Limits of Human Performance
In a wide variety of human activity, achievement is not possible without discomfort. So what is our relationship to that pain?
Two Hours: May 6, 2017
Michael Joyner, an ex-collegiate runner from the University of Arizona who was completing a medical residency at the Mayo Clinic in Minnesota, proposed a provocative thought experiment.
Limits of endurance running, according to physiologists, could be quantified with three parameters: aerobic capacity, also known as VO2max, which is analogous to the size of a car’s engine; running economy, which is an efficiency measure like gas mileage; and lactate threshold, which dictates how much of your engine’s power you can sustain for long periods of time.
What would happen, Joyner wondered, if a single runner happened to have exceptional — but humanly possible — values in all three parameters? His calculations suggested that this runner would be able to complete a marathon in 1: 57: 58.
Mind and Muscle
The Unforgiving Minute
Reaching the “limits of endurance” is a concept that seems yawningly obvious, until you actually try to explain it.
A suitably versatile definition that I like, borrowing from researcher Samuele Marcora, is that endurance is “the struggle to continue against a mounting desire to stop.” That’s actually Marcora’s description of “effort” rather than endurance.
The science of how we pace ourselves turns out to be surprisingly complex.
After Roger Bannister came the deluge — at least, that’s how the story is often told. Typical of the genre is The Winning Mind Set, a 2006 self-help book by Jim Brault and Kevin Seaman, which uses Bannister’s four-minute mile as a parable about the importance of self-belief.
“In the year after that, over 300 runners ran a mile in less than four minutes.”
But to draw any meaningful conclusions, it’s important to get the facts right. For one thing, Landy was the only other person to join the sub-four club within a year of Bannister’s run, and just four others followed the next year. It wasn’t until 1979, more than twenty years later, that Spanish star José Luis González became the three hundredth man to break the barrier.
Knowing (or believing) that your ultimate limits are all in your head doesn’t make them any less real in the heat of a race. And it doesn’t mean you can simply decide to change them.
“It should be mathematical,” is how U.S. Olympic runner Ian Dobson described the struggle to understand the ups and downs of his own performances, “but it’s not.”
Physiologists spent most of the twentieth century on an epic quest to understand how our bodies fatigue.
The Human Machine
It was early evening on January 9, 2009, one hundred years to the day since British explorer Ernest Shackleton had planted a Union Jack in the name of King Edward VII at this precise location on the Antarctic plateau: 88 degrees and 23 minutes south, 162 degrees east. In 1909, it was the farthest south any human had ever traveled, just 112 miles from the South Pole.
For Shackleton, 88° 23′ south was a bitter disappointment.
To Worsley, a century later, that moment epitomized Shackleton’s worth as a leader: “The decision to turn back,” he argued, “must be one of the greatest decisions taken in the whole annals of exploration.” Worsley was a descendant of the skipper of Shackleton’s ship in the Endurance expedition; Gow was Shackleton’s great-nephew by marriage; and Adams was the great-grandson of Shackleton’s second in command on the 1909 trek. The three of them had decided to honor their forebears by retracing the 820-mile route without any outside help.
Studies of modern polar travelers suggest they were burning somewhere between 6,000 and 10,000 calories per day — and doing it on half rations.
In August 1907, researchers at the University of Cambridge published an account of their research on lactic acid.
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. Berzelius noticed that the muscles of hunted stags seemed to contain high levels of this “lactic” acid.
We now know that lactate from muscle and blood, once extracted from the body, combines with protons to produce lactic acid.
In 1907, when Cambridge physiologists Frederick Hopkins and Walter Fletcher took on the problem.
Hopkins and Fletcher plunged the muscles they wanted to test into cold alcohol immediately after finishing whatever tests they wished to perform.
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.
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.
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.
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.
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.
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.
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.”
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 Minnesota Starvation Study put the volunteers through six months of “semi-starvation,” eating on average 1,570 calories in two meals each day while working for 15 hours and walking 22 miles per week.
