Alex Hutchinson.

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

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 worlds 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 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, its clear that the older generation of physiologistsNoakess peerswill never be convinced. On the other hand, says American Society of Exercise Physiologists cofounder Robert Robergs of Noakess 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 exercisea task that, until recently, was completely impossible. With advances in brain imaging, its 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 cant capture changes that occur in less than a second or two. You also have to remain perfectly still inside the bore of a powerful magneta restriction that presents serious challenges for exercise studies. During my visit to Cape Town, Noakes showed me video of a Rube Goldbergesque contraption, developed by collaborators in Brazil, that allows subjects to pedal an externally mounted bike (you cant 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, didnt manage to push subjects to exhaustion and produced unclear patterns of brain activity.

Other researchers have tried electroencephalography, or EEG, which uses a web of electrodes mounted on the head to measure the brains electrical activity. The advantage of EEG is that it can truly measure changes in real time; the disadvantage is that its highly sensitive to body or head motionjust blinking or letting your gaze wander garbles the results. Such studies are already yielding insights about the brain areas involved in fatigue, and (as well 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 isnt simply a dial in the brain; its 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 governors existence might also be the first and most obvious question that pops into peoples 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? Theres 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 brains subconscious decision to throttle back muscle recruitmentor is it, as a rival brain-centered theory of endurance posits, simply a matter of how badly you want it?

The Conscious Quitter

Since the days of Marco Polo, no trip along the Silk Road has ever been straightforwardand Samuele Marcoras 13,000-mile motorcycle ride from London overland to Beijing in 2013 was no exception. Unlike Polo, Marcora didnt 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 Kents Endurance Research Group, joined the eighty-day expedition, which was organized by adventure motorcycling outfitter GlobeBusters. Packed on the back of Marcoras BMW R1200GS Triple Black was his lab in a pannier, crammed with portable scientific equipment to perform daily measurements of the trips 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.

Marcoras 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 wasnt yet allowed to drive on. But he also nurtured an interest in bikes of the nonpowered varietyand, 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 mothera very important figure in any Italian mans life, he says, only half-jokinglywho 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 couldnt be clearly linked to any single physical root causea 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 brainand 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 universitys 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 brainan 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 linkeda brain-centered view of endurance, like Tim Noakess central governor, but with several key differences.

In 2011, I drove 120 miles through Australias Blue Mountains from Sydney, where I was living at the time, to an old gold-rush town in the countrys sparsely populated interior called Bathurst. The local campus of Charles Sturt University was hosting an international conference called The Future of Fatigue: Defining the Problema 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 Im 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 Id 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 Timesreading public, with a provocative study of mental fatigue. Hed 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. Its not a particularly difficult task, but it requires sustained focusand 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 TrainsThe Venice Simplon Orient Express and The History of FerrariThe 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 wasnt 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 samehelped 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, Borgs 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 youd 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, since it integrates information from muscles and joints, the cardiovascular and respiratory systems, and the central nervous system.

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 Marcoras psychobiological model (bottom), effort is what connects physical fatigue to performancewhich means that anything that alters your perception of effort (subliminal messages, mental fatigue, etc.) can alter your endurance, independent of whats happening in your muscles.

In his talk at the conference in Bathurst, Marcora took this argument a step further. Perceived exertionwhat well refer to in this book as your sense of effortisnt just a proxy for whats going on in the rest of your body, he argued. Its 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 its a profound statementbecause, as well discover, there are lots of ways you can alter your sense of effort, and thus your apparent physical limits, without altering whats 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 Marcoras 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 Marcoras psychobiological model, motivation is the yang. Were 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. 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 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 upwithin three to four secondsthey 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 didnt 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 Australias Great Ocean Road after the conference. At one point, he showed a bewilderingly complex slide taken from a recent paper describing the conventional model of endurance fatiguea flow chart with forty-four different boxes ranging from heart rate to mitochondrial density/enzyme activityand then compared it to the equations for general relativity and quantum mechanics. Physicists can explain the whole universe with two theories, and theyre not happy with that, he said. Endurance performance is complicated, but its 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 cuesdehydration, tired muscles, a pounding heartcontribute 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 feelsand 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, thenjust like the bodyit 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, Im going to improve your endurance performance by making you sit in front of a computer and do things on a keyboard, you will think Im nuts. But if something can fatigue you, and you repeat it over time systematically, youll adapt and get better at the task. Thats 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 trainingweeks of doing mentally fatiguing computer taskscould, 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 studiesits not nice, he said. Its 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 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 performancea clear indication that intellectual labor had sapped their muscular endurance.

Mossos 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 vulnerablefor instance, by limiting the workday of children between nine and eleven to at most eight hours.

Unlike Marcoras results 120 years later, Mossos mental-fatigue studies werent 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, Mossos insights were mostly forgotten and discussions of the brains role in endurance dropped out of exercise physiology textbooks. The torch passed instead to psychologists, who in the late 1800s began turning their attention to sports.

An 1898 study by Indiana University psychologist Norman Triplett, 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 draftingwhat Triplett termed the Suction Theory and the Shelter Theoryhe 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 didnt 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 brains role in endurance.

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