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

by Alex Hutchinson

Trying to make a clean divide between “brain fatigue” and “muscle fatigue,” in other words, is inevitably an oversimplification, because they’re inseparably linked.

At the point of exhaustion in a long endurance challenge, the legs are merely unwilling, not incapable.

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.

“You have to find the mental strength to continue,” he says. “I tell myself that if I feel pain, it means I’m still alive.”

Another key part of the dive reflex is massive peripheral vasoconstriction: the blood vessels in your arms and legs squeeze nearly shut, sending blood flooding back to your core, where it maintains the crucial oxygen supply to your heart and brain for as long as possible. This shift of blood volume to your torso also helps your lungs resist collapse under the pressure of deep dives, since fluids (unlike air) are nearly incompressible. All it takes to trigger these changes is dunking your face in cool water; in fact, the sensors appear to be primarily around the nose, lending credence to the idea that splashing cold water on your face really can calm you down.

Hanli Prinsloo, a South African freediving coach, divides the progress of a dive into four stages. First is a subtle “awareness phase,” where the urge to breathe begins to assert itself in your consciousness. If you push past that, you’ll start to feel involuntary contractions in your diaphragm—a response to the buildup of carbon dioxide in your blood rather than the lack of oxygen. This you can safely (but temporarily) ignore, if you’re willing to suffer. Then comes the welcome rush of fresh blood from the spleen, offering a psychological boost and allowing you to extend your dive. Finally, when your oxygen-hungry brain senses that its supply really is threatened, you black out, entering the neural equivalent of standby mode to conserve energy.

The fact that people can dive to three hundred feet or hold their breath for nearly twelve minutes tells us that oxygen’s absolute limits aren’t quite as constrictive as they feel—that we’re protected by layer upon layer of reflexive safety mechanisms.

Tim Noakes would call this “anticipatory regulation”: your brain uses knowledge that is gathered consciously, like an impending dive or a looming finish line, to activate or deactivate safety mechanisms that are otherwise purely unconscious.

“I am nothing more than a single, narrow, gasping lung, floating over the mists and the summits.”

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.

“Under normal circumstances, it’s very rare for people to reach the limits of their cold tolerance if they’re appropriately dressed,”

In nearly every ride by every rider, the thermometer read between 104.0 and 104.5 degrees at the moment of failure.16 It was as if, in crossing that critical threshold, a temperature-sensitive circuit-breaker had been tripped.

Just as the transformation of liquid water to vapor cools your skin when you sweat, the “phase change energy” of ice melting to water in your stomach provides an extra cooling boost beyond what you would get from simply drinking a cold drink.

So is it brain temperature or stomach temperature that matters most? It’s probably a bit of both—along

Heat doesn’t act like a light switch that flicks your muscles off; in most real-world situations, as Tucker explained to me, it’s a dimmer switch, controlled by the brain for your own protection.

Stephen Cheung, an avid cyclocross racer and environmental physiologist at Brock University in Canada, first explored this topic during his doctoral studies.25 In a military-funded experiment, he showed that fit, well-trained athletes could push to a higher core temperature during a treadmill test than less fit subjects—evidence that the brain’s temperature settings can indeed be altered.

“There seems to be a strong mental-psychological component to it.” The right frame of mind, in other words, allows you to push beyond your usual temperature limits: “Even if you’re already fit, you can still improve your perception of heat and how you perform in it.”

In the first trial, they drank as much as they wanted; in the other five, they were assigned varying levels of hydration ranging from nothing to enough to fully replace all their sweat losses. Sure enough, being hydrated improved performance: in the three trials where the cyclists were forced to drink less than they’d chosen to in the first trial, they were slower than the three higher-hydration trials. But there was no further improvement when they drank more than they had chosen to in the first trial. Avoiding thirst, rather than avoiding dehydration, seems to be the most important key to performance.

The study was double-blinded, meaning that neither the subjects nor the researchers knew how dehydrated a cyclist was allowed to get during each ride; instead, a paramedic hidden behind a curtain controlled how much (if any) saline solution was infused into their arms. The results showed that, in a twenty-kilometer time trial following ninety minutes of steady riding in the heat, even 3 percent dehydration had no effect on performance.

A famous 1997 study at Yale had subjects exercise for two hours to induce dehydration, then allowed them to drink and monitored the changes in perceived thirst and antidiuretic hormone, the two key regulators of plasma osmolality.33 Then they repeated the trial, but inserted a tube down through the nose into the stomach to vacuum out the water as soon as it was swallowed. The result: thirst and antidiuretic hormone secretion both decreased anyway, presumably in response to the sensation of water flowing down the throat. And when they reversed the experiment, sending the same amount of water down the nasogastric tube instead of letting the subjects swallow it, it was less effective in quenching thirst even though the water was allowed to stay in their stomachs.

