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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” hypothesis, an idea that can be traced back to Charles Darwin: just as emotions trigger a physical response, that physical response can amplify or perhaps even create the corresponding emotion. Related experiments have extended this finding to clusters of related mental states: smiling, for instance, makes you happier, but it also enhances feelings of safety and—intriguingly—cognitive ease, a concept intimately tied to effort.

      Does that also apply to the effort of exercise? Marcora used EMG electrodes to record the activity of facial muscles while subjects lifted leg weights or cycled, and found a strong link between reported effort and the activation of frowning muscles during heavy exercise. A subsequent study by Taiwanese researchers also linked jaw-clenching muscles to effort. It’s no coincidence, then, that coaches have long instructed runners to “relax your face” or “relax your jaw.” One of the most famous proponents of facial relaxation was the legendary sprint coach Bud Winter, who had honed his ideas while training pilots during World War II. “Watch his lower lip,” Winter instructed a Sports Illustrated reporter who visited one of his practices in 1959, as his star sprinter streaked past. “If his lower lip is relaxed and flopping when he runs, his upper body is loose.” 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.”

      In fact, smiles and other facial expressions can have even more subtle effects, as one of Marcora’s most remarkable experiments showed. With his colleagues Anthony Blanchfield and James Hardy, of Bangor University in Wales, he paid thirteen volunteers to pedal a stationary bike at a predetermined pace for as long as they could. Such time-to-exhaustion trials are a well-established method of measuring physical limits, but in this case there was also a hidden psychological component. As the cyclists pedaled, a screen in front of them periodically flashed images of happy or sad faces in imperceptible 16-millisecond bursts, ten to twenty times shorter than a typical blink. The cyclists who were shown sad faces rode, on average, for just over 22 minutes. Those who were shown happy faces rode for three minutes longer and reported a lower sense of effort at corresponding time points. Seeing a smiling face, even subliminally, evokes feelings of ease that bleed into your perception of how hard you’re working at other tasks, like pedaling a bike.

      With these results in mind, the idea that sports psychology

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