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      As these other battles rage, the central governor controversy has to some extent faded into the background. With their own retirements on the horizon, it’s clear that the older generation of physiologists—Noakes’s peers—will never be convinced. On the other hand, says American Society of Exercise Physiologists cofounder Robert Robergs of Noakes’s influence, “most of the younger breed of exercise physiologists, in which I would group myself, recognize that, boy, some of his challenges are correct.” Whether the brain plays a role in defining the limits of endurance is no longer in doubt; the debate now is how.

      One way to answer that question would be to peer inside the brain during strenuous exercise—a task that, until recently, was completely impossible. With advances in brain imaging, it’s now just very, very difficult. Functional magnetic resonance imaging, or fMRI, allows researchers to observe changes in blood flow to different regions of the brain with great spatial precision, but can’t capture changes that occur in less than a second or two. You also have to remain perfectly still inside the bore of a powerful magnet—a restriction that presents serious challenges for exercise studies. During my visit to Cape Town, Noakes showed me video of a Rube Goldberg–esque contraption, developed by collaborators in Brazil, that allows subjects to pedal an externally mounted bike (you can’t have metal parts in the same room as the MRI magnet) via a 10-foot-long driveshaft, while lying supine inside the cylindrical bore of the magnet, with cushions jammed around their heads to keep them still. But the initial results, published in 2015, didn’t manage to push subjects to exhaustion and produced unclear patterns of brain activity.

      Other researchers have tried electroencephalography, or EEG, which uses a web of electrodes mounted on the head to measure the brain’s electrical activity. The advantage of EEG is that it can truly measure changes in real time; the disadvantage is that it’s highly sensitive to body or head motion—just blinking or letting your gaze wander garbles the results. Such studies are already yielding insights about the brain areas involved in fatigue, and (as we’ll see in Chapter 12) even being used to identify promising regions for electrical stimulation in an attempt to enhance endurance.

      But these approaches are unlikely to ever truly pinpoint the central governor. “One of the big issues with the central governor is that it was initially portrayed to be a specific point, as if there was going to be one structure that did all this,” Tucker told me. “And people were like, show me the structure.” But endurance isn’t simply a dial in the brain; it’s a complex behavior that will involve nearly every brain region, Tucker suspects, which makes proving its existence (or nonexistence) a dauntingly abstract challenge.

      Ultimately, the most convincing route to proving the central governor’s existence might also be the first and most obvious question that pops into people’s minds when they first hear about the theory, which is: Can you change its settings? Can you gain access to at least some of the emergency reserve of energy that your brain protects? There’s no doubt that some athletes are able to wring more out of their bodies than others, and those who finish with the most in reserve would dearly love to be able to reduce that margin of safety. But is this really a consequence of the brain’s subconscious decision to throttle back muscle recruitment—or is it, as a rival brain-centered theory of endurance posits, simply a matter of how badly you want it?

CHAPTER 4

       The Conscious Quitter

      Since the days of Marco Polo, no trip along the Silk Road has ever been straightforward—and Samuele Marcora’s 13,000-mile motorcycle ride from London overland to Beijing in 2013 was no exception. Unlike Polo, Marcora didn’t encounter any dragons or men with dogs’ faces along the route, but he and his trip-mates did spend seventeen hours crossing the Caspian Sea on a rusty Soviet-era freighter; navigate the crumbling roads and stifling bureaucracy of Turkmenistan, Uzbekistan, Tajikistan, and Kyrgyzstan (the ’Stans, as he refers to them affectionately); skid along endless soft sand and mud trails in the thin air of the Tibetan plateau, up to 16,700 feet above sea level, for two weeks; and splash through monsoon-drenched roads on the final leg of their journey through China. Oh, and he also broke his ankle in Uzbekistan and shattered a rib on the road from Everest Base Camp, making the bone-rattling corrugated roads of Central Asia even more painful than normal.

      In a sense, all of these stressors were part of the plan. Their inevitability was the reason Marcora, an exercise scientist in the University of Kent’s Endurance Research Group, joined the eighty-day expedition, which was organized by adventure motorcycling outfitter GlobeBusters. Packed on the back of Marcora’s BMW R1200GS Triple Black was his “lab in a pannier,” crammed with portable scientific equipment to perform daily measurements of the trip’s mounting mental and physical toll, with himself and his thirteen fellow riders as lab rats: swallowable thermometer pills to record core temperature, “bioharness” straps to record heart rhythms and breathing rate, 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

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