Henry Longstreet Taylor. Taylor and two other scientists took on the task of developing a test protocol that “would eliminate both motivation and skill as limiting factors” in objectively assessing endurance. Your VO2max was your VO2max, regardless of how you felt that day or whether you were giving your absolute best. Taylor’s description of this protocol, published in 1955, marked the real start of the VO2max era.
Scientists gradually fine-tuned their models of endurance by incorporating other physiological traits like economy and “fractional utilization” along with VO2max.
It was in this context that Michael Joyner proposed his now-famous 1991 thought experiment on the fastest possible marathon.
Joyner finally pushed this train of thought to its logical extreme, he arrived at a very specific number: 1:57:58.
In a sense, Worsley’s death seemed a vindication of the mathematical view of human limits. “The machinery of the body is all of a chemical or physical kind. It will all be expressed some day in physical and chemical terms,” Hill had predicted in 1927.
The Central Governor
The brain’s role in endurance is, perhaps, the single most controversial topic in sports science. It’s not that anyone thinks the brain doesn’t matter.
In that view, the body sets the limits, and the brain dictates how close you get to those boundaries. But starting in the late 1990s, a South African physician and scientist named Tim Noakes began to argue that this picture is insufficiently radical — that it’s actually the brain alone that sets and enforces the seemingly physical limits we encounter during prolonged exercise.
Over the next decade, Noakes began searching for better ways of predicting and measuring endurance.
Noakes’s first idea for an alternative to VO2max, in the late 1980s, was that the limits might reside in the contractility of the muscle fibers themselves, but that theory fizzled.
Whatever our limits are, something must prevent us from exceeding them by too much. And that something, he reasoned, must be the brain.
In his keynote lecture at the 1996 ACSM conference, Noakes had argued that A.V. Hill’s concept of VO2max was fundamentally flawed: that physical exhaustion isn’t a consequence of the heart’s inability to pump enough oxygen to the muscles. Otherwise, he reasoned, the heart itself, and perhaps the brain, would also be starved of oxygen, with catastrophic results.
But if Hill’s ideas about oxygen were wrong, what was the alternative? Noakes felt the brain had to be involved, and in a 1998 paper he coined the term “central governor,” borrowing terminology that A.V. Hill himself had used seventy years earlier.
First, the limits we encounter during exercise aren’t a consequence of failing muscles; they’re imposed in advance by the brain to ensure that we never reach true failure. And second, the brain imposes these limits by controlling how much muscle is recruited at a given effort level.
The first point — the concept of “anticipatory regulation,” as Noakes and his colleagues refer to it — is subtle, so it’s worth pausing to unpack it.
Of the 66 world records in the 5,000 and 10,000 meters dating back to the early 1920s, the last kilometer was either the fastest of the race or the second fastest (behind the opening kilometer) in all but one.
In fact, there’s good reason to think that pacing is driven as much by instinct as by choice, according to Dominic Micklewright.
Sometime around the age of eleven or twelve, in other words, our brains have already learned to anticipate our future energy needs and hold back something in reserve — a relic, Micklewright speculated, of the delicate balance between searching for food and conserving energy deep in our evolutionary past.
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.
It’s only the brain that can respond to abstract incentives like breaking four hours for an arbitrary distance like 26.2 miles.
A further curious detail from this dataset: the faster the runners were, the less likely they were able to summon a finishing sprint.
Whether the brain plays a role in defining the limits of endurance is no longer in doubt; the debate now is how.
The Conscious Quitter
Samuele Marcora. Over the next few years, he formulated a new “psychobiological” model of endurance. Integrating exercise physiology, motivational psychology, and cognitive neuroscience. In his view, the decision to speed up, slow down, or quit is always voluntary, not forced on you by the failure of your muscles.
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.
If effort is the yin of Marcora’s psychobiological model, motivation is the yang. We’re not always willing to push to an effort of 20, which is one reason athletes rarely produce world records or even personal bests in training.
