Endure: Mind, Body and the Curiously Elastic Limits of Human Performance
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The importance of oxygen was confirmed the next year by Leonard Hill, a physiologist at the London Hospital Medical College, in the British Medical Journal. He administered pure oxygen to runners, swimmers, laborers, and horses, with seemingly astounding results. A marathon runner improved his best time over a trial distance of three-quarters of a mile by 38 seconds. A tram horse was able to climb a steep hill in two minutes and eight seconds instead of three and a half minutes, and it wasn’t breathing hard at the top.
One of Hill’s colleagues even accompanied a long-distance swimmer named Jabez Wolffe on his attempt to become the second person to swim across the English Channel. After more than thirteen hours of swimming, when he was about to give up, Wolffe inhaled oxygen through a long rubber tube, and was immediately rejuvenated. “The sculls had to be again taken out and used to keep the boat up with the swimmer,” Hill noted; “before, he and it had been drifting with the tide.” (Wolffe, despite being slathered head-to-toe with whiskey and turpentine and having olive oil rubbed on his head, had to be pulled from the water an agonizing quarter mile from the French shore due to cold. He ultimately made twenty-two attempts at the Channel crossing, all unsuccessful.)
As the mysteries of muscle contraction were gradually unraveled, an obvious question loomed: what were the ultimate limits? Nineteenth-century thinkers had debated the idea that a “law of Nature” dictated each person’s greatest potential physical capacities. “[E]very living being has from its birth a limit of growth and development in all directions beyond which it cannot possibly go by any amount of forcing,” Scottish physician Thomas Clouston argued in 1883. “The blacksmith’s arm cannot grow beyond a certain limit. The cricketer’s quickness cannot be increased beyond this inexorable point.” But what was that point? It was a Cambridge prot?g? of Fletcher, Archibald Vivian Hill (he hated his name and was known to all as A. V.), who in the 1920s made the first credible measurements of maximal endurance.
You might think the best test of maximal endurance is fairly obvious: a race. But race performance depends on highly variable factors like pacing. You may have the greatest endurance in the world, but if you’re an incurable optimist who can’t resist starting out at a sprint (or a coward who always sets off at a jog), your race times will never accurately reflect what you’re physically capable of.
You can strip away some of this variability by using a time-to-exhaustion test instead: How long can you run with the treadmill set at a certain speed? Or how long can you keep generating a certain power output on a stationary bike? And that is, in fact, how many research studies on endurance are now conducted. But this approach still has flaws.Most important, it depends on how motivated you are to push to your limits. It also depends on how well you slept last night, what you ate before the test, how comfortable your shoes are, and any number of other possible distractions and incentives. It’s a test of your performance on that given day, not of your ultimate capacity to perform.
In 1923, Hill and his colleague Hartley Lupton, then based at the University of Manchester, published the first of a series of papers investigating what they initially called “the maximal oxygen intake”—a quantity now better known by its scientific shorthand, VO2max. (Modern scientists call it maximal oxygen uptake, since it’s a measure of how much oxygen your muscles actually use rather than how much you breathe in.) Hill had already shared a Nobel Prize the previous year, for muscle physiology studies involving careful measurement of the heat produced by muscle contractions. He was a devoted runner—a habit shared by many of the physiologists we’ll meet in subsequent chapters. For the experiments on oxygen use, in fact, he was his own best subject, reporting in the 1923 paper that he was, at thirty-five, “in fair general training owing to a daily slow run of about one mile before breakfast.” He was also an enthusiastic competitor in track and cross-country races: “indeed, to tell the truth, it may well have been my struggles and failures, on track and field, and the stiffness and exhaustion that sometimes befell, which led me to ask many questions which I have attempted to answer here.”
The experiments on Hill and his colleagues involved running in tight circles around an 85-meter grass loop in Hill’s garden (a standard track, in comparison, is 400 meters long) with an air bag strapped to their backs connected to a breathing apparatus to measure their oxygen consumption. The faster they ran, the more oxygen they consumed—up to a point. Eventually, they reported, oxygen intake “reaches a maximum beyond which no effort can drive it.” Crucially, they could still accelerate to faster speeds; however, their oxygen intake no longer followed. This plateau is your VO2max, a pure and objective measure of endurance capacity that is, in theory, independent of motivation, weather, phase of the moon, or any other possible excuse. Hill surmised that VO2max reflected the ultimate limits of the heart and circulatory system—a measurable constant that seemed to reveal the size of the “engine” an athlete was blessed with.
