Endure. Alex Hutchinson

of Canberra and the neighboring Australian Institute of Sport, working with Marcora, decided to do. They recruited eleven elite professional cyclists and compared them with nine trained amateur cyclists. All the volunteers completed two 20-minute time trials, one preceded by a 30-minute Stroop task to deplete their response inhibition, the other preceded by a control task of simply gazing at a black cross on a white screen for 10 minutes.

      The first interesting finding was that the professionals were significantly better at the Stroop task, amassing an average of 705 correct responses during the 30-minute test compared to 576 for the amateurs. In other words, to the list of measurable traits that distinguish the pros from the rest of us—the size of their heart, the number of capillaries feeding their muscles, their lactate threshold, and so on—we can now add response inhibition.

      The second interesting finding was how the cyclists performed in the time trial after completing the response-inhibiting Stroop task. The amateurs, depleted by the mental effort of focusing on all those flashing letters, produced 4.4 percent less power than in their control ride. The pros, on the other hand, didn’t slow down at all. They were able to resist the effects of mental fatigue, at least in the doses produced by a 30-minute Stroop task, and cycle just as fast as when they were fresh.

      There are two ways to explain these findings. One is that the pros were born with superior response inhibition and resistance to mental fatigue, and that’s one of the reasons they’ve ended up as elite athletes. The other is that long years of training help the mind adapt to resist mental fatigue, just as the body adapts to resist physical fatigue. Which is it? I suspect a bit of both, and the smattering of evidence that exists supports the idea that these traits are partly inherited but also can be improved with training. And this, in turn, raises the really big question: What’s the best way to boost your mental endurance? Marcora’s idea, as he proposed back in 2011 at the conference in Bathurst, is that specially tailored cognitive challenges like the Stroop task, repeated over and over, constitute a form of “brain endurance training” that can give athletes an edge. As I’ll describe in Chapter 11, I visited the University of Kent for a brain-training boot camp, and then tried out the technique for twelve weeks while preparing for a marathon. Marcora has also run a series of military-funded trials of the technique—and the initial results suggest he’s onto something big.

      The studies described in this chapter make it clear that we can’t talk about the limits of endurance without considering the brain and perception of effort. But they don’t necessarily mean that Marcora’s psychobiological theory is right. In fact, not everyone agrees his theory is even new. Tim Noakes, when I asked him about Marcora’s ideas in 2010, dismissed them as a minor variation of his own central governor model: “The only distinction between our model and his model—and he has to differentiate, obviously—is that everything is consciously controlled,” he said.

      The distinction between conscious and unconscious has become a bitterly contested flashpoint between the two camps, but the differences aren’t as great as they appear. Marcora does indeed argue that the decision to speed up, slow down, or stop is always conscious and voluntary. But such “decisions,” he acknowledges, can be effectively forced on you by an intolerably high sense of effort. And crucially, they can still be influenced by any number of factors that you’re not consciously aware of, as demonstrated most clearly by his own experiment with subliminal images. Noakes and his colleagues, on the other side, don’t dispute the importance of effort, motivation, and conscious decision making. When you run a marathon, it’s not the central governor that prevents you from sprinting for the first 100 meters (a fact demonstrated by the enthusiastic souls who do, in fact, sprint at the start of marathons and later pay the price).

      It’s true, though, that there are some real contrasts between Noakes’s and Marcora’s theories, and they’re most obvious at the limits of total exhaustion—a state most people rarely, if ever, encounter. Imagine going to the gym, setting the treadmill to 10 miles per hour, and deciding to run for as long as you can. For most people, the decision to step off will be purely voluntary, a simple result of the effort becoming greater than they’re willing to tolerate. But if, instead, you’re running the final mile of the Olympic marathon, neck-and-neck with a rival for the gold medal, it’s harder to accept that the runner who slackens first does so because the effort feels too great or because she’s not motivated enough. Noakes would argue that the runner’s brain is overriding her conscious desires, reducing muscle recruitment in order to prevent damage to critical organs—and that process is not only unconscious, but is flatly contradicting the runner’s conscious decisions. To anyone who has raced seriously, it’s the latter explanation that feels right.

      Of course, the other option is that such scenarios of truly maximal effort and motivation push you to plain old physical limits—that, as A. V. Hill would have argued nearly a century ago, it’s muscle fatigue or the limits of oxygen delivery that hold you back in the final mile of the Olympics. When I first started planning this book, in 2009, it was going to be all about Tim Noakes and how his ideas had upended the conventional body-centric view of endurance. Then I discovered Marcora’s work, and realized that no explanation of endurance could be complete without considering the psychology involved. And then, as I dug deeper, I got to know some of the physiologists who don’t believe either of them, and whose views of human endurance are still rooted in the heart, lungs, and muscles—like University of Exeter physiologist Andrew Jones, who helped guide Paula Radcliffe to a marathon world record and whose Breaking2 lab data suggests Eliud Kipchoge is capable of a sub-two-hour run. And I discovered that they, too, have some powerful evidence to back their views.

      So who is right? The short answer is that scientists are currently fighting about it, strenuously and sometimes bitterly, with no end in sight. The longer—and to me, more interesting—answer is that, as the comparison above between running on a treadmill in the gym and racing in the Olympics illustrates, it depends. In Part II of the book, we’ll explore how specific factors like pain, oxygen, heat, thirst, and fuel define your limits in different contexts. We’ll encounter situations that seem to confirm Noakes’s view, like sports drinks that boost your endurance even if you don’t swallow them. We’ll explore whether it’s really possible for a panicked mother to lift a car off her child. And we’ll see what happens when an injection in the spine temporarily removes the limits imposed by the brain, allowing athletes to push their muscles all the way to the brink—a dream scenario that turns out to be more of a nightmare.