If you could train the brain to become more accustomed to mental fatigue, then — just like the body — it would adapt and the task of staying on pace would feel easier.
If something can fatigue you, and you repeat it over time systematically, you’ll adapt and get better at the task. That’s the basis of physical training.
There are several theories about how caffeine boosts strength and endurance. Some argue it directly enhances muscle contraction; others suggest it enhances fat oxidation to provide extra metabolic energy. To Marcora, the most convincing explanation relates to caffeine’s ability to shut down receptors in the brain that detect the presence of adenosine, a “neuromodulator” molecule associated with mental fatigue.
Cognitive process called “response inhibition” — the ability to consciously override your impulses. This is one of the skills that Stanford University psychologist Walter Mischel tested with his famous “marshmallow test” in the late 1960s.
Response inhibition really is an important mental component of endurance — and that it’s a finite resource that runs low if you use it too much.
We can’t talk about the limits of endurance without considering the brain and perception of effort. But they don’t necessarily mean that Marcora’s psychobiological theory is right.
Marcora does indeed argue that the decision to speed up, slow down, or stop is always conscious and voluntary. But such “decisions,” he acknowledges, can be effectively forced on you by an intolerably high sense of effort.
University of Exeter physiologist Andrew Jones, who helped guide Paula Radcliffe to a marathon world record and whose Breaking2 lab data suggests Eliud Kipchoge is capable of a sub-two-hour run.
Two Hours: November 30, 2016
The Breaking2 project.
In the end, the team zeroed in on five key areas: selecting the best athletes, optimizing the course and environment, executing the best possible training, delivering the right fuel and hydration, and deploying cutting-edge shoes and apparel.
The big gains, from what I am gathering, will come from two sources. First, they have a new shoe with a counterintuitively thick, cushioned sole made with an advanced foam that breaks all previous records for lightness and resilience. Embedded in the sole is a curved carbon-fiber plate that adds enough stiffness to avoid the energy loss that would otherwise result from running in such a marshmallowy shoe.
The second big factor is drafting. In my 2014 analysis, I had argued that the cost of overcoming air resistance, even on a perfectly still day, might amount to 100 seconds over the course of a two-hour marathon.
The contrast between the high-tech pursuit of marginal gains and the simple life and elemental grind of African marathon training is striking.
Limits
Pain
In the popular imagination (and the thesaurus), endurance and suffering are inextricably linked. “No pain, no gain” is a motto across most sports, but in skill sports this relationship is more negotiable, says Wolfgang Freund, a researcher at University Hospitals Ulm in Germany who studies pain in athletes.
For cyclists and other endurance athletes, though, pain is unavoidable, and how you handle it is intimately tied to how well you perform.
“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.
Among the first to study pain perception in athletes was Karel Gijsbers, a psychologist at the University of Stirling, in Scotland, who (with a graduate student) published an influential paper in the British Journal of Medicine in 1981.
Athletes, and especially endurance athletes, are consistently willing to tolerate more pain.
Simply getting fitter doesn’t magically increase your pain tolerance; how you get fit matters: you have to suffer. Pain in training leads to greater tourniquet tolerance, and greater tourniquet tolerance predicts better race performance. At least in recreational athletes, pain tolerance is both a trainable trait and a limiting factor in endurance.
Alexis Mauger. In the real world, Mauger argued, we don’t just run to the point of failure; we pace ourselves to go as fast as possible while never reaching failure. This process of managing fatigue over a prolonged period of time — enduring the rack rather than submitting to the guillotine — puts a greater emphasis on managing pain.
Their pain-blocking powers rely on the “gate control” theory of pain, which was first proposed in the 1960s. If you whack your shin against a chair, your first instinct will be to rub your bruised shin with your hand. Why? Because the nonpainful sensation of rubbing competes with the pain of the bruise for the same neural signaling pathways that report back to your brain.
