Eliud Kipchoge, the Olympic marathon champion from Kenya, is famous for his incongruously serene smile during the grueling late stages of his marathon triumphs. It’s a deliberate tactic: “When you smile and you’re happy,” he says, “you can trigger the mind to feel your legs.” Sounds like wishful thinking? That’s what I thought back in college, in the 1990s, when my 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 sports psychology and exercise physiology is what Samuele Marcora, an exercise physiologist at the University of Kent’s Endurance Research Group, was hoping to address when he decided to spend his mid-career sabbatical term studying psychology in 2006. A truly universal theory of endurance, he felt, should be able to explain how both mental and physical factors—self-talk and sports drinks, say—alter your performance. And in the model that he eventually came up with, the link between old-school sports psychology techniques and actual physiological outcomes does seem plausible. Your subjective 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. While the replicability of these results has been questions, the findings are consistent with 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 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.

Adapted from the book, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance. Copyright ©2018 by Alex Hutchinson. Reprinted by permission of William Morrow, an imprint of HarperCollins Publishers.