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href="#litres_trial_promo">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.

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