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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 can also alter your sense of effort no longer seems quite so far-fetched. To prove it, Marcora and his colleagues tested a simple self-talk intervention—precisely the approach my teammates and I had laughed at two decades earlier. They had twenty-four volunteers complete a cycling test to exhaustion, then gave half of them some simple guidance on how to use positive self-talk before another cycling test two weeks later. The self-talk group learned to use certain phrases early on (“feeling good!”) and others later in a race or workout (“push through this!”), and practiced using the phrases during training to figure out which ones felt most comfortable and effective. Sure enough, in the second cycling test, the self-talk group lasted 18 percent longer than the control group, and their rating of perceived exertion climbed more slowly throughout the test. Just like a smile or frown, the words in your head have the power to influence the very feelings they’re supposed to reflect.

      As Marcora and his fellow motorcyclists rumbled across Europe and Central Asia, they were gradually becoming fitter: losing weight, increasing grip strength, gaining aerobic fitness. But they were also getting increasingly tired. Before and after each day’s ride, Marcora administered a Psychomotor Vigilance Test to his subjects, who had to tap a button as quickly as possible on a small handheld device in response to an irregular series of flashing lights. On average, their reaction time slowed from about 300 milliseconds in the morning to 350 milliseconds after nine or more hours in the saddle—a significant decrease if you’re whipping around a blind corner on a mountain road or swerving to avoid a wandering goat. The decline was most pronounced as they crossed the Tibetan plateau, where the thin air magnified the effects of mental fatigue: average end-of-ride scores on the Psychomotor Vigilance Test ballooned to 450 milliseconds.

      Fortunately, Marcora had a potent countermeasure. Tucked into his pannier of lab equipment was a stash of Military Energy Gum, a chewing gum containing 100 milligrams of caffeine that is quickly absorbed through the inner lining of your mouth. Half of the gums were the standard-issue rocket fuel; the other half were specially prepared caffeine-free placebos. Starting after lunch each day, Marcora chewed six pieces of gum, having organized and disguised them so that even he didn’t know if he was getting caffeine or not that day. When he crunched the data after the trip, the results were striking: the slowdown in reaction time between the beginning and end of the day was completely eliminated on the days his gum contained caffeine.

      Caffeine’s perk-up powers aren’t exactly a secret—without even considering coffee, caffeine pills are already one of the most widely used legal supplements among athletes—but the results illustrate how, in Marcora’s view, everything comes down to the perception of effort. There are several theories about how caffeine boosts strength and endurance. Some argue it directly enhances muscle contraction; others suggest it enhances fat oxidation to provide extra metabolic energy. To Marcora, the most convincing explanation relates to caffeine’s ability to shut down receptors in the brain that detect the presence of adenosine, a “neuromodulator” molecule associated with mental fatigue. Warding off mental fatigue, in turn, keeps your sense of effort lower, allowing you to exert yourself harder and longer.

      The demands of riding a motorcycle may seem far removed from typical tests of endurance, but in fact they closely mimic the demands encountered by soldiers, Marcora points out. In both cases, you have to maintain high levels of focus and concentration for hours at a time while doing moderate physical activity in bulky, poorly ventilated gear. And in both cases, even a brief lapse can be fatal. As a result, much of the funding for Marcora’s research, from caffeine gum to “brain endurance training,” comes from Britain’s Ministry of Defence, who are interested in ways of fighting both mental and physical fatigue.

      Closely linked to the sustained attention required by adventure motorcyclists and soldiers is another cognitive process called “response inhibition”—the ability to consciously override your impulses. This is one of the skills that Stanford University psychologist Walter Mischel tested with his famous “marshmallow test” in the late 1960s. The experimenters offered preschoolers a choice between one treat right away, or two treats if they waited for fifteen minutes. Over decades of follow-up, the children who resisted temptation the longest ended up with better test scores, more education, and lower body-mass index. Other studies have linked low response inhibition to higher risk of outcomes like divorce and even crack cocaine addiction.

      No one has checked whether the kids who aced the marshmallow test were more likely to become champion endurance athletes—but they should. For motorcyclists and soldiers, impulse inhibition matters because you have to suppress the urge to let your mind wander, and a similar challenge faces marathoners and other endurance athletes. Think of it this way: If you stick your finger in a candle flame, your natural response will be to yank it out as soon as you start feeling heat. The essence of pushing to your limits in endurance sports is learning to override that instinct so that you can hold your finger a little closer to the flame—and keep it there, not for seconds but for minutes or even hours.

      Marcora and his colleagues tested this idea in an experiment in 2014, using a technique called the Stroop task to tax their subjects’ response inhibition. The task involves words flashing on a screen in various colors; you have to press a particular button in response to each color. What’s tricky is that the words themselves are colors: you might see the word green in blue letters, and you have to overcome your initial impulse to press the button corresponding to green instead of blue. In the study, subjects performed the task twice: once with the words and colors mismatched, requiring response inhibition, and once with the words and colors matched, as a control. In both cases, after 30 minutes of the cognitive task, they ran a 5K as fast as possible on a treadmill.

      The results were clear. Even though the subjects weren’t aware of any mental fatigue, they started their 5K slower after the response inhibition version of the task, rated their level of effort higher throughout the run, and finished with times 6 percent slower. That suggests that response inhibition really is an important mental component of endurance—and that it’s a finite resource that runs low if you use it too much. Holding your finger to the flame

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