The pain you experience in the extremes of sustained exercise is fundamentally different, from your brain’s perspective, from the pain you experience while dunking your hand in ice water. All pleasure is alike, as Leo Tolstoy might have put it, but each pain hurts in its own unique way.
The way most of us think of pain was most famously articulated by French philosopher René Descartes in his 1664 Treatise of Man: you whack your thumb with a hammer, and this sends a message that, in Descartes’s imagery, rings a bell in your brain
There are many ways of delineating the boundary between short and uncomfortable high-intensity exercise and longer, more pleasant efforts. One of the most familiar is lactate threshold, the point at which you’re working hard enough that levels of lactate in your blood start creeping inexorably upward. A more recently developed concept is critical power, which is the point beyond which your muscles can no longer stay in the sustainable “steady state” equilibrium fetishized by Harvard Fatigue Laboratory researchers.
“Riders in the Hour have to exercise above lactate threshold, but very slightly below the critical power — in other words, ride with the highest metabolic rate that is also steady state.”
The experiments that Alexis Mauger and Samuele Marcora have done trying to untangle the difference between “pain” and “effort” make me think that pain, in most contexts, is a warning light on the dashboard. It instructs you (sometimes very insistently) to slow down, and in most contexts you heed that warning without even realizing you’re doing it. But it’s not an absolute limit. For that, we have to look elsewhere.
Muscle
Even in short, supposedly all-out maximal contractions, when we’re explicitly told to hold nothing in reserve , we pace ourselves — a finding that helps explain why Ikai and Steinhaus were seemingly able to tap into hidden reserves of strength, but doesn’t explain how a human can lift a car.
Most human movements use several different muscle groups triggered by different nerve pathways, unlike the crude single-muscle twitch produced by an electric shock.
The basic measure of fatigue Guillaume Millet uses in his studies is simple: how much does the biggest force you can produce with a given muscle decline? Not surprisingly, he has found that the force produced by two key muscle groups in the legs, the quadriceps and the calves, gets progressively weaker as the distance of a running race increases — up to a point. By the time you’ve been out there for about 24 hours, your leg muscles will be 35 to 40 percent weaker, and they won’t lose much more.
For ultra-endurance runs, it turns out, the muscles themselves typically only lose about 10 percent of their force-producing capacity; the rest is central, reflecting a progressive decline in the brain’s voluntary activation of muscle.
When Millet compared muscle fatigue following three hours of running to similar durations of cycling and cross-country skiing, he found that voluntary activation declined by 8 percent in running but didn’t change in cycling or skiing.
Trying to make a clean divide between “brain fatigue” and “muscle fatigue,” in other words, is inevitably an oversimplification, because they’re inseparably linked.
“We are rarely running to death,” Millet says. Factors like excessive heat, drugs, and prolonged sleep deprivation — the likely culprit in Couleaud’s ordeal — can alter the body’s delicate balance, but “our brain protects us against our own excess — almost always.”
So where is the crossover between short, muscle-limited acts of strength and prolonged tests of will?
Norwegian researcher Christian Frøyd, working under the joint supervision of Guillaume Millet and Tim Noakes,
The results, which were published in 2016, echoed some of the patterns in Millet’s ultra-endurance data: muscle fatigue dominated in the shortest trials, while central fatigue was increasingly important in the longer ones.
If you’re looking for the midpoint between the muscle’s role in hoisting a car and the brain’s role in running an ultra, this is as good a definition as any: that agonizing point, about 600 meters into an 800-meter race, where you’re holding nothing back but can feel yourself slowing anyway.
So why is it that your muscles fail you when you rig? The traditional explanation has long been that they are overwhelmed by a flood of lactic acid, which is produced when you’re working so hard that oxygen-fueled aerobic energy supplies can’t keep up with demand.
The results suggest that lactic burn isn’t literally the feeling of acid dissolving your muscles; instead, it’s a cautionary signal created in the brain by nerve endings that are triggered only in the presence of three key metabolites.
