What are the limits of endurance? And is it the body or the mind that sets those limits? In Endure, athlete and scientist Alex Hutchinson travels the globe and interviews hundreds of sports scientists, coaches, and athletes to explore how the brain and body work together to set limits on our endurance—and how those limits can be broken.

In 1954, Roger Bannister was the first man in history to break the four-minute mile barrier. Just weeks later, John Landy, a stocky Australian and unlikely athlete, surprised the world by accomplishing the same feat. In the weeks leading up to this milestone, Landy had consistently finished the mile with a time of 4:02. He compared those last two seconds to a brick wall, and was beginning to conclude that he just had to acknowledge the limits of his endurance. But then, for reasons he himself was unable to pinpoint, he finished with a time of 3:57.9—about four seconds faster than his previous times.

What changed in Landy? Sudden shifts like this, those inexplicably remarkable runs (or bike rides, or swims, or mountain climbs) raise questions about the limits of endurance. Many people work backward from their achievement, assuming that success lay with their particular workout routine, diet, or rest cycles. The physiologists would explain the limits in terms of maximum heart rate, lactic acid, and muscle strength, but no single factor adequately explains our ability to endure.

Can you view stories of epic survival in the wilderness, for example, as a case for either physical or psychological endurance? It hardly seems possible to compartmentalize the matter so neatly.

After a century of physiological tests about fatigue, a rather formulaic approach to physical limits emerged. The human body was compared to a car that ran until fuel was used up or the pistons stopped firing. But this is not the entire story. Increasingly advanced technology enables us to examine the brain’s features in great detail. What scientists are discovering is that, whatever the signals being sent to the brain about pain, shortness of breath, and so on, how the brain perceives those messages is just as important.

This discovery of the brain’s interpretation of signals has led to new opportunities and new risks—opportunities for researchers not only to test the boundaries of human performance, but also the potential of expanding or breaking those boundaries. Scientists have run tests on performance-enhancing electric currents or chemicals to energize motivational parts of the brain and subdue the parts of the brain that inhibit actions. Time will tell if these methods will be helpful and without harmful side-effects. 

Henry Worsley was a rough-around-the-edges veteran of the British Special Air Forces—the equivalent of a US Navy Seal. Inspired by an intrepid Briton named Shacktleton, who almost made it to the southernmost tip of Antarctica about a century ago, Worsley followed in his hero’s footsteps and made it even farther. He then proceeded to make the trip to the southernmost point from a number of different routes, each more ambitious than the trip before it. These four-month trips were grueling. Worsley and his men each pulled sleds with 500 pounds of provisions, and scientists estimate that they were expending between 6,000 and 10,000 calories each day while living on only half-day rations.

Unlike his companions, Worsley was older and increasingly unable to keep pace. He would show up to the camp ten or twenty minutes after the others, and the gap kept growing. The final challenge that did him in was his attempted 900-mile solo trek across Antarctica. With only thirty miles remaining, he radioed for help, but help came too late.

On the face of it, the story of Worsley appears to corroborate the body-as-machine thesis—that Worsley’s endurance reserves were used up just as a car uses up the gas in the tank. During the twentieth century, there were some scientists brimming with optimism that it was just a matter of time before the capabilities of humans would be mathematically and chemically expressible.

But if this were the case, why aren’t there more deaths by endurance? The case rests on anomalies—not the typical experience among athletes and adventurers. The majority do not die and that is what is truly remarkable.

The most controversial discussion within human performance circles is almost certainly the role of the brain in endurance. The case of Diane Van Deren gives us some insight into the discussion, specifically the interrelations between the brain and the rest of the body.

One of the most remarkable sporting events is the 1,000-mile trail. The standing record for completion was 24 days, 3 hours, and 50 minutes—until Diane Van Deren, a woman in her early fifties, beat the record in 2012. This is the same woman who lugged a 50-pound sled across 400 miles of tundra. She accomplished similarly amazing feats on numerous occasions. So how was someone decades beyond her prime able to beat the record for one of the world’s most grueling challenges?

 Not only was experience on her side, but there was also a physiological advantage: a small portion of her brain had been removed to curtail the epileptic fits to which she had been prone. The seizures stopped, but one of the unintended consequences of the surgery was a poor sense of direction and time. One runner’s magazine referred to her as “The Disoriented Express.” Her tweaked sense of time means that she has no internal grid for gauging her capabilities. She doesn’t have to deal with the lengthy inner dialogue that athletes have about what they’re capable of, and how much father they can go. In the case of Van Deren, she doesn’t know how long, or how far, or how hard she’s been running. She could run for hundreds of miles but have no clue that she’d actually run that far.

Van Deren is a hare without a trace of tortoise, but, unlike the fable, she has the drive. This serves her well in races, and shows us that the brain’s interpretation of time and distance covered is at least as important as the time and distance actually covered.

