Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

As John L. Parker Jr. wrote in his cult running classic, Once a Runner, “A runner is a miser, spending the pennies of his energy with great stinginess, constantly wanting to know how much he has spent and how much longer he will be expected to pay. He wants to be broke at precisely the moment he no longer needs his coin.”

You judge what’s sustainable based not only on how you feel, but on how that feeling compares to how you expected to feel at that point in the race.

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.4 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 eff...

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.17 Just like a smile or frown, the words in your head have the power to influence the very feelings they’re supposed to reflect.

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.18 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.

all else being equal, the gold medal goes to whoever is willing to suffer a bit more than everyone else. Freund isn’t the only one to find that well-trained athletes can tolerate more pain; others have shown that regular physical training, especially if it involves unpleasant high-intensity workouts, increases your pain tolerance. But the link between what’s happening in your muscles and what you feel in your head turns out to be much more indirect than you might assume. “Pain is more than one thing,” says Dr. Jeffrey Mogil, the head of the Pain Genetics Lab at McGill University. It’s a sensation, like vision or touch; it’s an emotion, like anger or sadness; and it’s also a “drive state” that compels action, like hunger. For athletes, the role of pain depends on how these different effects mingle together in their specific situation. Sometimes pain slows them to a halt; other times it drives them to even greater heights.

First, pain tolerance increased by 41 percent in the high-intensity group, while the medium-intensity subjects didn’t see any change. This shows that simply getting fitter doesn’t magically increase your pain tolerance; how you get fit matters: you have to suffer. Second, despite the similar fitness gains, the high-intensity group saw much bigger improvements in their racing performance, as measured by a series of time-to-exhaustion tests at different intensities. In one test, the interval group lasted 148 percent longer on the bike, compared to a mere 38 percent gain for the medium-intensity trainers. Intriguingly, the individual improvements in the time-to-exhaustion tests were correlated with the individual gains in pain tolerance, meaning that the cyclists who learned to handle more pain from the tourniquet test were the same ones who managed to cycle faster. This is a profound finding: pain in training leads to greater tourniquet tolerance, and greater tourniquet tolerance predicts better race performance.

doctors and pain researchers have concluded that pain is fundamentally a subjective, situation-dependent phenomenon.21 For example, stress, fear, and anxiety activate an impressive array of brain chemicals, including endorphins (the body’s store-brand opioid drugs) and endocannabinoids (the body’s cannabis), to dull or completely block pain that would overwhelm you in other circumstances. In evolutionary terms, pain may serve a valuable function by telling you to stop and allow an injury to heal. “But if you’re a deer being chased by a wolf and you trip and break a leg,” says Mogil, the McGill pain researcher, “you need to forget about that pain until later.”

There are many ways of delineating the boundary between short and uncomfortable high-intensity exercise and longer, more pleasant efforts. One of the most familiar is lactate threshold, the point at which you’re working hard enough that levels of lactate in your blood start creeping inexorably upward. A more recently developed concept is critical power, which is the point beyond which your muscles can no longer stay in the sustainable “steady state” equilibrium fetishized by Harvard Fatigue Laboratory researchers. Sixty minutes of all-out exercise, for a well-trained athlete, sits in the excruciating gap between these two markers, explains Mark Burnley, a physiologist in the University of Kent’s Endurance Research Group. “Riders in the Hour have to exercise above lactate threshold, but very slightly below the critical power—in other words, ride with the highest metabolic rate that is also steady state.” Done right, then, the Hour is literally the longest bout of painful high-intensity exercise you can endure.

Overall, according to Roger Enoka, who directs the University of Colorado Boulder’s Neurophysiology of Movement Lab, the modern consensus from these studies echoes Merton’s finding: most healthy people can achieve “voluntary activation scores” of close to 100 percent. In Mark Burnley’s lab, at the University of Kent, typical scores for all-out quadriceps contractions are 92 to 97 percent, and anything less than 90 percent suggests something has gone wrong with the test. Under normal conditions, in other words, we’re utilizing pretty much all the strength our muscles have to offer. There are, however, two possible loopholes, Enoka notes. One is that you can’t sustain 100 percent activation indefinitely, so the idea of a hidden muscle reserve makes more sense for cyclists and rowers and runners than for piano movers. The other is the difference between twitching your thumb and, say, deadlifting a car—a movement that requires a deceptively intricate synchronized pattern of activation involving at least thirteen different muscle groups. It’s possible that these complex real-world actions make it harder to reach full voluntary activation in all the relevant muscle groups, meaning there might be some reserve accessible in stressful situations. This prospect hasn’t been tested, Enoka says, and it’s not clear that such a test is even possible with current technology (which is why Zatsiorsky’s claims are so intriguing).

