This feedback loop is a relatively new model of endurance. Living creatures were long thought to be powered by some inscrutable, vital force. That belief gave way in the 20th century to what Hutchinson calls a “mechanistic—almost mathematical—view of human limits: Like a car with a brick on its gas pedal, you go until the tank runs out of gas or the radiator boils over, then you stop.” But more-recent research into the mind’s influence has made for much trickier analogies. All runners know that the racing experience is rarely linear. You might feel strong at the start, pained in the middle, then catch a second wind and charge to the finish. Some days you float; others you barely crawl. Physiologists broadly concur—despite plenty of heated debate over the specifics—that how your brain interprets your body’s signals sets the limit on the effort you can put in at any given moment. Tweak your mentality, and your sense of that limit can change.

William Morrow

Hutchinson tallies the conventional tools that any coach would recommend: positive thinking and visualization, good diet and hydration. He also delves into the more unsettling vanguard of expertise on breaking through the brain’s barriers. It includes “brain endurance training,” a weeks-long program of painfully boring computer tasks designed to help people fight off mental fatigue. Researchers, as well as an ever-growing scene of DIY enthusiasts, are trying transcranial direct-current stimulation (or “brain zapping”), which involves sticking dual electrodes to a subject’s skull in an effort to unlock the brain’s hidden reserves. (The practice is controversial, but some evidence indicates that it can enhance endurance and power.) In one series of experiments, scientists injected the powerful opioid fentanyl into the spines of cyclists so that they couldn’t register pain at all. The volunteers rode so hard that they couldn’t walk afterward—and they ended up pacing themselves so poorly that their times weren’t faster than usual.

Still, Hutchinson suggests, the single greatest impetus for stretching beyond your limits appears to be good old belief. No out-of-shape runner can crush a four-minute mile with motivation alone. But research shows that having an unshakable confidence in one’s ability and commitment reliably compels athletes to find that extra gear. “Training is the cake and belief is the icing,” Hutchinson reflects, “but sometimes that thin smear of frosting makes all the difference.” He offers a long list of studies that have sneakily goaded subjects into better performances:

Telling runners they look relaxed makes them burn measurably less energy to sustain the same pace. Giving rugby players a postgame debriefing that focuses on what they did right rather than what they did wrong has effects that continue to linger a full week later, when the positive-feedback group will have higher testosterone levels and perform better in the next game. Even doing a good deed—or simply imagining yourself doing a good deed—can enhance your endurance by reinforcing your sense of agency.

But beyond the boosts of trickery and experimental nudges, how is such belief instilled? How do you get the unrelenting sense of purpose that sustains, say, one of the world’s greatest ultra-marathoners? Not the way you might think: Avoiding introspection seems to be key. Hutchinson, a creditable runner himself (though his career never came close to matching Jurek’s), spends long passages puzzling over the mysteries of his own peak performances and dissecting his failures. Jurek, meanwhile, gives the impression that doubting his commitment hardly ever even occurred to him—until he hit the Appalachian Trail.