Humans are capable of extraordinary things. We can climb mountains, run marathons, and more. At least, some humans can. Just how much can a human accomplish physically, though? What is the limit of human endurance? A recent study might have found the answer.
20 weeks of marathons
Data on endurance events like triathlons, ultramarathons, and the Tour de France already existed when a research team decided to find the limit of human endurance. However, they were inspired to collect more data when they heard a former anthropologist was organizing 20 weeks of marathons called Race Across the USA in 2015. Scientists took the chance to measure six athletes’ endurance by adding harmless isotopes to their drinking water, so they could track how much carbon dioxide they produced, which correlates to burned calories.
The team measured the BMRs of the athletes before their races as reference. BMR stands for “basal metabolic rate,” which is the amount of calories the body needs to perform its most basic functions. After analyzing the data, the research team learned that over the course of the 20-day Race Across the USA, energy use plateaued at 2.5 times the initial BMR. This doesn’t mean an athlete can’t exceed that. During a single marathon, runners can use over 15 times their BMR, but what the data showed was that after a certain length of time and activity, the BMR has to level out or it’s unsustainable. Someone can sprint really fast for a short length of time, but they aren’t able to sprint at that top speed for miles and miles.
The limit explained
Why is 2.5 times the limit of human endurance? Researchers believe it’s actually because of the digestive system, and not the heart, lungs, or muscles. Basically, the body can only digest and absorb a certain amount of calories. If you were to exceed that 2.5 times in the long-term, your body is burning too many calories and isn’t able to absorb more to sustain you. For short periods of intense exercise, this can happen, too, and that’s when the body starts relying on fat reserves and even muscles. As soon as you stop your activity, the body is able to recover. However, if you were to try and push yourself harder without recovery time, your body starts to shut down.
Another interesting takeaway from the study: pregnant women’s energy use pushes right against the limit of human endurance. They can peak at 2.2 times their BMR. Anyone who has ever been pregnant probably agrees with the conclusion that creating new life is as hard as an ultramarathon.
Pushing the envelope
What are some of the hardest events in the world that test the limit of human endurance? In Canada, there are 120-mile and 380-mile races through freezing temperatures and powerful winds. In Tennessee, the Barkley marathons send off runners with a conch shell trumpet and 120,000 feet of elevation. One loop is about the same as one marathon, and to be honored as a finisher, you have to complete five loops in less than 60 hours. Within 30 years, there’s only been 15 finishers. If you like the heat, head over to the Marathon des Sables in the Sahara desert. The length varies year to year, but it’s around 150 miles over the course of six days. You only get one rest day.
If you’re a marathon runner (or an ultramarathon runner), you know the pain of injuries. Is there a way to avoid them? Click here to check out the research.