and 10,000 calories per day—and doing it on half rations. By the end of their journey, they would have consumed close to a million calories over the course of four relentless months, similar to the totals of the subsequent Scott expedition of 1911–12. South African scientist Tim Noakes argues these two expeditions were “the greatest human performances of sustained physical endurance of all time.”
Shackelton’s understanding of these various factors was limited. He knew that he and his men needed to eat, of course, but beyond that the inner workings of the human body remained shrouded in mystery. That was about to change, though. A few months before Shackleton’s ship, the Nimrod, sailed toward Antarctica from the Isle of Wight in August 1907, researchers at the University of Cambridge published an account of their research on lactic acid, an apparent enemy of muscular endurance that would become intimately familiar to generations of athletes. While the modern view of lactic acid has changed dramatically in the century since then (for starters, what’s found inside the body is actually lactate, a negatively charged ion, rather than lactic acid), the paper marked the beginning of a new era of investigation into human endurance—because if you understand how a machine works, you can calculate its ultimate limits.
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. (To be fair to Berzelius, chemists 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
