Cliff Young and Dean Karnazes May Set the Limits.

A professional relay race. Photograph by Thomas Wolter.

You were born to run barefoot, with nothing consoling your feet from the harsh earth. Your DNA was made for this. Your ancestors chased, hunted and outlasted even the most rapid of prey. Gazelles, horses, elk: nothing was a match for the upstanding bipeds.

Somewhere along the way, humans relinquished their genetic fabric. We displaced what made us human in the first place. We stopped running extraordinary distances. We no longer had to chase prey, instead opting for long-range projectiles and clever traps. Our intellect replaced our physical prowess. And yet, we still hold tight to the unique attributes that made us such great long-distance runners in the first place. We dissipate heat by sweating and the shape of our feet — and small toes especially — nearly “double the mechanical work of the foot”, according to a study in the Journal of Experimental Biology.

But not all is lost and there are still those that can run extraordinary distances. Over the last two hundred years, humans have time and again returned to their roots. We still run marathons, a race inspired by the story of a Greek soldier, despite it being invented for the 1896 Olympic Games.

We don’t just run against other people, either. In 1818, an Englishman bet a friend that he could beat a horse in a 48-hour race. He lost (running 158 miles vs. the horse’s 179), but the obsession with long-distance running never faded. By 1880, during a 6.5-day long race of man vs. horse held in Chicago, a man named Michael J. Byrne successfully ran 578 miles against a horse, beating it by 15 miles.

In the 21st century, people have continued to stretch the physical limits of their endurance, displaying their feats in international competitions. The Badwater 135 is a 135-mile race through Death Valley, held in the climax of the desert heat. The Barkley Marathons are a grueling, 100 mile race with 54,200 feet of vertical climb (the height of Mt. Everest…twice) through Frozen Head State Park, which must be completed in 60 hours. The Marathon des Sables is a blistering 156-mile race across the Sahara Desert.

Potato farmer and ultra-runner, Cliff Young.

Though these races sound crazy, there are those that make extreme distances look easy. Dean Karnazes made headlines in 2016 for running 350 miles in 80 hours and 44 minutes without stopping for sleep. He has a rare genetic condition that accelerates his clearing of lactic acid, a molecule that builds up in the body during exercise and fatigues muscles. Cliff Young was an Australian potato farmer that won the inaugural Sydney to Melbourne Ultramarathon in 1983. At the age of 61, Cliff ran the 544 miles in about five days, wearing sheep-herding ‘gumboots’ instead of sneakers. The Rarámuri, or Tarahumara, indigenous peoples of Mexico’s Copper Canyon are also among the most elite ultra-runners in the world. They routinely run upwards of 100 miles in a single day, with a diet consisting mainly of maize and root vegetables.

While all of these examples are exceptional, they are also rare. There are (probably) far fewer people capable of running over a hundred miles in a session today compared to 5000 years ago. Are modern people anywhere near approaching the feats capable of our ancestors? What are the true limits of human endurance?

Tarahumara girls. Photo by Diego Fernandez Briseño.

Though humans were literally born to run, there are still plenty of factors that set our theoretical limits, among them the toxic build-up of lactic acid, which causes muscle fatigue during exercise, as well as the amount of energy available in the form of glycogen, which our body readily uses for energy. Other limits are set not by training, but by genetics. Bone and muscle structure, lung size, and even the length of our toes all play a role in a person’s maximum running distance.

The biggest limits for long-distance running, as stated by Joel Gheen, are energy availability and sleep. Humans need to have enough energy to continue running. The body requires glycogen (a glucose polymer) for energy, but burns it relatively quickly. This means that, despite eating a meal ahead of time, an ultra-runner will eventually deplete glycogen stores and switch to anaerobic exercise, which causes lactic acid to build-up in the muscles. Anaerobic running is not sustainable over long distances (unless you have desirable genetics like Dean Karnazes). Most glycogen is stored in the liver, and this reserve is only large enough to maintain blood glucose levels for about 8 to 10 hours before it is completely consumed (while at rest).

Previous estimates, when accounting for glycogen depletion, suggest that a human could run at about a 10 minute per mile pace, which allows existing fat stores to be converted to glycogen, forever. The only limit to our eventual mileage, therefore, is our need for sleep.

After a few days, our mind gets tired and demands sleep. The world record for consecutive time spent awake is a staggering 264 hours (more than 11 days), set by Randy Gardner in 1964. But most people cannot achieve anywhere near this. After only 36 hours, our body’s sleep-wake cycle becomes arrhythmic and alters the release of specific hormones, dismantling our metabolism and appetite. After 72 hours, people may experience paranoia or hallucinations.

The physiological effects of sleep deprivation.

Given our intimate reliance on both sleep and energy availability, it is safe to say that Cliff Young’s momentous, consecutive 544 mile run over 5 days will be upheld for some time. It may even be the upper limit of human endurance. Nevertheless, there is room for improvement. We may someday see a human achieve upwards of 600 miles in a single running session. For that, we just need to find a rare individual that possesses the endurance of Cliff Young and the immunity to sleep deprivation of Randy Gardner.

Science journalism at NYU. Previously Caltech, Imperial College. #SynBio newsletter: Web:

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