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A physicist reveals the secrets of human endurance

A new book explains how our brains can hold our performance back, and how to get around it.

A cross-country skier trains ahead of the Pyeongchang 2018 Winter Olympic Games on February 7 in Pyeongchang-gun, South Korea.
Al Bello/Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

Watch the end of a long race, and you’ll notice a strange pattern: The athletes speed up. This happens to 5K runners, Olympic cross-country skiers, and ultramarathoners, who all suddenly find enough gas in the tank to sprint in the home stretch.

What was holding them back the rest of the race?

Alex Hutchinson, a physicist, author, and distance runner who represented Canada in international competitions, has thought deeply about questions like this. And he explores them in a fascinating new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.

One of Hutchinson’s obsessions is what our minds do under the stresses of performance and how we can trick them to shave seconds off times.

I spoke with Hutchinson about our current understanding of mind over muscle, brain-boosting tactics, why you shouldn’t do your taxes before a marathon, and his favorite sport to watch at the Winter Olympics.

A lightly edited transcript of our conversation follows.

Umair Irfan

Can you explain the conventional wisdom around endurance and how your thinking on it shifted while writing the book?

Alex Hutchinson

The default assumption is that limits are about the body, and the body is a machine. And so if you understand the components of the machine and how it works, you should understand your limits. In terms of endurance, we think of things like VO2 max, the maximum amount of oxygen a body can use during intense exercise. It governs how quickly you can deliver oxygen to your muscles.

More than a decade ago, I came across some research that people were rethinking this idea, and finding that, actually, the brain determines your limits. There’s a guy named Tim Noakes at the University of Cape Town who first advanced this idea really in a very literal way: It’s not that your muscles don’t matter, it’s that muscle fatigue only matters in that it provides information to your brain which then decides to what your limits are.

So as I got into that area of research, I got really enthusiastic about the brain-centered research. But the deeper you go into any research, the less simple the story gets.

Umair Irfan

What finding on how the brain governs physical limits jumped out to you as especially meaningful?

Alex Hutchinson

One of the lightbulb moments for me was a study by a guy named Samuele Marcora, a researcher at the University of Kent, where they used subliminal images — flashing images of faces on a screen for 16 milliseconds at a time, which is a tenth of the time of a blink — so people were totally unaware of these images. Athletes were exposed to these images during a cycling test. If there was a smiling face compared to a frowning face, they improved their performance. They were about 12 percent better with the smiling faces than with the frowning faces.

It’s very, very hard to separate out the power of suggestion and the power of placebo in all these brain-centered studies. A study like that really confirmed to me that there’s something deeper here. It’s not just a question of a giant placebo effect. Fundamentally your brain is interpreting signals and making decisions on what your physical limits are.

Umair Irfan

There’s the old proverb that if you want to go fast, go alone; if you want to go far, go together. What do we know about how other people influence our endurance?

Alex Hutchinson

Emma Cohen, an evolutionary anthropologist at the University of Oxford, did a study where the university rowing team did a pain tolerance test before and after doing a stationary rowing machine workout.

What she found is if they did the workout alone in a room with a rowing machine, they would get one increase in pain tolerance. If they did the workout with their teammates, still on stationary rowing machines, but sitting next to each other — with these teammates with whom they have been pursuing a common goal for a year or more — they got a much bigger pain tolerance from exactly the same workout.

There’s something powerful in our brains that responds to the prospect of being on a team, whether that’s an Olympic team representing a country or whether it’s your local group of buddies with whom you play soccer or run.

Umair Irfan

One of the findings that startled me was Guillaume Millet at the University of Calgary’s works showing the level of muscle fatigue was actually lower in runners in a 200-mile race than it was in runners in a 100-mile race. How do you square that with what we know about the brain and endurance?

Alex Hutchinson

Four days out, after 100 miles or 200 miles, it doesn’t really matter how much you can deadlift fresh because muscular strength is only relevant insofar as you’re able and willing to push yourself to use it. Once you’re getting up to these very long durations, it’s all about whether your brain can keep you going and less about whether your legs are capable of going.

If you stop someone 150 miles in one of those races and hook them up to an electric shock machine, their muscles have plenty of juice in them. It’s their brain that’s unable to keep them going forward as quickly as they might like.

Umair Irfan

What does this mean for how we train, as amateurs and professionals?

Alex Hutchinson

All physical training is also mental training, and athletes tend to have greater pain tolerance and things like that. But there’s no evidence that they’re born that way. It’s the act of committing to a regular training program over a long period of time. That exposure to discomfort, learning to get comfortable being uncomfortable, teaches you to handle these sorts of uncomfortable situations.

Let’s say you’re starting an exercise training program, you’re going to run a 5K in six months, and you go and run three times a week. In six months, your body is definitely going to be fitter, but you’re also going to be able to push yourself to a harder place.

You’re going to learn that when you’re out of breath, when your legs are hurting, that doesn’t mean you have to stop, it doesn’t mean you’re out of oxygen. That’s a yellow light rather than a red light, and you can ignore the yellow light for a while.

There are ways of specifically training your brain. I wouldn’t say that’s the first priority for someone that isn’t looking for the last 0.1 percent of performance, but there are some things people don’t think about.

The research over the past 10 years into mental fatigue and physical performance makes it clear that how fresh your brain is also makes a big difference. Let’s say you’re training less because you’re about to run a marathon. You don’t want to use that free time to do your taxes the day before the marathon. You want to make sure your brain is ready for a big effort.

Umair Irfan

Where will we see this play out at the Winter Olympics?

