Endure: Mind, Body and the Curiously Elastic Limits of Human Performance
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When I was in university, in the 1990s, our 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 psychology and exercise physiology is what Marcora, trained as an exercise physiologist, was hoping to address when he spent his mid-career sabbatical term studying psychology. A truly universal theory of endurance, he felt, should be able to use the same theoretical framework to explain how both mental and physical factors—self-talk and sports drinks, say—alter your performance. And in the psychobiological model that he came up with, the link between old-school sports psychology techniques and actual physiological outcomes suddenly seems much more plausible. After all, the 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. This illustrates 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.
As Marcora and his fellow motorcyclists rumbled across Europe and Central Asia, they were gradually becoming fitter: losing weight, increasing grip strength, gaining aerobic fitness. But they were also getting increasingly tired. Before and after each day’s ride, Marcora administered a Psychomotor Vigilance Test to his subjects, who had to tap a button as quickly as possible on a small handheld device in response to an irregular series of flashing lights. On average, their reaction time slowed from about 300 milliseconds in the morning to 350 milliseconds after nine or more hours in the saddle—a significant decrease if you’re whipping around a blind corner on a mountain road or swerving to avoid a wandering goat. The decline was most pronounced as they crossed the Tibetan plateau, where the thin air magnified the effects of mental fatigue: average end-of-ride scores on the Psychomotor Vigilance Test ballooned to 450 milliseconds.
Fortunately, Marcora had a potent countermeasure. Tucked into his pannier of lab equipment was a stash of Military Energy Gum, a chewing gum containing 100 milligrams of caffeine that is quickly absorbed through the inner lining of your mouth. Half of the gums were the standard-issue rocket fuel; the other half were specially prepared caffeine-free placebos. Starting after lunch each day, Marcora chewed six pieces of gum, having organized and disguised them so that even he didn’t know if he was getting caffeine or not that day. When he crunched the data after the trip, the results were striking: the slowdown in reaction time between the beginning and end of the day was completely eliminated on the days his gum contained caffeine.
Caffeine’s perk-up powers aren’t exactly a secret—without even considering coffee, caffeine pills are already one of the most widely used legal supplements among athletes—but the results illustrate how, in Marcora’s view, everything comes down to the perception of effort. There are several theories about how caffeine boosts strength and endurance. Some argue it directly enhances muscle contraction; others suggest it enhances fat oxidation to provide extra metabolic energy. To Marcora, the most convincing explanation relates to caffeine’s ability to shut down receptors in the brain that detect the presence of adenosine, a “neuromodulator” molecule associated with mental fatigue. Warding off mental fatigue, in turn, keeps your sense of effort lower, allowing you to exert yourself harder and longer.
The demands of riding a motorcycle may seem far removed from typical tests of endurance, but in fact they closely mimic the demands encountered by soldiers, Marcora points out. In both cases, you have to maintain high levels of focus and concentration for hours at a time while doing moderate physical activity in bulky, poorly ventilated gear. And in both cases, even a brief lapse can be fatal. As a result, much of the funding for Marcora’s research, from caffeine gum to “brain endurance training,” comes from Britain’s Ministry of Defence, who are interested in ways of fighting both mental and physical fatigue.
Closely linked to the sustained attention required by adventure motorcyclists and soldiers is another cognitive process called “response inhibition”—the ability to consciously override your impulses. This is one of the skills that Stanford University psychologist Walter Mischel tested with his famous “marshmallow test” in the late 1960s. The experimenters offered preschoolers a choice between one treat right away, or two treats if they waited for fifteen minutes. Over decades of follow-up, the children who resisted temptation the longest ended up with better test scores, more education, and lower body-mass index. Other studies have linked low response inhibition to higher risk of outcomes like divorce and even crack cocaine addiction.
No one has checked whether the kids who aced the marshmallow test were more likely to become champion endurance athletes—but they should. For motorcyclists and soldiers, impulse inhibition matters because you have to suppress the urge to let your mind wander, and a similar challenge faces marathoners and other endurance athletes. Think of it this way: If you stick your finger in a candle flame, your natural response will be to yank it out as soon as you start feeling heat. The essence of pushing to your limits in endurance sports is learning to override that instinct so that you can hold your finger a little closer to the flame—and keep it there, not for seconds but for minutes or even hours.
Marcora and his colleagues tested this idea in an experiment in 2014, using a technique called the Stroop task to tax their subjects’ response inhibition. The task involves words flashing on a screen in various colors; you have to press a particular button in response to each color. What’s tricky is that the words themselves are colors: you might see the word green in blue letters, and you have to overcome your initial impulse to press the button corresponding to green instead of blue. In the study, subjects performed the task twice: once with the words and colors mismatched, requiring response inhibition, and once with the words and colors matched, as a control. In both cases, after 30 minutes of the cognitive task, they ran a 5K as fast as possible on a treadmill.
The results were clear. Even though the subjects weren’t aware of any mental fatigue, they started their 5K slower after the response inhibition version of the task, rated their level of effort higher throughout the run, and finished with times 6 percent slower. That suggests that response inhibition really is an important mental component of endurance—and that it’s a finite resource that runs low if you use it too much. Holding your finger to the flame (or simply focusing on a tricky computer task) takes mental effort, and that effort is just as real as the effort of moving your legs.
It has long been a clich? that the best athletes are defined as much by their superior minds as by their muscle. With response inhibition, we have a way of testing this, which is what a team based at the University 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.
A homeless man is asleep in the doorway, his grungy brown sleeping bag zipped up to his nose to keep the drizzle off. Next to his head, stowed neatly out of the weather, is a crisp, spotless pair of brightly colored Nike trainers with fluorescent yellow laces. This, I tell myself, is peak Portland. I jog a few more blocks back to my downtown hotel, shower up, and head out with David Willey to the manicured mega-campus of Nike World Headquarters to find out how, exactly, the company plans to leapfrog a half-century ahead of my predicted marathon timeline.
It’s immediately clear that the Breaking2 project isn’t just a passing whim cooked up by the marketing department. As we’re ushered through security into the Nike Sport Research Lab—an area, our escorts breathlessly assure us, that is strictly off-limits even to the vast majority of Nike employees on the site—we pass a massive mural at the end of a hallway that doubles as a two-lane rubberized running track. It reads, in pixelated scoreboard font, “1:59:59.” Some twenty people have been working on the secret project, more or less full-time, for nearly two years, with a total cost that the company won’t disclose but clearly extends to millions, if not tens of millions, of dollars.
The barrier-breaking science behind the plan? You name it, they’re willing to try it. In a series of meetings that stretches late into the evening, we hear from the company’s top physiologists, biomechanists, and product designers about the lengths they’ve gone to in contemplating how to squeeze extra inches from exhausted muscles. Some of the crazier ideas have, perhaps mercifully, been left on the cutting-room floor—like pinning your arms to your sides to save wasted motion and energy. Tests on former elite runner Matt Tegenkamp using a specially designed elastic sling showed a measurable efficiency boost, but “he wouldn’t wear it,” Matthew Nurse, the lab’s director, tells us. “It looked like a Three Stooges
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