how much does the biggest force you can produce with a given muscle decline? Not surprisingly, he has found that the force produced by two key muscle groups in the legs, the quadriceps and the calves, gets progressively weaker as the distance of a running race increases—up to a point. By the time you’ve been out there for about 24 hours, your leg muscles will be 35 to 40 percent weaker, and they won’t lose much more. In fact, his Tor des Géants subjects, who took more than 100 hours, on average, to complete the race, ended up losing just 25 percent of their pre-race leg strength—a result that, on the surface, makes little sense. “Okay,” Millet jokes, “so if I run 200 miles, I’m less fatigued than if I run 100 miles!” This counterintuitive finding, on its own, already hints that leg muscles aren’t what ultimately limit ultra-endurance athletes. And there are some more subtle details. The electrically stimulated twitch data allows Millet to estimate how much of the force loss is due to fatigue in the muscles themselves, and how much is “central,” reflecting either diminished output from the brain or losses in transmission via the spinal cord. For ultra-endurance runs, it turns out, the muscles themselves typically only lose about 10 percent of their force-producing capacity; the rest is central, reflecting a progressive decline in the brain’s voluntary activation of muscle. “The brain is able to do more, but it doesn’t,” Millet says. But, he adds, that doesn’t necessarily me...

muscle fatigue dominated in the shortest trials, while central fatigue was increasingly important in the longer ones. In fact, the maximal force measurements taken during the longer trials showed that fatigue in the muscles themselves soon reached a fairly stable plateau at about 80 percent of full strength, which persisted until the subjects launched into their finishing kick at the end of the trial. That suggests that the importance of purely muscle-based fatigue in long events has been, if anything, overestimated by previous studies. If your leg muscles are really shot at the end of a one-hour race, it’s largely because you high-stepped down the final straightaway.

In a three-minute trial (and presumably in 800-meter races), the brain still calls for a sprint as the finish line approaches; the muscles are simply unable to obey. If you’re looking for the midpoint between the muscle’s role in hoisting a car and the brain’s role in running an ultra, this is as good a definition as any: that agonizing point, about 600 meters into an 800-meter race, where you’re holding nothing back but can feel yourself slowing anyway.

In 1935, an international team of scientists led by David Bruce Dill of the Harvard Fatigue Lab ventured to Chile, where they outfitted a mobile laboratory in a train car and journeyed from sea level to a sulfur mine on the upper slopes of a 20,000-foot-high volcano called Aucanquilcha, putting themselves and other volunteers through exhaustive experiments at various elevations along the route. In the process, they identified a puzzling and still-controversial phenomenon known as the “lactate paradox.”43 Under ordinary circumstances, you produce high levels of lactate in your muscles and blood when you “go anaerobic”—that is, when you’re exercising so hard that your muscles can’t get fuel quickly enough from the usual oxygen-dependent aerobic pathways. As you go to higher altitudes, with less oxygen in the air, you would expect to go anaerobic sooner, and produce more lactate at a given pace or power output. Instead, Dill’s team observed the opposite: the higher they went, the lower the lactate levels they were able to produce at exhaustion. Extrapolating from their data (which has since been reproduced and reconfirmed many times) suggests that by the time you reach 23,000 feet, where oxygen levels are less than half their sea-level values, you won’t be able to raise your lactate levels at all. A series of studies by Markus Amann and his colleagues, involving 5K cycling time trials and rides to exhaustion at a range of simulated altitudes, offers a possible explanation for...

assigning the blame to mind or muscles is an often hopeless and sometimes misleading task. After all, the brain is part of the body. This was a point emphasized by Michio Ikai and Arthur Steinhaus in 1961, when they studied the psychological effects of surprise gunshots on muscle strength (see Chapter 6): “[P]sychology,” they wrote, “is a special case of brain physiology.” In other words, feelings and emotions and urges are as physiologically real as changes in core temperature or decreases in hydration, and are mediated by chemical signals. So when oxygen levels in the brain drop, are we compelled by failing neurons or safety circuitry to slow down, or do we simply decide to slow down? Is there a difference? Whatever the answers (and I don’t think we know them at this point), the outcome is clear. We slow down.

