tests, the subjects spent 90 minutes performing a mentally fatiguing computer task that involved watching a series of letters flash on a screen, and clicking different buttons as quickly as possible depending on which letters appeared. It’s not a particularly difficult task, but it requires sustained focus—and doing it for 90 minutes is definitely draining. Before the other cycling test, the subjects spent the same 90 minutes watching a pair of bland documentaries (“World Class Trains—The Venice Simplon Orient Express” and “The History of Ferrari—The Definitive Story”), specifically chosen to be “emotionally neutral.”

Depending on how you look at it, the results were either utterly predictable or, from the perspective of textbook physiology, inexplicable. After the mentally draining computer game, the subjects gave up 15.1 percent sooner in the cycling test, stopping on average at 10 minutes and 40 seconds compared to 12 minutes and 34 seconds. It wasn’t because of any detectable physiological fatigue: heart rate, blood pressure, oxygen consumption, lactate levels, and a host of other metabolic measurements were identical during the two trials. Motivation levels, as measured by psychological questionnaires immediately before the cycling tests, were the same—helped along by a ?50 prize for top performance. The only difference was that, right from the very first pedal stroke, the mentally fatigued subjects reported higher levels of perceived exertion. When their brains were tired, pedaling a bike simply felt harder.

The system Marcora used to measure perceived exertion was called the Borg Scale, named for Swedish psychologist Gunnar Borg, who pioneered its use in the 1960s. Though there are many variations, Borg’s original scale ran from 6 (“no effort at all”) to a maximum of 20 (the penultimate value, 19, was defined as “very, very hard”), with the numbers corresponding very roughly to your expected heart rate divided by ten. A Borg score of 13 to 14, for example, corresponds to an effort you’d call “somewhat hard,” which would produce a heart rate of 130 to 140 beats per minute in most people. But Borg viewed the effort scale as far more than a convenient shortcut for researchers whose heart-rate monitor ran out of batteries. “In my opinion,” he wrote, “perceived exertion is the single best indicator of the degree of physical strain,” (#litres_trial_promo) since it integrates information from muscles and joints, the cardiovascular and respiratory systems, and the central nervous system.

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In the conventional “human machine” view of endurance (top), physical fatigue in the muscles directly causes you to slow down or stop; how hard the effort feels is merely an incidental by-product. In Samuele Marcora’s psychobiological model (bottom), effort is what connects physical fatigue to performance—which means that anything that alters your perception of effort (subliminal messages, mental fatigue, etc.) can alter your endurance, independent of what’s happening in your muscles. (#ulink_8d18bfde-b0c5-5e55-a26f-6c37e9212e25)

In his talk at the conference in Bathurst, Marcora took this argument a step further. Perceived exertion—what we’ll refer to in this book as your sense of effort—isn’t just a proxy for what’s going on in the rest of your body, he argued. It’s the final arbiter, the only thing that matters. If the effort feels easy, you can go faster; if it feels too hard, you stop. That may sound obvious, or even tautological, but it’s a profound statement—because, as we’ll discover, there are lots of ways you can alter your sense of effort, and thus your apparent physical limits, without altering what’s happening in your muscles. Case in point: getting mentally fatigued increases your sense of effort (by between one and two points on the Borg scale, in Marcora’s protocol) and thus reduces endurance. By definition, the cyclists always decided to quit as their perceived exertion approached the maximum of 20; they just reached that point sooner when they were mentally fatigued.

If effort is the yin of Marcora’s psychobiological model, motivation is the yang. We’re not always willing to push to an effort of 20, which is one reason athletes rarely produce world records or even personal bests in training. In his talk, Marcora offered a now-classic illustration of this, from a 1986 experiment by French researcher Michel Cabanac (#litres_trial_promo). Cabanac asked volunteers to sit bent-legged against a wall with no chair for as long as they could, offering varying rewards for each 20-second period they stayed in position. When the subjects were offered 0.2 francs per 20 seconds, their quads gave out after just over two minutes, on average; when they were offered 7.8 francs per 20 seconds, their endurance magically doubled. If the moment of collapse was dictated by a failure of the muscles, how did the muscles know about the richer payoff?

Marcora himself produced a similar mind-over-muscle demonstration (#litres_trial_promo) with a group of elite rugby players who competed in a time-to-exhaustion cycling test. At an average target power of 242 watts, which corresponded to 80 percent of their peak power, the players lasted for about 10 minutes, with cash prizes to ensure they fully exhausted themselves. As soon as they gave up—within three to four seconds—they were asked to see how much power they could generate in a single 5-second burst of pedaling. Curiously, although they had just declared themselves incapable of producing 242 watts, they managed to average 731 watts during this five-second sprint. It follows that the subjects didn’t stop the test because their muscles were physically incapable of producing the required power; instead, the researchers argued, it was perception of effort that mattered.

At the exercise physiology conference in Bathurst, Marcora laid out his case with characteristic zeal. Amid the mostly uniform crowd of tracksuit-clad ex-athletes, he cut a swashbuckling figure, with untucked shirt, perma stubbled jaw, and casual asides about his plan to motorcycle along Australia’s Great Ocean Road after the conference. At one point, he showed a bewilderingly complex slide taken from a recent paper (#litres_trial_promo) describing the conventional model of endurance fatigue—a flow chart with forty-four different boxes ranging from heart rate to “mitochondrial density/enzyme activity”—and then compared it to the equations for general relativity and quantum mechanics. “Physicists can explain the whole universe with two theories, and they’re not happy with that,” he said. “Endurance performance is complicated, but it’s not more complicated than the entire universe!”

