Endure. Alex Hutchinson

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 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 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 and discussions of the brain’s 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 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.

      This schism between psychology and exercise physiology is what Marcora, trained as an exercise physiologist, was hoping to address when he spent his mid-career sabbatical term studying psychology. A truly universal theory of endurance, he felt, should be able to use the same theoretical framework to explain how both mental and physical factors—self-talk and sports drinks, say—alter your performance. And in the psychobiological model that he came up with, the link between old-school sports psychology techniques and actual physiological outcomes suddenly seems much more plausible. After all, the perception of effort—the master controller of endurance, in Marcora’s view—is a fundamentally psychological construct.

      For example, a famous 1988 experiment conducted by psychologists at the University of Mannheim and the University of Illinois asked volunteers to hold a pen either in their teeth, like a dog with a bone, which required activating some of the same muscles involved in smiling; or in their lips, as if they were sucking on a straw, which activated frowning muscles. Then they were asked to rate how funny a series of Far Side cartoons were. Sure enough, the subjects rated the cartoons as funnier, by about one point on a 10-point scale, when they were (sort of) smiling. This illustrates what’s known as the “facial feedback”