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

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, 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,” 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 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.

      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. 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 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 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