Some scientists have made similar arguments about cycling in the mountains, where the benefits of staying light might exceed the benefits of staying hydrated.

Thirst, not dehydration, increases your sense of perceived effort and in turn causes you to slow down.

Cheung points to the disappointing performance of American cyclist Taylor Phinney after he dropped a water bottle at the world championships in 2013. The race was only an hour long, so it shouldn’t have mattered—but since Phinney believed it was a problem, it hurt his ride.

British researchers found that skipping breakfast resulted in a 4.5 percent drop in 30-minute cycling time trial performance at 5 p.m. that afternoon, even though the subjects had been allowed to eat as much as they wanted at lunch.

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.

In theory, the math behind this sort of fueling plan is simple: the number of calories you need to ingest is the difference between how many you already have stored in your body and how many you want to burn.

glycogen stores in your muscles don’t just act as energy reservoirs; they also help individual muscle fibers contract efficiently.27 That means your muscles will weaken as you burn through your glycogen stores, sapping your strength long before you’re actually out of fuel.

simply having sports drink in your mouth seemed to be more important than getting it into your bloodstream and to your muscles.

Rinsing and spitting sports drink produced the biggest benefit in the depleted condition, a smaller benefit in the fasted condition, and none at all in the fed condition.

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.)

“Nutrition is a cyclical science,” Burke says. “You’d be surprised at how many ‘new ideas’ are simply old ideas reimagined. So there is always the chance that it’s simply ‘hula hoop season’ again, and it will be a craze until it’s not. But there’s also a chance that new science will emerge.”

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.

Samuele Marcora would argue that this growing sense of effort is all that matters—that we pace ourselves to keep the effort manageable, and quit when it gets higher than we’re willing to tolerate.

In contrast, Noakes, drawing on the work of collaborators such as Alan St. Clair Gibson, sees the sense of effort as a conscious manifestation of hardwired neural circuitry that kicks in to hold us back from the precipice.

In 2009, one of Noakes’s former students, Ross Tucker, published a paper in the British Journal of Sports Medicine on the “anticipatory regulation of performance,” in which he tried to explain how, exactly, the brain knows in advance to slow you down before catastrophe strikes.

The answer, Tucker suggested, was Borg’s rating of perceived exertion, or RPE, which he described as “the conscious/verbal manifestation of the integration of these psychological and physiological cues.”

when I felt unable to maintain my pace, it was because of a mismatch between anticipated and actual effort, not because I was hitting a physical limit. That’s why, in the final laps where I expected effort to be near-maximal, I was suddenly able to speed up again. Is this really an explanation of how endurance is regulated, or is it simply a description of how it feels?

Marcora believes such feedback contributes to feelings of pain and discomfort but not effort, which is instead dictated by the brain’s outgoing signals to the muscles.

So anything that moves the “effort dial” in your head up or down will affect your endurance, even if it has no effect on your muscles or heart or VO2max.

Mauger’s subjects ran or cycled at steadily increasing levels of self-determined effort. The results, which remain highly contentious, showed that subjects reached higher VO2max values in the effort-based test than in the traditional test—an impossible paradox if you believe that VO2max represents a physical ceiling on oxygen consumption.

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.

In this sense, all training is brain training, even if it doesn’t specifically target the brain.

Anxious people, he found, tend to overreact to negative stimuli, producing a distinct pattern of brain activity. Elite endurance, athletes, on the other hand, display a completely opposite response pattern.

Before the breathing restriction starts, the athletes already have higher levels of activity in their insular cortex—consistent with the idea that they’ve become adept at monitoring their own signals.

“Typically, athletes are pretty in tune with their body awareness,” Lori Haase, another of Paulus’s colleagues, told me. They’re in a state of watchful anticipation, ready to handle any discomfort that arises.

The result was mPEAK—Mindful Performance Enhancement, Awareness & Knowledge—another eight-week program modeled on Kabat-Zinn’s stress-reduction course.

Between 2013 and 2016 alone, researchers published more than two thousand studies exploring the technique’s potential for goals as varied as enhancing learning, fighting addiction and depression, and improving walking ability in patients with neurological diseases.

he started with a relatively simple experiment in which volunteers were tested on handgrip strength. They repeated a series of thirteen-second contractions, with the required strength carefully manipulated so that they would fail to hold it about half the time. Functional MRI scans showed that two regions of the brain, the insular cortex and the thalamus, were more active during the failed contractions.

Shortly before the cyclists gave up, there was an increase in communication between the insular cortex, which was monitoring their internal condition, and the motor cortex, which issued the final commands to their leg muscles. The brain, in other words, knew that the cyclists were about to reach their limits before their legs actually failed, seemingly demonstrating Noakes’s anticipatory regulation in action.

tDCS is, after all, a blunt tool: it’s impossible to limit stimulation to a single brain area, since the current has to flow from one electrode to the other through multiple brain regions.