it’s worth considering what thirst is for. The simplest explanation is that it’s the body’s way of ensuring that you keep your fluid levels topped up. In this picture, voluntary dehydration is a failure of the system: it indicates that your thirst sensation isn’t very good at its job, because it fails to notice that you’re losing fluids. But physiologists have shown that this isn’t how thirst works. Instead of monitoring fluid levels, your body monitors “plasma osmolality,” which is the concentration of small particles like sodium and other electrolytes in your blood.25 As you get dehydrated, your blood gets more concentrated, and your body responds by secreting an antidiuretic hormone that causes your kidneys to start reabsorbing water, and by making you thirsty. Unlike your body’s fluid levels, plasma osmolality is very tightly regulated: when you’re looking at the right variable, your thirst sensation (along with other homeostatic mechanisms like antidiuretic hormone) doesn’t make mistakes. This means that what looks like a potential problem—voluntary dehydration—may actually be completely normal from the body’s perspective. In a 2011 study, eighteen South African Special Forces soldiers undertook a sixteen-mile march carrying 57-pound packs, including rifles and water supplies, in temperatures that peaked at 112 degrees Fahrenheit.26 The soldiers were permitted to drink as much water as they wanted, but—as expected—they nonetheless lost an average of six pounds, corres...

During easy exercise, like a gentle walk, you burn mostly fat from the supplies circulating in your bloodstream.9 As you speed up, you begin to add more carbohydrate to the mix, and by the time you’re panting heavily, the proportions have flipped and you’re burning mostly carbohydrate. The precise blend depends on a variety of factors: the fitter you are, for example, the greater the proportion of fat you burn at any given speed. (That’s simply because maintaining a given speed gets easier as you get fitter. As John Hawley, an exercise metabolism researcher at Australian Catholic University, points out, no matter how fit you are you’ll burn the same fat-carb mix at any given relative intensity.) Eating a diet high in either fat or carbohydrate also tilts your preferred fuel mix in that direction. But even taking these factors into account, carbohydrates dominate for any intense exercise: one study found that over the marathon distance, running at 2:45 pace relied on 97 percent carbohydrate fuel, while slowing down to 3:45 pace reduced the carbohydrate mix to 68 percent.

It wasn’t until 2009 that researchers at the University of Birmingham settled the debate, with a study that confirmed the performance benefits of swishing and spitting a carbohydrate drink—and used functional magnetic resonance imaging to show that brain areas associated with reward were lighting up as soon as the subjects had carbohydrate in their mouth.30 Crucially, neither the brain scan nor cycling performance showed any effects when the drink was artificially sweetened, but the benefits returned when maltodextrine, a tasteless and undetectable carbohydrate, was added to the artificially sweetened drink. The sweet taste of sugar, in other words, is not enough to trigger the benefits. Instead, the mouth appears to contain previously unknown (and as yet unidentified) sensors that relay the presence of carbohydrate directly to the brain. In Tim Noakes’s central governor framework, it’s as if the brain relaxes its safety margin when it knows (or is tricked into believing) that more fuel is on the way.

In practice, these findings mean that the benefits of sports drinks and other mid-race carbohydrates for short bouts of exercise are irrelevant as long as you don’t start out with an empty stomach and depleted fuel stores. (Pro tip: you shouldn’t.) On a more theoretical level, the results are among the strongest evidence we have that your brain is looking out for your well-being in ways that are outside your conscious control and that kick in long before you reach a point of actual physiological crisis.

endurance athletes on a three-week high-fat diet became fat-burning machines to an extent few had imagined possible. By the end of a 25-kilometer time trial at their expected 50-kilometer race pace, the athletes were burning through 1.57 grams of fat per minute, which is two and a half times greater than the “normal” values seen in athletes eating a standard carbohydrate diet.37 That was the good news. The problem was that the fat-adapted athletes became less efficient, requiring more oxygen to sustain their race pace. This, it turns out, is a consequence of the cascade of metabolic reactions required to transform either fat or carbohydrate into ATP, the final form of fuel used for muscle contractions: the fat reactions require more oxygen molecules. If you’re out for a leisurely stroll, that’s no big deal, but if you’re running (or walking) a race at a pace that leaves you out of breath, anything that forces you to consume more oxygen is a liability. As a result, it was no surprise that the LCHF athletes ended up performing worse than the high-carbohydrate athletes in the Supernova study’s final and most important real-world test: a 10-kilometer racewalk.

now, Burke is betting on a “periodized” approach to carbohydrate and fat during training—that is, carefully selecting certain workouts to perform with full carbohydrate reserves and others to do on empty. The goal isn’t necessarily to boost fat usage in competition; instead, the carbohydrate-depleted workouts function as the nutritional equivalent of a weighted vest, forcing the body to work harder and triggering greater fitness gains in response. The problem with these bonk-prone depleted workouts is that they tend to be poor quality, which is why they need to be mixed with other workouts where you have enough carbohydrate to sustain high intensities. Burke and others published a pair of studies in 2016 using a protocol dubbed “sleep low,” which involved a high-quality carbohydrate-fueled workout in the late afternoon, followed by a carbohydrate-free dinner; then, the next morning, a carbohydrate-depleted moderate workout before breakfast.38 Repeating this cycle just three times, for a total of six days, produced a 3 percent improvement in 20-kilometer cycling times.