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.
In his twenties, Hill reported, he had run best times of 53 seconds for the quarter mile, 2:03 for the half mile, 4:45 for the mile, and 10:30 for two miles—creditable times for the era, though, he modestly emphasized, not “first-class.” (Or rather, in keeping with scientific practice at the time, these feats were attributed to an anonymous subject known as “H.,” who happened to be the same age and speed as Hill.) The exhaustive tests in his back garden showed that his VO2max was 4.0 liters of oxygen per minute, and his lactic acid tolerance would allow him to accumulate a further “oxygen debt” of about 10 liters. Using these numbers, along with measurements of his running efficiency, he could plot a graph that predicted his best race times with surprising accuracy.
Hill shared these results enthusiastically. “Our bodies are machines, whose energy expenditures may be closely measured,” he declared in a 1926 Scientific American article titled “The Scientific Study of Athletics.” He published an analysis of world records in running, swimming, cycling, rowing, and skating, at distances ranging from 100 yards to 100 miles. For the shortest sprints, the shape of the world record curve was apparently dictated by “muscle viscosity,” which Hill studied during a stint at Cornell University by strapping a dull, magnetized hacksaw blade around the chest of a sprinter who then ran past a series of coiled-wire electromagnets—a remarkable early system for precision electric timing. At longer distances, lactic acid and then VO2max bent the world-record curve just as predicted.
But there was a mystery at the longest distances. Hill’s calculations suggested that if the speed was slow enough, your heart and lungs should be able to deliver enough oxygen to your muscles to keep them fully aerobic. There should be a pace, in other words, that you could sustain pretty much indefinitely. Instead, the data showed a steady decline: the 100-mile running record was substantially slower than the 50-mile record, which in turn was slower than the 25-mile record. “Consideration merely of oxygen intake and oxygen debt will not suffice to explain the continued fall of the curve,” Hill acknowledged. He penciled in a dashed near-horizontal line showing where he thought the ultra-distance records ought to be, and concluded that the longer records were weaker primarily because “the greatest athletes have confined themselves to distances not greater than 10 miles.”
By the time Henry Worsley and his companions finally reached the South Pole in 2009, they had skied 920 miles towing sleds that initially weighed 300 pounds. Entering the final week, Worsley knew that his margin of error had all but evaporated. At forty-eight, he was a decade older than either Adams or Gow, and by the end of each day’s ski he was struggling to keep up with them. On New Year’s Day, with 125 miles still to go, he turned down Adams’s offer to take some weight off his sled. Instead, he buried his emergency backup rations in the snow—a calculated risk in exchange for a savings of eighteen pounds. “Soon I was finding each hour a worrying struggle, and was starting to become very conscious of my weakening condition,” he recalled. He began to lag behind and arrive at camp ten to fifteen minutes after the others.
On the eve of their final push to the pole, Worsley took a solitary walk outside the tent, as he’d done every evening throughout the trip before crawling into his sleeping bag. Over the course of the journey, he had sometimes spent these quiet moments contemplating the jagged glaciers they had just traversed and distant mountains still to come; other times, the view was simply “a never-ending expanse of nothingness.” 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. It was the first clear display of sun dogs during the entire journey. Surely, Worsley told himself, this was an omen—a sign from the Antarctic that it was finally releasing its grip on him.
The next day was anticlimactic, a leisurely five-mile coda to their epic trip before entering the warm embrace of the Amundsen-Scott South Pole Station. They had done it, and Worsley was flooded with a sense of relief and accomplishment. The Antarctic, though, was not yet finished with him after all. Worsley had spent three decades in the British Army, including tours in the Balkans and Afghanistan with the elite Special Air Service (SAS), the equivalent of America’s SEALs or Delta Force. He rode a Harley, taught needlepoint to prison inmates, and had faced a stone-throwing mob in Bosnia. The polar voyage, though, had captivated him: it demanded every ounce of his reserves, and in doing so it expanded his conception of what he was capable of. In challenging the limits of his own endurance, he had finally found a worthy adversary. Worsley was hooked.
