On Oct. 13, a 25-year-old Kenyan woman named Brigid Kosgei managed to do what many runners consider an unthinkable feat for a female athlete. In just over two hours and 14 minutes Kosgei completed—and won—the Chicago Marathon, a race that spans 26.2 miles across the city. Even more astounding, by the end of her run, Kosgei had smashed the previous women's marathon world record by a full 81 seconds. (A day earlier, fellow Kenyan runner Eliud Kipchoge had set another astonishing mark, becoming the first man to run a marathon in under two hours.)
Kosgei's and Kipchoge's superhuman runs have boggled the minds not only of other marathoners but also of scientists who study sports physiology and endurance. For both groups, the questions are the same: How are some athletes able to accomplish near-impossible feats of endurance and stamina? How can any athlete extend his or her ordinary capacity for endurance? And is there a limit to how far, how fast, or how long human beings can perform?
To answer these questions, says Alex Harrison, PhD, a sports performance consultant based in Washington, athletes (and the scientists who study them) first need to hone in on what facet of endurance they want to measure.
Endurance—which is commonly defined as an athlete's ability to retain a standard of physical performance for an extended period of time—can be measured in terms of cardiorespiratory endurance and muscular endurance. Cardiorespiratory endurance measures how efficiently your heart, lungs, and muscles work together to process oxygen during exercise, while muscular endurance measures the ability of a muscle group to sustain repeated contractions against resistance.
“As scientists, we measure cardiorespiratory endurance through something called maximal oxygen uptake ability, or VO2 max,” which is measured in milliliters of oxygen consumed per minute. Harrison says. Accomplished long-distance runners have a high VO2 max, meaning they're able to consume a high amount of oxygen and deliver it to their muscles to fuel performance—and generally, the higher the VO2 max, the greater an athlete's level of endurance.
A sedentary person who doesn't exercise much might have a VO2 max of 20, Harrison explains, while an average athletic person might have a VO2 max of 45. A VO2 max in the 80s is considered in the range of a world-class athlete (Lance Armstrong's VO2 max is rumored to be in the low- to mid-eighties) but again and again, athletes keep proving that the limits of human potential can be nearly infinite: In 2012, an 18-year-old Norwegian cyclist named Oskar Svendsen underwent metabolic tests to determine his VO2 max and clocked an unprecedented score of 97.5.
To measure muscular endurance, experts look at something called a lactate threshold, Harrison explains, which is “the point at which lactate starts to accumulate in the blood at an ever-increasing rate, faster than the body is able to clear it without decreasing the pace of running.” Once a runner or weightlifter starts to approach their lactate threshold, they experience fatigue, shortness of breath, and cramping—signs the body is shutting down its performance. Lactate threshold is expressed as a certain percentage of an athlete's maximum oxygen intake or heart rate. In other words, a less-established runner might reach his lactate threshold at 50 percent of their maximum VO2 level, while world-class athletes start to tap out at 80 or 90 percent of their max VO2 rate— meaning that they can perform at 80 to 90 percent of the absolute limit of what their body is capable.
With these parameters in mind, how do athletes increase things like VO2 max and lactate threshold, enabling them to break world records and push themselves for farther, faster, and longer?
“The fast way is to do interval training,” Harrison says, which is short periods of intense activity followed by even shorter periods of recovery time—so, for example, running moderately hard for two minutes, followed by a one-minute walk. This, Harrison says, is “really well evidenced to increase one's VO2 max quickly over the span of a few weeks,” sometimes by a factor of 10 or 20 mls/minute—a “huge” improvement, Harrison says. The downside? “You're flirting with overtraining and cumulative fatigue,” which results in muscle aches and reduced performance.
So what's the alternative? Like the tortoise and the hare, exercising slow and steady for months at a time is usually an athlete's best bet.
“The best way to improve performance is to do 60-90 percent of your training at longer distances and slower velocities than your velocity at VO2 max,” Harrison says. “Doing around 10 to 30 percent of your training as intervals is a good way to avoid cumulative fatigue and it increases your VO2 more reliably than trying to hammer out intervals every day.”
But even though athletes keep pushing themselves to reach new limits of endurance, scientists have recently discovered that there may be a hard limit on what they can endure: Researchers from Duke University who tested energy expenditure in athletes during sporting events found that humans were only able to burn calories at 2.5 times their resting metabolic rate—something not even the world's fastest runners were able to exceed. Beyond that, researchers said, the body starts to break down its own tissue.
This weekend, as Brigid Kosgei advanced toward the finish line, spectators at the Chicago marathon lined up at the edge of the streets, holding up signs and cheering her on. One of the signs read “No human is limited”—and while that might not be scientifically accurate, in terms of endurance, humans can still continue to amaze.