How hard it feels dictates, in a true and literal sense and with greater accuracy than any physiological measurement yet devised, how long you can sustain it.

That study, which was led by Noakes’s student Fernando Beltrami, used a similar “reverse” protocol that started fast and gradually slowed down just enough to enable the subjects to stay on the treadmill as they tired. One of the curious details of Beltrami’s study was that, when the subjects returned to the lab for a follow-up test using the conventional accelerating VO2max protocol, their values stayed at the new, higher value.7 To Beltrami, who also coaches runners, this suggests that the mere fact of having attained the higher level of oxygen consumption somehow adjusts the brain’s settings.

the boundary between “real” ergogenic (performance-enhancing) aids and “fake” belief effects is much fuzzier than most people, even scientists, realize. They cited an observation by sports scientist Trent Stellingwerff, who also coaches athletes including his wife, Hilary, a two-time Olympic 1,500-meter runner. At a conference in 2013, Stellingwerff noted the wide variety of supplements and training methods that have been shown to produce a 1–3 percent boost in performance, from caffeine to beet juice to altitude training. In theory, combining all these approaches should create a superathlete; in practice, studies that combine multiple interventions in elite athletes tend to see overall improvements of . . . 1 to 3 percent. If 1 + 1 + 1 = 1, the implication is that many different “proven” training aids act, at least in part, on the same target: the brain.

They compared fifteen-minute post-cycling-workout soaks in either cold water, lukewarm water, or lukewarm water with the addition of a special “recovery oil.” “We made sure that we put the recovery oil in the water in plain sight of the participants,” recalls David Bishop, the study’s senior author, “and we gave them a glossy summary of some made-up research about scientifically proven benefits of ‘recovery oils.’”6 Over the following two days, the researchers tested their subjects’ leg strength—which, in the end, is the most important recovery outcome. Sure enough, the ice bath significantly outperformed the lukewarm bath throughout the two-day recovery period. But the recovery oil was just as good, and perhaps even marginally better than the ice bath—even though the oil was, in fact, a liquid soap called Cetaphil Gentle Skin Cleanser. This, you might think, debunks the value of ice baths once and for all—except for the fact that the athletes who had either ice or oil really did seem stronger in the two days following the workout.

placebos can produce measurable biochemical changes. The paradigm-altering demonstration of this phenomenon came in a 1978 study, from the University of California, San Francisco, of people recovering from dental surgery. The patients were given IV drips of either morphine or a plain saline solution to block their pain; as expected, some “placebo responders” had reductions in pain even though they only received saline. The researchers then added a drug called naloxone, which counteracts overdoses of morphine and heroin by blocking the body’s opioid receptors. This immediately shut off the painkilling effect of the saline solution, suggesting that its painkilling powers were the result of a surge of endorphins, the body’s internal version of morphine.

according to a 2006 analysis by University of Nottingham researcher David Gardner, winning times at major races like the Kentucky Derby and the Epsom Derby have remained stagnant since about 1950. Over the same period, winning times at major marathons such as the Olympics continued to drop by more than 15 percent.20 A champion marathoner and a champion horse are both physiological marvels; the difference is that the marathoner can look beyond the present moment. Secretariat’s Kentucky Derby record of 1:59.4 has stood since 1973. Nearly thirty years later, in 2001, Monarchos became only the second horse to dip under 2:00 at the Derby, winning by five lengths. Could he have challenged Secretariat’s record? Perhaps if Secretariat had been there in front of him. But only humans can make the abstract leap to virtual competition: if you know that someone, somewhere, has covered a given distance in 1:59.4, you know that it’s possible to cover that distance in 1:59.3—and you can guide your training and execute your race plan accordingly.

If I could go back in time to alter the course of my own running career, after a decade of writing about the latest research in endurance training, the single biggest piece of advice I would give to my doubt-filled younger self would be to pursue motivational self-talk training—with diligence and no snickering.