Three years later, in late 2011, Worsley returned to the Antarctic for a centenary reenactment of Robert Falcon Scott and Roald Amundsen’s race to the South Pole. Amundsen’s team, skiing along an eastern route with 52 dogs that hauled sleds and eventually served as food, famously reached the Pole on December 14, 1911. Scott’s team, struggling over the longer route that Shackleton had blazed, with malfunctioning mechanical sleds and Manchurian ponies that couldn’t handle the ice and cold, reached it thirty-four days later only to find Amundsen’s tent along with a polite note (“As you probably are the first to reach this area after us, I will ask you kindly to forward this letter to King Haakon VII. If you can use any of the articles left in the tent please do not hesitate to do so. The sledge left outside may be of use to you. With kind regards I wish you a safe return …”) awaiting them. While Amundsen’s return journey was uneventful, Scott’s harrowing ordeal showed just what was at stake. A combination of bad weather, bad luck, and shoddy equipment, combined with a botched “scientific” calculation of their calorie needs, left Scott and his men too weak to make it back. Starving and frostbitten, they lay in their tent for ten snowy days, unable to cover the final eleven miles to their food depot, before dying.
A century later, Worsley led a team of six soldiers along Amundsen’s route, becoming the first man to complete both classic routes to the pole. Still, he wasn’t done. In 2015, he returned for yet another centenary reenactment, this time of the Imperial Trans-Antarctic Expedition—Shackleton’s most famous (and most brutally demanding) voyage of all.
In 1909, Shackleton’s prudent decision to turn back short of the pole had undoubtedly saved him and his men, but it was still a perilously close call. Their ship had been instructed to wait until March 1; Shackleton and one other man reached a nearby point late on February 28 and lit a wooden weather station on fire to get the ship’s attention and signal for rescue. In the years after this brush with disaster, and with Amundsen having claimed the South Pole bragging rights in 1911, Shackleton at first resolved not to return to the southern continent at all. But, like Worsley, he couldn’t stay away.
Shackleton’s new plan was to make the first complete crossing of the Antarctic continent, from the Weddell Sea near South America to the Ross Sea near New Zealand. En route to the start, his ship, the Endurance, was seized by the ice of the Weddell Sea, forcing Shackleton and his men to spend the winter of 1915 on the frozen expanse. The ship was eventually crushed by shifting ice, forcing the men to embark on a now-legendary odyssey that climaxed with Shackleton leading an 800-mile crossing over some of the roughest seas on earth—in an open lifeboat!—to rugged South Georgia Island, where there was a tiny whaling station from which they could call for rescue. The navigator behind this remarkable feat: Frank Worsley, Henry Worsley’s forebear and the origin of his obsession. While the original expedition failed to achieve any of its goals, the three-year saga ended up providing one of the most gripping tales of endurance from the great age of exploration—Edmund Hillary, conqueror of Mount Everest, called it “the greatest survival story of all time”—and again earned Shackleton praise for bringing his men home safely. (Three men did die on the other half of the expedition, laying in supplies at the trek’s planned finishing point.)
Once more, Worsley decided to complete his hero’s unfinished business. But this would be different. His previous polar treks had covered only half the actual distance, since he had flown home from the South Pole both times. Completing the full journey wouldn’t just add more distance and weight to haul; it would also make it correspondingly harder to judge the fine line between stubborn persistence and recklessness. In 1909, Shackleton had turned back not because he couldn’t reach the pole, but because he feared he and his men wouldn’t make it back home. In 1912, Scott had chosen to push on and paid the ultimate price. This time, Worsley resolved to complete the entire 1,100-mile continental crossing—and to do it alone, unsupported, unpowered, hauling all his gear behind him. On November 13, he set off on skis from the southern tip of Berkner Island, 100 miles off the Antarctic coast, towing a 330-pound sled across the frozen sea.
That night, in the daily audio diary he uploaded to the Web throughout the trip, he described the sounds he had become so familiar with on his previous expeditions: “The squeak of the ski poles gliding into the snow, the thud of the sledge over each bump, and the swish of the skis sliding along … And then, when you stop, the unbelievable silence.”
