Endure

Highlights

The nineteenth-century Swedish chemist Jöns Jacob Berzelius is now best remembered for devising the modern system of chemical notation— H2O and CO2 and so on— but he was also the first, in 1807, to draw the connection between muscle fatigue and a recently discovered substance found in soured milk. Berzelius noticed that the muscles of hunted stags seemed to contain high levels of this “lactic” acid, and that the amount of acid depended on how close to exhaustion the animal had been driven before its death.
With this advance, Hill now had the means to calculate the theoretical maximum performance of any runner at any distance. At low speeds, the effort is primarily aerobic (meaning “with oxygen”), since oxygen is required for the most efficient conversion of stored food energy into a form your muscles can use. Your VO2max reflects your aerobic limits. At higher speeds, your legs demand energy at a rate that aerobic processes can’t match, so you have to draw on fast-burning anaerobic (“ without oxygen”) energy sources. The problem, as Hopkins and Fletcher had shown in 1907, is that muscles contracting without oxygen generate lactic acid. Your muscles’ ability to tolerate high levels of lactic acid— what we would now call anaerobic capacity— is the other key determinant of endurance, Hill concluded, particularly in events lasting less than about ten minutes.
On this final night, he was greeted by a spectacular display in the polar twilight: the sun was shaped like a diamond, surrounded by an incandescent circle of white-hot light and flanked on either side by matching “sun dogs,” an effect created when the sun’s rays are refracted by a haze of prism-shaped ice crystals.
In 2014, a group of economists from the University of Southern California; the University of California, Berkeley; and the University of Chicago mined a massive dataset containing the finish times of more than nine million marathoners from races around the world spanning four decades. 27 The distribution of finishing times looks a bit like the classic bell-shaped curve, but with a set of spikes superimposed. Around every significant time barrier— three hours, four hours, five hours— there are far more finishers than you’d expect just below the barrier, and fewer than you’d expect just above. Similar but smaller spikes show up at half-hour intervals, and there are barely perceptible ripples even at ten-minute increments. The cruel metabolic demands of the marathon, which inevitably depletes your stores of readily available fuel, mean that most people are slowing in the final miles. But with the right incentive, some are able to speed up— and it’s only the brain that can respond to abstract incentives like breaking four hours for an arbitrary distance like 26.2 miles.
The system Marcora used to measure perceived exertion was called the Borg Scale, named for Swedish psychologist Gunnar Borg, who pioneered its use in the 1960s. Though there are many variations, Borg’s original scale ran from 6 (“ no effort at all”) to a maximum of 20 (the penultimate value, 19, was defined as “very, very hard”), with the numbers corresponding very roughly to your expected heart rate divided by ten.
“In my opinion,” he wrote, “perceived exertion is the single best indicator of the degree of physical strain,” since it integrates information from muscles and joints, the cardiovascular and respiratory systems, and the central nervous system.
Perceived exertion— what we’ll refer to in this book as your sense of effort— isn’t just a proxy for what’s going on in the rest of your body, he argued. It’s the final arbiter, the only thing that matters. If the effort feels easy, you can go faster; if it feels too hard, you stop. That may sound obvious, or even tautological, but it’s a profound statement— because, as we’ll discover, there are lots of ways you can alter your sense of effort, and thus your apparent physical limits, without altering what’s happening in your muscles.
A subsequent study by Taiwanese researchers also linked jaw-clenching muscles to effort. It’s no coincidence, then, that coaches have long instructed runners to “relax your face” or “relax your jaw.”
When Kipchoge tiptoes with exaggerated care onto the treadmill, one of the scientists edges around to the back of the machine, ready to be a spotter if needed. It’s only the second time Kipchoge has run on a treadmill— the first time was during the initial selection process— and it’s hard not to think of Bambi flailing around on the ice. Kipchoge’s lab data, Jones later confides, was surprisingly ordinary, presumably because he was so uncomfortable on the treadmill. For the Olympic champion, they decided to look past this mediocre lab data.
“Pain is more than one thing,” says Dr. Jeffrey Mogil, the head of the Pain Genetics Lab at McGill University. It’s a sensation, like vision or touch; it’s an emotion, like anger or sadness; and it’s also a “drive state” that compels action, like hunger. For athletes, the role of pain depends on how these different effects mingle together in their specific situation. Sometimes pain slows them to a halt; other times it drives them to even greater heights.