The best way to test the mind-over-matter versus the body-as-machine debate is to look at the instances where people are pushed to limits. Where there is pain, muscle strain, oxygen deprivation, excessive heat, dehydration, and hunger, the body’s self-preserving mechanisms cry out (or shriek) for the deprivations to be addressed.

There’s the cyclist Jens Voight, long believed to be immune to pain. What he was really doing was swearing at his pain during races, shoving it to the back corners of consciousness, only to have it make a resurgence the moment the race was over. Studies have shown that hydration, while important, is rarely a matter of life and death for athletes and hikers. Interestingly, the scientists who have reached these conclusions still bring an extra bottle of water on treks and rides because they derive some sort of psychological boost from the experience of rehydrating. Unless you’re running across Death Valley, though, it’s unlikely that thirst is the crisis you’ve been led to believe it is.

On one side, there are very clearly limits to human capability, but on the other side, belief that one can go further is critical for one to go further. Whatever questions are still up in the air about the brain, we can be confident that effort is the difference-maker.

As you push the limits of your endurance while you  train for an athletic competition or event, your brain starts sensing trouble is approaching. A risk-averse organ, it sends distress signals well before an emergency. To improve endurance, training your brain is part of the process. So, then, how do you train your brain?  A significant part of training the brain is training the body. What happens when you consistently run a mile at a faster pace than that to which you’re accustomed?  Do it often enough, and you will get stronger. Your body will produce more mitochondria, which supply more power and energy to your body’s cells; more capillaries will emerge to support a greater flow of oxygen. This all leads to less mental strain and focus required to sustain a pace that had, at one point, been physically and psychologically taxing. Here again, we run into the interconnections between brain and body.

People are trying to set the brain’s effort dial to a higher setting. One approach is with focus-building exercises designed to retrain pathways—simple games that do not build in complexity, like pressing a button every time a certain combination of shapes appears. Under such circumstances, the mind tends to wander, which is where the strain comes in. Interestingly, Zen Buddhism has made inroads into athletics with its mindfulness practices to address precisely these aspects of human performance. 

The downside of this approach is that it’s a time-gouger—a luxury many athletes can’t afford to add to their training regimen. Because these practices require a level of time and patience that many don’t possess, artificial means of achieving new endurance limits are also being researched. Corporations and militaries are exploring technologies like chewing gums that block the brain’s fatigue response, or electrical currents that keep the motor neural systems firing to such an extent that they override fatigue signals.

Belief is also an important component of breaking the limits of endurance. This is not total capitulation to the “It’s all in your head” mentality that naïve self-help books advocate. A slew of people beat the four-minute mile barrier after Bannister, for instance, but this wasn’t simply because a psychological barrier was broken. 

Still, we see that belief makes a difference. This can be clearly seen from placebos. “Placebo” has become an obscenity of sorts in the ivory tower of sports science, where the placebo effect skews results. But “belief effects”—a phrase gaining currency in athletic circles—is a real thing, and is embraced outside the academy where athletes are looking for any kind of edge.

Consider the experiment where cyclists were given pills with varying amounts of caffeine. When cyclists believed that they had been given a pill with a small amount of caffeine, they biked 1.3 percent faster. When they believed they’d received a pill with high amounts of caffeine, their pace was 3.1 percent faster. When cyclists believed that they’d taken a placebo, their pace dropped by 1.4 percent. But here’s the kicker: none of the pills had caffeine in them; they were all sugar pills. But results were substantially different—at a competitive level, it would be the difference between a gold medal and not qualifying.

At a physiological level, too, the body responds to the placebo. The neural pathways respond in anticipation of a supplement that they believe is helping them. One study showed that patients all experienced a lessening of pain after dental surgeries—whether they were given morphine or a saline drip. This suggested to researchers that the body produces a higher quantity of endorphins to accompany an outside source of healing—real or pretend.

So really, the placebo effect is not fake or imagined at all; when someone buys into a placebo, there are often real biological changes occurring that the psychological belief initiates.

At the end of the day, who cares if the sugar pill (or the ice bath or the new app) is just a placebo if it yields positive results?

Filmmaker and former top runner, Michael De Monte, did a documentary on the running culture in Kenya. So many long-distance champions have been Kenyan, and Del Monte wanted to uncover what accounted for it. 

What he saw in Kenya was that aspiring runners got up in the morning with a buoyant optimism that today was the big day—the day that they would not only keep pace with the leaders, but pass them on the final leg of the race. Even with repeated losses and coming up against their limits, they retain a resilient, hopeful outlook about tomorrow. Maybe this sounds like wishful thinking, but the long-standing legacy of Kenyan marathon champions should be a check on our dismissiveness.