Endure. Alex Hutchinson

a finger-mounted oximeter to measure oxygen saturation in the blood, a grip-strength tester to measure muscular fatigue, a portable reaction-time device to assess cognitive fatigue, and more.

      Marcora’s interest in adventure motorcycling dates back to his teens. His first long trip, as a fourteen-year-old growing up in northern Italy, was a solo ride of more than 100 miles from his hometown outside Milan to Lake Maggiore, near the Swiss border, to visit his girlfriend. He taped a map to the gas tank of his 50cc Fantic Caballero dirt bike and navigated on back roads, to avoid the highways he wasn’t yet allowed to drive on. But he also nurtured an interest in bikes of the nonpowered variety—and, more broadly, in the enduring riddle of endurance. He trained as an exercise physiologist, and early in his career served as a consultant for Mapei Sport Service, a research center charged with providing a scientific edge for one of the top road cycling teams in the world in the 1990s and early 2000s, publishing research on mountain biking and soccer. His focus, as for thousands of other physiologists around the world, was on figuring out how to extend the limits of the human body by a percent here and a fraction of a percent there.

      It was his mother—a very important figure in any Italian man’s life, he says, only half-jokingly—who gave his career trajectory a crucial nudge in a new direction. In 2001 she was diagnosed with thrombotic thrombocytopenic purpura, a rare autoimmune disorder that causes tiny blood clots to form in small blood vessels throughout the body. After one attack, she was left with kidney damage that necessitated seven years of dialysis and, eventually, a transplant. What puzzled her son was the seemingly subjective nature of the extreme fatigue that she and other patients with similar conditions endured, which fluctuated rapidly and couldn’t be clearly linked to any single physical root cause—a disconnect reminiscent of other enigmatic conditions like chronic fatigue syndrome. The feeling of fatigue was debilitating, but from the usual below-the-neck perspective of an exercise physiologist, there was seemingly nothing to fix.

      This riddle led Marcora to the brain—and to tackle it, he decided he needed to learn more about what brain experts already knew. In 2006, he took a sabbatical from his teaching position at the University of Bangor, in Wales, to take courses in the university’s psychology department. Over the next few years, he formulated a new “psychobiological” model of endurance, integrating exercise physiology, motivational psychology, and cognitive neuroscience. In his view, the decision to speed up, slow down, or quit is always voluntary, not forced on you by the failure of your muscles. Fatigue, in other words, ultimately resides in the brain—an insight as relevant to motorcyclists as to marathoners. As Marcora rolled along the Silk Road collecting data on the mental and physical performance of his fellow adventure riders, he was gathering support for his contention that mind and muscle are inextricably linked—a brain-centered view of endurance, like Tim Noakes’s central governor, but with several key differences.

      In 2011, I drove 120 miles through Australia’s Blue Mountains from Sydney, where I was living at the time, to an old gold-rush town in the country’s sparsely populated interior called Bathurst. The local campus of Charles Sturt University was hosting an international conference called “The Future of Fatigue: Defining the Problem”—a title that reflected the continuing controversy and confusion surrounding even the most basic concepts in endurance research. “Every time I say the word ‘fatigue’ I have to put it in quotes,” joked one of the hosts, “because I’m not even sure what it means.” Scientists from around the world had gathered to present their ideas and try to hash out their differences. One of the featured speakers, and the main reason I’d decided to make the trip, was Samuele Marcora.

      Marcora had made his first big splash two years earlier, not just among researchers but among the New York Times–reading public, with a provocative study of mental fatigue. He’d asked sixteen volunteers to complete a pair of time-to-exhaustion tests on a stationary bike. Before one of the tests, the subjects spent 90 minutes performing a mentally fatiguing computer task that involved watching a series of letters flash on a screen, and clicking different buttons as quickly as possible depending on which letters appeared. It’s not a particularly difficult task, but it requires sustained focus—and doing it for 90 minutes is definitely draining. Before the other cycling test, the subjects spent the same 90 minutes watching a pair of bland documentaries (“World Class Trains—The Venice Simplon Orient Express” and “The History of Ferrari—The Definitive Story”), specifically chosen to be “emotionally neutral.”

      Depending on how you look at it, the results were either utterly predictable or, from the perspective of textbook physiology, inexplicable. After the mentally draining computer game, the subjects gave up 15.1 percent sooner in the cycling test, stopping on average at 10 minutes and 40 seconds compared to 12 minutes and 34 seconds. It wasn’t because of any detectable physiological fatigue: heart rate, blood pressure, oxygen consumption, lactate levels, and a host of other metabolic measurements were identical during the two trials. Motivation levels, as measured by psychological questionnaires immediately before the cycling tests, were the same—helped along by a £50 prize for top performance. The only difference was that, right from the very first pedal stroke, the mentally fatigued subjects reported higher levels of perceived exertion. When their brains were tired, pedaling a bike simply felt harder.

      The system Marcora used to measure perceived exertion was called the Borg Scale, named for Swedish psychologist Gunnar Borg, who pioneered its use in the 1960s. Though there are many variations, Borg’s original scale ran from 6 (“no effort at all”) to a maximum of 20 (the penultimate value, 19, was defined as “very, very hard”), with the numbers corresponding very roughly to your expected heart rate divided by ten. A Borg score of 13 to 14, for example, corresponds to an effort you’d call “somewhat hard,” which would produce a heart rate of 130 to 140 beats per minute in most people. But Borg viewed the effort scale as far more than a convenient shortcut for researchers whose heart-rate monitor ran out of batteries. “In my opinion,” he wrote, “perceived exertion is the single best indicator of the degree of physical strain,” since it integrates information from muscles and joints, the cardiovascular and respiratory systems, and the central nervous system.

In the conventional “human machine” view of endurance (top), physical fatigue in the muscles directly causes you to slow down or stop; how hard the effort feels is merely an incidental by-product. In Samuele Marcora’s psychobiological model (bottom), effort is what connects physical fatigue to performance—which means that anything that alters your perception of effort (subliminal messages, mental fatigue, etc.) can alter your endurance, independent of what’s happening in your muscles.

      In his talk at the conference in Bathurst, Marcora took this argument a step further. Perceived exertion—what we’ll refer to in this book as your sense of effort—isn’t just a proxy for what’s going on in the rest of your body, he argued. It’s the final arbiter, the only thing that matters. If the effort feels easy, you can go faster; if it feels too hard, you stop. That may sound obvious, or even tautological, but it’s a profound statement—because, as we’ll discover, there are lots of ways you can alter your sense of effort, and thus your apparent physical limits, without altering what’s happening in your muscles. Case in point: getting mentally fatigued increases your sense of effort (by between one and two points on the Borg scale, in Marcora’s protocol) and thus reduces endurance. By definition, the cyclists always decided to quit as their perceived exertion approached the maximum of 20; they just reached that point sooner when they were mentally fatigued.

      If effort is the yin of Marcora’s psychobiological model, motivation is the yang. We’re not always willing to push to an effort of 20, which is one reason athletes rarely produce world records or even personal bests in training. In his talk, Marcora offered a now-classic illustration of this, from a 1986 experiment by French researcher Michel Cabanac. Cabanac asked volunteers to sit bent-legged against a wall with no chair for as long as they could, offering varying rewards for each 20-second period they stayed in position. When the subjects