Amann’s theory is that the lactate-proton-ATP feedback is the brain’s way of ensuring that the muscles themselves never exceed a critical level of stress and disruption. If you disable this protective system, for example with fentanyl, then you become capable of pushing your muscles closer to their real limits. At that point, elevated levels of other metabolites such as phosphate begin to interfere directly with the ability of muscle fibers to contract.
Oxygen
There is no limit more fundamental — to endurance, and to life itself — than oxygen.
When the body wants more oxygen and when it needs it.
The urge to breathe (which is actually driven by a build-up of carbon dioxide rather than a lack of oxygen) turns out to be a warning signal that you can choose to ignore — up to a point.
The current record holder in static apnea is a Frenchman named Stéphane Mifsud, who on a Monday afternoon in 2009 managed to stay submerged in his local pool for a hard-to-fathom 11 minutes and 35 seconds.
The record for breath-holding after inhaling pure oxygen, a feat made famous by magician David Blaine’s 17-minute hold in 2008, now stands at 24:03, by Spanish freediver Aleix Segura.
Hanli Prinsloo, a South African freediving coach, divides the progress of a dive into four stages. First is a subtle “awareness phase,” where the urge to breathe begins to assert itself in your consciousness. If you push past that, you’ll start to feel involuntary contractions in your diaphragm — a response to the buildup of carbon dioxide in your blood rather than the lack of oxygen. Then comes the welcome rush of fresh blood from the spleen. Finally, when your oxygen-hungry brain senses that its supply really is threatened, you black out.
The fact that people can dive to three hundred feet or hold their breath for nearly twelve minutes tells us that oxygen’s absolute limits aren’t quite as constrictive as they feel — that we’re protected by layer upon layer of reflexive safety mechanisms.
Freedivers offer a graphic illustration of how the human body copes when its oxygen supply is completely shut off.
Endurance athletes have hearts that pump so powerfully that their blood barely has time to load up with oxygen as it rushes past the lungs.
A good distance runner might sustain an average of 85 percent of her VO2max over the course of 13.1 miles; a marathoner might average 80 percent.
That increases in VO2max aren’t necessarily proportional to increases in race performance.
Bjørn Dæhlie. He was reportedly able to process and use 96 milliliters of oxygen per kilogram of body weight each minute; a typical healthy adult might manage 40.
Stephen Seiler. In 2017, Seiler and several other Norwegian sports scientists published a manuscript called “New Records in Human Power,” echoing the title of a famous 1937 study from the Harvard Fatigue Lab, in which they pegged the highest reliably reported VO2max values at around 90 ml/kg/min, seen in cyclists and cross-country skiers.
The highest reported value was about 78 ml/kg/min, again in a cross-country skier.
Dæhlie lost the unofficial VO2max record in the fall of 2012 to another Norwegian, an eighteen-year-old cyclist named Oskar Svendsen, who according to Norwegian media reports notched a lab score of 97.5 ml/kg/min and went on to win the junior time trial at the world cycling championships a few weeks later.
A link between cerebral oxygenation and the limits of endurance.
So is oxygen a “real” limiting factor in endurance? It seems convenient to make a distinction between ironclad limits imposed by your muscles and softer, more negotiable ones imposed by your mind.
In practice, assigning the blame to mind or muscles is an often hopeless and sometimes misleading task. After all, the brain is part of the body.
When oxygen levels in the brain drop, are we compelled by failing neurons or safety circuitry to slow down, or do we simply decide to slow down? Is there a difference? Whatever the answers (and I don’t think we know them at this point), the outcome is clear. We slow down.
Heat
The human body, as Thompson’s experiment suggested, is quite literally a furnace. It transforms the energy from food into mechanical work — and this transformation generates heat as a sometimes useful and sometimes inconvenient by-product. The harder you work, the more heat you generate.
