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were still almost a century away from figuring out what “acids” really were. We now know that lactate from muscle and blood, once extracted from the body, combines with protons to produce lactic acid. That’s what Berzelius and his successors measured, which is why they believed that it was lactic acid rather than lactate that played a role in fatigue. For the remainder of the book, we’ll refer to lactate except in historical contexts.)

      What the presence of lactic acid in the stags’ muscles signified was unclear, given how little anyone knew about how muscles worked. At the time, Berzelius himself subscribed to the idea of a “vital force” that powered living things and existed outside the realm of ordinary chemistry. But vitalism was gradually being supplanted by “mechanism,” the idea that the human body is basically a machine, albeit a highly complex one, obeying the same basic laws as pendulums and steam engines. A series of nineteenth-century experiments, often crude and sometimes bordering on comical, began to offer hints about what might power this machine. In 1865, for example, a pair of German scientists collected their own urine while hiking up the Faulhorn, an 8,000-foot peak in the Bernese Alps, then measured its nitrogen content to establish that protein alone couldn’t supply all the energy needed for prolonged exertion. As such findings accumulated, they bolstered the once-heretical view that human limits are, in the end, a simple matter of chemistry and math.

      These days, athletes can test their lactate levels with a quick pinprick during training sessions (and some companies now claim to be able to measure lactate in real time with sweat-analyzing adhesive patches). But even confirming the presence of lactic acid was a formidable challenge for early investigators; Berzelius, in his 1808 book, Föreläsningar i Djurkemien (“Lectures in Animal Chemistry”), devotes six dense pages to his recipe for chopping fresh meat, squeezing it in a strong linen bag, cooking the extruded liquid, evaporating it, and subjecting it to various chemical reactions until, having precipitated out the dissolved lead and alcohols, you’re left with a “thick brown syrup, and ultimately a lacquer, having all the character of lactic acid.”

      Not surprisingly, subsequent attempts to follow this sort of procedure produced a jumble of ambiguous results that left everyone confused. That was still the situation in 1907, when Cambridge physiologists Frederick Hopkins and Walter Fletcher took on the problem. “[I]t is notorious,” they wrote in the introduction to their paper, “that … there is hardly any important fact concerning the lactic acid formation in muscle which, advanced by one observer, has not been contradicted by some other.” Hopkins was a meticulous experimentalist who went on to acclaim as the codiscoverer of vitamins, for which he won a Nobel Prize; Fletcher was an accomplished runner who, as a student in the 1890s, was among the first to complete the 320-meter circuit around the courtyard of Cambridge’s Trinity College while its ancient clock was striking twelve—a challenge famously immortalized in the movie Chariots of Fire (though Fletcher reportedly cut the corners).

      Hopkins and Fletcher plunged the muscles they wanted to test into cold alcohol immediately after finishing whatever tests they wished to perform. This crucial advance kept levels of lactic acid more or less constant during the subsequent processing stages, which still involved grinding up the muscle with a mortar and pestle and then measuring its acidity. Using this newly accurate technique, the two men investigated muscle fatigue by experimenting on frog legs hung in long chains of ten to fifteen pairs connected by zinc hooks. By applying electric current at one end of the chain, they could make all the legs contract at once; after two hours of intermittent contractions, the muscles would be totally exhausted and unable to produce even a feeble twitch.

      The results were clear: exhausted muscles contained three times as much lactic acid as rested ones, seemingly confirming Berzelius’s suspicion that it was a by-product—or perhaps even a cause—of fatigue. And there was an additional twist: the amount of lactic acid decreased when the fatigued frog muscles were stored in oxygen, but increased when they were deprived of oxygen. At last, a recognizably modern picture of how muscles fatigue was coming into focus—and from this point on, new findings started to pile up rapidly.

      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 protégé 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

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