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

by Alex Hutchinson

In a wide variety of human activity, achievement is not possible without discomfort.

So what is our relationship to that pain? How do the signals of protest from our brain interact with the physical will to keep moving?

The limits of endurance running, according to physiologists, could be quantified with three parameters: aerobic capacity, also known as VO2max, which is analogous to the size of a car’s engine; running economy, which is an efficiency measure like gas mileage; and lactate threshold, which dictates how much of your engine’s power you can sustain for long periods of time.

To their frustration, physiologists have found that the will to endure can’t be reliably tied to any single physiological variable.

endurance is “the struggle to continue against a mounting desire to stop.”

What’s crucial is the need to override what your instincts are telling you to do (slow down, back off, give up), and the sense of elapsed time. Taking a punch without flinching requires self-control, but endurance implies something more sustained: holding your finger in the flame long enough to feel the heat; filling the unforgiving minute with sixty seconds’ worth of distance run.

any task lasting longer than a dozen or so seconds requires decisions, whether conscious or unconscious, on how hard to push and when.

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.

If anything, my head held me back as often as it pushed me forward during those years, to my frustration and befuddlement.

Using this newly accurate technique, the two men investigated muscle fatigue by experimenting on frog legs hung in long chains of ten to fifteen pairs connected by zinc hooks. By applying electric current at one end of the chain, they could make all the legs contract at once; after two hours of intermittent contractions, the muscles would be totally exhausted and unable to produce even a feeble twitch.

It’s a test of your performance on that given day, not of your ultimate capacity to perform.

VO2max.17 (Modern scientists call it maximal oxygen uptake, since it’s a measure of how much oxygen your muscles actually use rather than how much you breathe in.)

“indeed, to tell the truth, it may well have been my struggles and failures, on track and field, and the stiffness and exhaustion that sometimes befell, which led me to ask many questions which I have attempted to answer here.”

The polar voyage, though, had captivated him: it demanded every ounce of his reserves, and in doing so it expanded his conception of what he was capable of. In challenging the limits of his own endurance, he had finally found a worthy adversary.

“Scientist Does It Because It’s Amusing.”

Taylor and two other scientists took on the task of developing a test protocol that “would eliminate both motivation and skill as limiting factors” in objectively assessing endurance.

your VO2max was your VO2max, regardless of how you felt that day or whether you were giving your absolute best. Taylor’s description of this protocol, published in 1955, marked the real start of the VO2max era.

“You have to listen to your body sometimes,”

Worsley, in trying to cross Antarctica on his own, had embarked on a mission that exceeded his body’s capacity, and no amount of mental strength and tenacity could change that calculation.

The brain’s role in endurance is, perhaps, the single most controversial topic in sports science.

she doesn’t focus on the challenge ahead of her. Hampered by poor short-term memory, she doesn’t dwell on the effort already expended, either.

Around every significant time barrier—three hours, four hours, five hours—there are far more finishers than you’d expect just below the barrier, and fewer than you’d expect just above.

Whether the brain plays a role in defining the limits of endurance is no longer in doubt; the debate now is how.

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.

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.

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.

It follows that the subjects didn’t stop the test because their muscles were physically incapable of producing the required power; instead, the researchers argued, it was perception of effort that mattered.

“Physicists can explain the whole universe with two theories, and they’re not happy with that,” he said. “Endurance performance is complicated, but it’s not more complicated than the entire universe!”

If you could train the brain to become more accustomed to mental fatigue, then—just like the body—it would adapt and the task of staying on pace would feel easier.

For Mosso, the working-class son of an impoverished carpenter, the conditions in sulfur mines and Sicilian farms, particularly for child laborers, amounted to an injustice “worse than slavery, worse than the dungeon.” Just as mental fatigue sapped physical strength, he argued, physical fatigue stunted mental growth in overworked child miners, so that “those who survive become wicked, villainous, and cruel.”

An 1898 study by Indiana University psychologist Norman Triplett, in which he explored why cyclists ride faster with others than alone, is often pegged as the debut of sports psychology as a distinct discipline.

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.

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

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.

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?

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.

“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.

Among the first to study pain perception in athletes was Karel Gijsbers, a psychologist at the University of Stirling, in Scotland,

“Pain threshold” was defined as the number of contractions needed to produce a sensation that registered as pain rather than merely discomfort; “pain tolerance” was quantified as the total number of contractions before the subject gave up.

Subsequent studies have mostly confirmed these findings: athletes, and especially endurance athletes, are consistently willing to tolerate more pain.

Less pain made the effort feel easier, allowing the cyclists to push closer to their true physiological limits, the researchers argued.

There are other reasons to avoid dulling the pain too much. In a series of experiments starting in 2009, researcher Markus Amann, then at the University of Wisconsin, investigated what happens to cyclists when they can’t feel pain at all. Amann and his colleagues injected the nerve blocker fentanyl into the spines of their volunteers, preventing signals from traveling up from the leg muscles to the brain, and asked them to ride 5K as hard as they could on a stationary bike.15 The effects were dramatic. The volunteers had been given a gift that many athletes dream of—the ability to push as hard as they wanted without feeling pain—and they took advantage of the opportunity to ride themselves into a smoking ruin. By the end of the time trial, the riders couldn’t even get off the bikes by themselves. Some couldn’t even unclip their feet from the pedals, Amann recalls, “and not a single one was able to walk.”

But the results told a different, and unexpected, story. Despite their temporary superhuman status, the subjects didn’t ride any faster than when they received a placebo injection, thanks to erratic and overly ambitious pacing.

Without pain, in other words, they’re incapable of pacing themselves.

“gate control” theory of pain, which was first proposed in the 1960s. If you whack your shin against a chair, your first instinct will be to rub your bruised shin with your hand. Why? Because the nonpainful sensation of rubbing competes with the pain of the bruise for the same neural signaling pathways that report back to your brain. The more you rub, the less bandwidth is left for pain signals.

The results suggested that the pain you experience in the extremes of sustained exercise is fundamentally different, from your brain’s perspective, from the pain you experience while dunking your hand in ice water. All pleasure is alike, as Leo Tolstoy might have put it, but each pain hurts in its own unique way.

“the end point of any performance is never an absolute fixed point but rather is when the sum of all negative factors such as fatigue and muscle pain are felt more strongly than the positive factors of motivation and will power.”

In practice, though, expectations mattered. Within a few reps, those who thought they were only doing 6 were producing slightly more force than the 12-rep control group; and those with no information about how long they would be expected to continue were producing less force than the other groups.

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.