For every 100 calories of food you eat, in others words, you might get 25 calories of useful work and 75 calories of heat.
“Under normal circumstances, it’s very rare for people to reach the limits of their cold tolerance if they’re appropriately dressed.”
For athletes, the biggest cold-related problems arise when your activity level changes, which happens if you get too tired to maintain the effort level that has been keeping you warm. And it’s worse if your clothes get wet and lose their insulative powers.
The body is like a car with no air-conditioning: you’ve got no way of actively cooling yourself, so the best you can do is get rid of excess heat as quickly as possible. At rest, about 250 milliliters (half a pint) of blood per minute flows through the vessels near your skin, carrying heat away from your core and releasing it to the environment primarily through radiation (in the form of electromagnetic waves) and convection (as moving air carries it away). As a result, you’re always giving off heat at a rate of about 100 watts, just like a lightbulb.
Because of the body’s imperfect efficiency, cycling at 250 watts generates as much as 1,000 watts of excess heat. Running at 10 miles per hour produces a sizzling 1,500 watts. In response, the blood vessels in your skin dilate dramatically, allowing up to eight liters of blood per minute — a thirty-fold increase — to course through them and dump heat to the air around you.
You also begin to sweat: the transformation of liquid water to vapor as sweat evaporates consumes energy, creating a powerful cooling effect on the skin.
When you exercise repeatedly in hot conditions, your body’s protective responses get progressively better: you sweat more heavily, starting at a lower temperature; your vessels dilate even wider to deliver heat-laden blood to the skin; and the total volume of blood in your body increases, allowing your heart rate to stay lower during exercise. This acclimatization process takes about two weeks.
So is it brain temperature or stomach temperature that matters most? It’s probably a bit of both — along with temperature signals from other parts of the body, like the skin. There’s a reason athletes don ice-filled vests and cooling sleeves and drape ice towels over their necks: these interventions don’t alter your core temperature, but they do influence how hot you feel — and that, in turn, dictates how hard you’re able to push.
The right frame of mind, in other words, allows you to push beyond your usual temperature limits: “Even if you’re already fit, you can still improve your perception of heat and how you perform in it.”
An organ-melting 43 degrees Celsius, three full degrees above the usual limit. We’ve traditionally viewed heatstroke as the last stop on a continuum: first you feel warm, then you’re uncomfortably hot, then you get heat exhaustion, and finally, if you don’t stop, you develop heatstroke.
The body’s defenses against heat, as we learned earlier, involve shunting blood toward the skin, where it releases heat. The flip side of this response is that the gut and other internal organs are starved of blood and oxygen. Eventually, this allows toxins that are normally corralled in the gut to begin leaking into the bloodstream, where they trigger a system-wide inflammatory surge. Heatstroke isn’t just about getting hot; it’s about a surge of inflammation that disables the body’s normal temperature defenses.
Contrary to the intuition drummed into us by a generation of public health messages, drinking more would not have saved Max Gilpin. And that, it turns out, is not the only piece of conventional wisdom about hydration that is wrong.
Thirst
The human body is about 50 to 70 percent water, and it needs pretty much all of it. You’re constantly losing water, not just from sweat but also from urine and more subtle leaks like the moisture in your breath. And, under normal circumstances, you’re constantly replacing it by eating and drinking.
According to calculations by U.S. Army researchers in a wilderness medicine textbook, you might last, in theory, for about seven days without water under ideal indoor conditions before reaching this critical point. If you’re lost in a hot desert and travel only by night, your expected survival plummets to twenty-three hours; if you also travel during the day, it’s sixteen hours.
No topic of advice in modern sports science has provoked more whiplash than hydration. A century ago, the prevailing advice to endurance athletes was to avoid drinking at all costs.
Robert Cade. Eventually came up with a drink containing water, sugar, and salts. The drink that became known as Gatorade never looked back.
But the rise of Gatorade kicked off a new era of interest in hydration for athletes, with generously funded research seemingly confirming its importance.