The simple alternative, Marcora argued, is that anything that moves the “effort dial” in your head up or down affects how far or fast you can run. All the usual physical cues—dehydration, tired muscles, a pounding heart—contribute to how hard an effort feels. Athletes train their bodies to adapt to those cues, and over time the effort of running at a given pace gets lower. But less obvious factors, like mental fatigue, also contribute to how hard your run feels—and trying to hold marathon pace for hours and hours, for example, is pretty taxing on the brain. This, Marcora told the conference, leads to a radical idea: If you could train the brain to become more accustomed to mental fatigue, then—just like the body—it would adapt and the task of staying on pace would feel easier. “I have an eye for things that at a superficial level seem crazy,” he said. “If I tell somebody, okay, I’m going to improve your endurance performance by making you sit in front of a computer and do things on a keyboard, you will think I’m nuts. But if something can fatigue you, and you repeat it over time systematically, you’ll adapt and get better at the task. That’s the basis of physical training. So my reasoning is simple: We should be able to get the same effect by using mental fatigue.”

This was an unexpectedly bold prediction, so I cornered Marcora during a break after his talk to find out more. He was designing a study to test whether “brain endurance training”—weeks of doing mentally fatiguing computer tasks—could, without any change in physical training, make people faster. I pestered him for details and asked if I could try it. He patiently answered my questions, then added a warning. “People who have done these mental fatigue studies—it’s not nice,” he said. “It’s really bad. They hate you at the end of the task.”

In June 1889, as the academic term at the University of Turin drew to a close, a physiologist named Angelo Mosso conducted a series of experiments (#litres_trial_promo) on his fellow professors before and after they administered their year-end oral exams. He attached a two-kilogram weight to a string, and asked the professors to raise and lower the weight every two seconds by flexing their middle fingers, and then repeated the task using electric shocks to force the fingers to contract. The number of contractions they managed after three and a half hours of grilling their students was dramatically reduced compared to their baseline performance—a clear indication that “intellectual labor” had sapped their muscular endurance.

Mosso’s results, which were collected in an influential text called La Fatica (“Fatigue”) in 1891, were the first scientific demonstration of the physical effects of mental fatigue. Like later fatigue researchers such as A. V. Hill and David Bruce Dill, Mosso was motivated by concerns about industrial working conditions. For Mosso, the working-class son of an impoverished carpenter, the conditions in sulfur mines and Sicilian farms, particularly for child laborers, amounted to an injustice “worse than slavery, worse than the dungeon.” Just as mental fatigue sapped physical strength, he argued, physical fatigue stunted mental growth in overworked child miners, so that “those who survive become wicked, villainous, and cruel.” By rigorously measuring the effects of fatigue, he hoped to encourage the passage of laws to protect the vulnerable—for instance, by limiting the workday of children between nine and eleven to at most eight hours.

Unlike Marcora’s results 120 years later, Mosso’s mental-fatigue studies weren’t seen as particularly surprising. This was before the idea of the “human machine” had become entrenched, so the idea that physical performance might depend as much on willpower as on muscle power seemed natural. As time passed, though, Mosso’s insights were mostly forgotten (#litres_trial_promo) and discussions of the brain’s role in endurance dropped out of exercise physiology textbooks. The torch passed instead to psychologists (#litres_trial_promo), who in the late 1800s began turning their attention to sports.

An 1898 study by Indiana University psychologist Norman Triplett (#litres_trial_promo), in which he explored why cyclists ride faster with others than alone, is often pegged as the debut of sports psychology as a distinct discipline. In addition to the aerodynamics of drafting—what Triplett termed the “Suction Theory” and the “Shelter Theory”—he considered psychological explanations such as “brain worry” for the link between mind and muscle, as well as the idea that heavy exercise “poisons” the blood, which in turn “benumbs the brain and diminishes its power to direct and stimulate the muscles.” He even speculated that a cyclist following behind another cyclist might become hypnotized by the motion of the wheel in front of him, producing performance-enhancing “muscular exaltation.” The field didn’t take off immediately: the first dedicated sports psychology lab in the United States, founded in 1925 at the University of Illinois, petered out in 1932 due to a lack of interest and funding. Still, by the second half of the twentieth century, sports psychology was established as a legitimate sub-field, with its own entirely separate body of knowledge about the brain’s role in endurance.

When I was in university, in the 1990s, our track team giggled through group sessions with a sports psychologist who introduced us to an arsenal of techniques meant to help us perform optimally—visualization, relaxation, and so on. We memorized a five-step self-talk technique for stopping negative thoughts that might arise during a race: Recognize, Refuse, Relax, Reframe, Resume. That’s what we would yell to anyone who started to drift off the pace during a long, grueling workout. It was a joke to us. None of us actually tried to apply these techniques with any seriousness—because victory, we knew, was the straightforward result of pumping the most oxygen to the fittest muscles.