Alex Hutchinson

Muscle fatigue is a bigger factor in running than it is in something cross-country skiing, where you’re going more smoothly, and as a result you can push your body a little harder before your muscles hold you back. One thing that’s really interesting in cross-country skiing, these guys will do races up to 50K, long races where they’re out there for hours, and when they get to the finish, they’re going to sprint for the medal.

That’s always to me a reminder that at the end of the race, if fatigue was all about your muscles and your heart and your body temperature and things like that, you’d be going slower and slower and slower as the race got on. But once you see the finish line, you see these guys sprinting like crazy, and then you really know it’s their brains that were holding them back earlier and there was always a hidden reserve.

Umair Irfan

How does the temperature outside affect endurance?

Alex Hutchinson

People will be worried about the conditions far more in the summer Olympics than in the winter Olympics. The human body is actually fairly similar to an internal combustion engine. We’re at best 25 percent efficient. If I burn 100 calories doing exercise, 25 of those calories go to actually moving my muscles and the other 75 just become heat.

If you dress appropriately and it’s not super, super cold, then, for the most part, your body keeps you warm enough that it’s not an issue.

Biathlon is an amazing sport. They’re doing this extremely demanding cross-country skiing. Then they have to stop and shoot at a target. The problem with the way you keep yourself warm when it’s cold is that your body will prioritize the muscles you’re using and it will prioritize your core and your brain. Things like your hands tend to get cold. But then when you shoot, obviously you need to get your heart rate down, but you need your fingers, your hands to be agile.

It’s actually a real challenge for biathletes for keeping their bodies warm but not too warm so they don’t sweat, because otherwise they will get cold. But then they have to keep their hands warm enough that they can stop, drop their gloves, and shoot. I may get excommunicated as a Canadian for saying this, but biathlon is probably my favorite winter Olympic sport to watch.

Umair Irfan

Where do you anticipate records being broken in winter Olympic sports? Are we starting to hit an upper limit of human performance?

Alex Hutchinson

The Olympics keep adding new sports. There’s these relatively young sports, like big air snowboarding, which is starting this year. The records in sports that are a decade or two old are still pretty open for rewriting.

The other thing is sports like skiing and skating, technology continues to evolve. In speed skating, for instance, these clap-skates came in in the 1990s, where the technology actually took a step change and all of a sudden every world record in the book was wiped out in a short period of time.

In all these sports that have technology, there is the opportunity to make incremental gains. The other thing is, even if you look at sports that have virtually no technology involved, if you look at running in the summer Olympics, the records keep edging downward.

If you look at say horse racing or dog racing records, they plateaued in the 1950s for the most part. Because of betting, there are huge amounts of money that go into training horses and dogs. But they’re not really getting any faster. Humans still are in every endeavor that we measure and I think that again was a lot to do with the brain.

If someone runs a marathon in 2 hours and 25 seconds, the next great marathoner knows it’s possible to run in 2 hours and 24 seconds. I don’t think we’ve hit plateaus. We’ve hit diminishing returns for sure, but there’s no event where I’d be surprised to see a world record.

Umair Irfan

How do improvements in endurance factor into Winter Olympics team sports or judged sports?

Alex Hutchinson

There’s no doubt that sports like hockey have a huge dependence on endurance. Figure skating too. It’s constant arms race. If you land a triple spin in competition last year, you want to land a quadruple this time. It’s not about whether can you do the jump once; it’s whether can you do that jump after 90 seconds where you’ve already been pushing yourself to the limit.

You’ll notice the figure skaters when they’ve finished their routine and are smiling for the camera, they’re panting. Part of the sport is not to show how tired they are.

Umair Irfan

It’s hard to talk about performance at the winter Olympics without talking about doping, particularly the Russian national team’s suspension. What kinds of performance-enhancing strategies do you anticipate given our current understanding of endurance?

Alex Hutchinson

One of the topics that I’ve been thinking about lately is transcranial direct current stimulation. It is something that has been shown over the past few years to enhance endurance. It changes your brain’s perception of how hard a physical activity is, with effects lasting about an hour.

There’s a company in Silicon Valley called Halo Neuroscience that has taken this to market. They’re using this technique to help them in training to push a little farther, but it’s something that could conceivably be used in competition. There’s no way to test for it at this point.

From the antidoping perspective, there are three pillars to consider: Does it enhance performance, is it potentially harmful, and does it violate the spirit of the sport. Meeting two out of three determines whether a substance or technique is banned.

You can reasonably ask whether transcranial direct current stimulation is in the spirit of sport to be doing that. I think that’s something that athletes in Pyeongchang will be doing and I think it’s something that international sports authorities need to carefully think about whether this is a road they want to go down.

Umair Irfan

How do we bring a more diverse cadre of athletes to the Olympics? Should Kenya’s legendary runners from the Kalenjin tribe be encouraged ski?

Alex Hutchinson

That has actually been attempted. Nike ran a project where they took a few second-string Kenyan runners and tried to train them to ski, and one actually competed in the Olympics.

There are some fundamental challenges. There’s obviously socioeconomic issues, but also climate issues. There’s a lot of places in the world that don’t have winter. Ultimately that limits how diverse Olympics are going to be. Countries that do have winter are getting more diverse. You look at the NHL, it’s still pretty white, but it’s not as white as it was 20 years ago.

But in addition to snow, for a lot of these sports, you need facilities and you need equipment. Even in a country like Canada, I don’t know if I’ve ever met anybody who’s been in a bobsled or has done the skeleton. They’re very, very specific to the places where there is infrastructure.

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