By 1996, the Gatorade-sponsored American College of Sports Medicine’s official position was that athletes should drink early and often in an attempt to “replace all the water lost through sweating … or consume the maximal amount that can be tolerated.”
Then came hyponatremia.
She had drunk as much as she could stomach during her run, causing the levels of sodium in her blood to become diluted (that’s what “hyponatremia,” sometimes referred to as “water intoxication,” means). Her lungs filled with fluid, and her brain began to swell, which after a few hours led to her death.
In 2003, U.S.A. Track and Field rewrote their guidelines to suggest that runners should drink when they’re thirsty rather than striving to replace all sweat losses or consuming “the maximal amount that can be tolerated.”
Dehydration is a greater concern in longer races, because you have more time to sweat; heatstroke, in contrast, is most common in shorter races. That’s because your body temperature is primarily determined by your “metabolic rate” — that is, how hot your engine is running.
Like Salazar, Gebrselassie sweats at a prodigious rate: in one lab test, he hit a rate of 3.6 liters per hour, which is among the highest ever recorded. By the end of his world – record run, he had lost nearly 10 percent of his body weight, dropping from 128 to 115.5 pounds.
Salazar’s “gastric emptying rate,” which determines how much fluid can pass through the stomach for absorption from the small intestine, was about one liter per hour while running. Given that his sweat rate was three times higher than that, there was never any chance he would be able to limit his fluid loss to 2 percent.
And since gastric emptying rarely exceeds 1.3 liters per hour, the same is true for many people, meaning that in prolonged exercise in the heat the 2 percent rule is more a theoretical ideal than a realistic plan.
At marathons, triathlons, and cycling races around the world, researchers have tried a simple test: weigh athletes before and after the race, and look for a relationship between race finish and degree of dehydration. The results are consistently the opposite of what you would expect: the fastest finishers tend to be the most dehydrated.
Instead of monitoring fluid levels, your body monitors “plasma osmolality,” which is the concentration of small particles like sodium and other electrolytes in your blood. As you get dehydrated, your blood gets more concentrated, and your body responds by secreting an antidiuretic hormone that causes your kidneys to start reabsorbing water, and by making you thirsty. Unlike your body’s fluid levels, plasma osmolality is very tightly regulated: when you’re looking at the right variable, your thirst sensation (along with other homeostatic mechanisms like antidiuretic hormone) doesn’t make mistakes.
Disconnect between thirst and water loss may actually be an evolutionary advantage rather than a bug.
Our ability to run long distances over the hot savanna gave us a crucial advantage over other species. To do that, we needed to be able to tolerate temporary periods of dehydration without negative effects.
In 2007, British scientists at the University of Loughborough estimated that a marathoner could conceivably lose 1 to 3 percent of his or her body mass without any net loss of water.
The result is that you can be “dehydrated,” at least in the sense that you’ve lost weight, without hurting your performance. What matters, instead, is how thirsty you are.
Avoiding thirst, rather than avoiding dehydration, seems to be the most important key to performance.
A 2013 meta-analysis in the British Journal of Sports Medicine concluded than any losses of less than 4 percent are “very unlikely to impair [endurance performance] under real-world exercise conditions.”
To me, the primary message is that, like oxygen and heat and (as we’ll discover) fuel, the loss of fluids first makes itself felt via the brain. Thirst, not dehydration, increases your sense of perceived effort and in turn causes you to slow down. Eventually, the physiological consequences of dehydration assert themselves, increasing the strain on your cardiovascular system and pushing your core temperature up as the volume of blood in your arteries decreases. But that only happens if you’ve already ignored the signs of thirst.
Fuel
The LCHF debate, which has been roiling the weight-loss world since the early 2000s, had recently made the leap to endurance sport.
