In nearly every athletic maneuver, from sprinting to jumping to fighting, human beings are pretty abysmal when compared to most other animals. The cheetah runs faster, the flea jumps higher (relative to its size), and the primate can absolutely kick ass. But there is one event in which humans trounce nearly every other animal on Earth: distance running.
Over thousands of years, humans have evolved to run extremely long distances for extended periods of time. Our muscles, tendons, and ligaments help store and release energy with every stride; our large percentage of slow-twitch muscle fibers, the type utilized for endurance, help us avoid fatigue; and unlike most other animals, humans can evade overheating by sweating, allowing us to dissipate body heat faster than many of our long-distance rivals.
Because of these anatomical abilities, humans have executed a number of remarkable feats. In 2015, Scott Jurek ran the Appalachian Trail in 46 days, 8 hours, and 7 minutes, averaging nearly 50 miles a day. Nebraskan ultrarunner Pete Kostelnick ran 3,067 miles across the U.S. in 42 days, 6 hours, and 30 minutes (an average of 72 miles per day). And competitive ultras of similar stature have grown in popularity over the past two decades as humans push the limits of distance like never before.
Yet despite all this physiological information alongside such staggering statistics, scientists have long pondered one simple question:
What’s the ultimate limit of human endurance?
To reach some form of conclusion that satisfied such a question, a team led by Herman Pontzer of Duke University and John Speakman of the University of Aberdeen collected data from the Race Across USA, a 140-day running event that crossed the continent from Huntington Beach, California, to Washington, D.C, in 2015.
To collect the much-needed data, Run Across USA participants volunteered to undergo routine assessments throughout the duration of the event, which lasted for 14-20 weeks; they were poked and prodded, examined and inspected. The data gathered was then plotted and compared to ultra runs of varying lengths to find patterns amongst the clutter.
But rather than simply ask “What’s the ultimate limit of human endurance?”, the team of scientists were a bit more, ahem, scientific in their approach, instead asking “what is the maximum rate at which you can burn calories over a sustained period of time?”
To find an answer, they expressed the results in terms of Basal Metabolic Rate (the rate of energy expenditure per unit time, and referred to henceforth as metabolic scope), or multiples of your metabolic scope, which is the number of calories you burn in a sedentary state. For instance, if an individual that burns 2,000 calories per day just to stay alive was asked to go for a run requiring 4,300 calories per day, their metabolic scope would be 2.15 (4,300 divided by 2,000). And in finding such a number, scientists and healthcare professionals can compare individuals of different sizes and shapes along a similar scale.
The monitored runners from Run Across USA began by burning approximately 6,000 calories during the first week of running. This corresponded to an average metabolic scope of 3.76, whereas their pre-race metabolic scope was measured to be 1.76. As the race went on, however, their caloric burn declined, resulting in an average metabolic scope of 3.11 over 140 days.
But 3.11 wasn’t the final magic number, the Holy Grail that is the true limit of endurance. You see, because humans can easily burn more calories over a shorter span of time, such as Tour de France cyclists that maintain a metabolic scope of 4.9 for 22 days, Pontzer and Speakman needed to understand the relationship between how high a caloric burn you can sustain and how long you can sustain that burn for. So the researchers went back to older data, looking at varying events of impressive sustained caloric burn, from an 800 meter run in world record time to a marathon, and even to pregnancy.
When plotted, the data collected can be seen from the graph on the far left. Events that lasted less than a tenth of a day (less than -1 on the horizontal axis) are represented by the blue line, and illustrate an increased metabolic scope over a shorter period of time. Events that lasted greater than a half day, on the other hand, are represented by the red line, and encompass events such as a multi-day race or hike.
When plotted on a linear scale rather than a logarithmic one (the middle graph), the red curve representing long-term bouts of exercise begins to flatten out with a metabolic scope somewhere around 2.5, thus suggesting that if you want to sustain a workload indefinitely (or for as long as possible), you’ll need to be sure that you’re not burning more than 2.5x more calories than your basal metabolic rate. This is the limit of human endurance.
But what happens if you do burn more than that magic number? Colin O’Brady, who broke the record for the fastest Antarctic crossing, consumed about 8,000 calories per day but still didn’t replenish what he’d lost. Instead his body dipped into reserves of fat and muscle to compensate, and in turn he lost weight, thus suggesting that the body simply can’t digest more than 2.5x your metabolic rate over prolonged periods of time. It can always consume more, but it fails to digest more.
As is often the case, outliers exist that can’t be easily accounted for. Michael Phelps was said to maintain a 12,000-calorie diet during the height of his Olympic training, yet he never lost a single pound. Pete Kostelnick broke the record for the fastest crossing of the U.S. and was reported to down anywhere from 9,000–14,000 calories per day, yet he too sustained weight. As it would seem, some elite athletes can simply maintain a higher metabolic scope without losing weight, and the potential reasons behind such abilities remain numerous.
For the average Joe that isn’t running across the country in 42 days, however, this research is telling. While there may not be a hard limit to human endurance — as illustrated by Phelps and his superhuman comrades — most of us can apply this magic number with some degree of certainty.
So does the limit of human endurance exist? Probably so. And it’s somewhere around 2.5.