At first, A. V. Hill’s attempts to calculate the limits of human performance were met with bemusement. In 1924, he traveled to Philadelphia to give a lecture at the Franklin Institute on “The Mechanism of Muscle.” “At the end,” he later recalled, “I was asked, rather indignantly, by an elderly gentleman, what use I supposed all these investigations were which I had been describing.” Hill first tried to explain the practical benefits that might follow from studying athletes but soon decided that honesty was the best policy: “To tell you the truth,” he admitted, “we don’t do it because it is useful but because it’s amusing.” That was the headline in the newspaper the next day: “Scientist Does It Because It’s Amusing.”
In reality, the practical and commercial value of Hill’s work was obvious right from the start. His VO2max studies were funded by Britain’s Industrial Fatigue Research Board, which also employed his two coauthors. What better way to squeeze the maximum productivity from workers than by calculating their physical limits and figuring out how to extend them? Other labs around the world soon began pursuing similar goals. The Harvard Fatigue Laboratory, for example, was established in 1927 to focus on “industrial hygiene,” with the aim of studying the various causes and manifestations of fatigue “to determine their interrelatedness and the effect upon work.” The Harvard lab went on to produce some of the most famous and groundbreaking studies of record-setting athletes, but its primary mission of enhancing workplace productivity was signaled by its location—in the basement of the Harvard Business School.
Citing Hill’s research as his inspiration, the head of the Harvard lab, David Bruce Dill, figured that understanding what made top athletes unique would shed light on the more modest limits faced by everyone else. “Secret of Clarence DeMar’s Endurance Discovered in the Fatigue Laboratory,” the Harvard Crimson announced in 1930, reporting on a study in which two dozen volunteers had run on a treadmill for twenty minutes before having the chemical composition of their blood analyzed. By the end of the test, DeMar, a seven-time Boston Marathon champion, had produced almost no lactic acid—a substance that, according to Dill’s view at the time, “leaks out into the blood, producing or tending to produce exhaustion.” In later studies, Dill and his colleagues tested the effects of diet on blood sugar levels in Harvard football players before, during, and after games; and studied runners like Glenn Cunningham and Don Lash, the reigning world record holders at one mile and two miles, reporting their remarkable oxygen processing capacities in a paper titled “New Records in Human Power.”
Are such insights about endurance on the track or the gridiron really applicable to endurance in the workplace? Dill and his colleagues certainly thought so. They drew an explicit link between the biochemical “steady state” of athletes like DeMar, who could run at an impressive clip for extended periods of time without obvious signs of fatigue, and the capacity of well-trained workers to put in long hours under stressful conditions without a decline in performance.
At the time, labor experts were debating two conflicting views of fatigue in the workplace. As MIT historian Robin Scheffler recounts, efficiency gurus like Frederick Winslow Taylor argued that the only true limits on the productive power of workers were inefficiency and lack of will—the toddlers-on-a-plane kind of endurance. Labor reformers, meanwhile, insisted that the human body, like an engine, could produce only a certain amount of work before requiring a break (like, say, a weekend). The experimental results emerging from the Harvard Fatigue Lab offered a middle ground, acknowledging the physiological reality of fatigue but suggesting it could be avoided if workers stayed in “physicochemical” equilibrium—the equivalent of DeMar’s ability to run without accumulating excessive lactic acid.
Dill tested these ideas in various extreme environments, studying oxygen-starved Chilean miners at 20,000 feet above sea level and jungle heat in the Panama Canal Zone. Most famously, he and his colleagues studied laborers working on the Hoover Dam, a Great Depression–era megaproject employing thousands of men in the Mojave Desert. During the first year of construction, in 1931, thirteen workers died of heat exhaustion. When Dill and his colleagues arrived the following year, they tested the workers before and after grueling eight-hour shifts in the heat, showing that their levels of sodium and other electrolytes were depleted—a telling departure from physico-chemical equilibrium. The fix: one of Dill’s colleagues persuaded the company doctor to amend a sign in the dining hall that said THE SURGEON SAYS DRINK PLENTY OF WATER, adding AND PUT PLENTY OF SALT ON YOUR FOOD. No more men died of heat exhaustion during the subsequent four years of construction, and the widely publicized results helped enshrine the importance of salt in fighting heat and dehydration—even though, as Dill repeatedly insisted in later years, the biggest difference from 1931 to 1932 was moving the men’s living quarters from encampments on the sweltering canyon floor to air-conditioned dormitories on the plateau.
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