The records for longest survival without food are both grim and confusing, depending on the precise circumstances and the trustworthiness of the witnesses. A frequently cited benchmark is Kieran Doherty, an Irish Republican Army prisoner at the infamous Maze Prison near Belfast, who refused food for 73 days in 1981 before dying. If you bend the rules a bit to allow vitamins in addition to water, then you’ll be able to continue accessing your body’s fat stores for much longer.
It’s not just how much fuel is in the tank, in other words. Endurance performance also depends on what types of fuel you have available, where it’s stored, and how quickly you can access it.
Importance has been debated for more than a century. Early experiments in the first half of the twentieth century showed that the balance between fat and carbohydrate use depends on how hard you’re working.
Subsequent biopsy studies confirmed that the amount of glycogen you can stuff into your muscles is a pretty good predictor of how long you’ll last on a treadmill or stationary bike test to exhaustion. There are other sources of carbohydrate in the body; your liver, for example, can store 400 or 500 calories of glycogen for use throughout the body, compared to about 2,000 for fully loaded leg muscles.
Your muscles can also dip into the glucose circulating in your blood, though the total amount of glucose in circulation at any given moment is very small.
One study found that Kenyan runners, who currently hold 60 of the top 100 men’s marathon times in history, typically get 76.5 percent of their calories from carbohydrate, including 23 percent from ugali, a sticky and stomach-filling cornmeal mash, and 20 percent from the copious spoonfuls of sugar they heap into their tea and porridge. Another 35 times on the top-100 list are held by Ethiopians; a similar study found that they get 64.3 percent of their calories from carbohydrate, with the biggest contribution from injera, a sourdough flatbread made from a local grain called teff.
The role that fuel stores play in the limits of endurance depends, of course, on what we mean by endurance. If you’re simply concerned with covering the greatest distance possible, without a particular focus on time or outsprinting rivals, then you might not care about pyruvate dehydrogenase.
But if your view of endurance involves racing — squeezing as much distance as possible out of the unforgiving minute — then it turns out that your primary fuel-related concern is not how much but rather how fast. How quickly do your muscles burn fuel? How quickly can they access the various sources of fuel scattered throughout your body? And how quickly can you refill those reservoirs as you go?
Researchers in Scandinavia have recently shown that glycogen stores in your muscles don’t just act as energy reservoirs; they also help individual muscle fibers contract efficiently.
The mouth appears to contain previously unknown (and as yet unidentified) sensors that relay the presence of carbohydrate directly to the brain. In Tim Noakes’s central governor framework, it’s as if the brain relaxes its safety margin when it knows (or is tricked into believing) that more fuel is on the way.
Your brain is looking out for your well-being in ways that are outside your conscious control and that kick in long before you reach a point of actual physiological crisis.
One of the biggest challenges for ultra-runners is refueling: convincing your recalcitrant stomach to accept yet another sports gel or banana or whatever else you’re trying to force down your gullet after twelve hours on the trail, without sending you scurrying to the bushes.
Jeff Volek of Ohio State University (and including LCHF pioneer Stephen Phinney) recruited twenty elite ultra-runners and Ironman triathletes. The results, published in the journal Metabolism in 2016, showed that the fat-adapted runners were able to burn fat twice as quickly as the non-fat-adapted control group.
Carbohydrate as a fast fuel with limited storage capability, fat as an inexhaustible but rate-limited alternative.
The Supernova results, which were published in 2017, confirmed that endurance athletes on a three-week high-fat diet became fat-burning machines to an extent few had imagined possible. By the end of a 25-kilometer time trial at their expected 50-kilometer race pace, the athletes were burning through 1.57 grams of fat per minute, which is two and a half times greater than the “normal” values seen in athletes eating a standard carbohydrate diet. That was the good news. The problem was that the fat-adapted athletes became less efficient, requiring more oxygen to sustain their race pace. This, it turns out, is a consequence of the cascade of metabolic reactions required to transform either fat or carbohydrate into ATP, the final form of fuel used for muscle contractions: the fat reactions require more oxygen molecules.
“Nutrition is a cyclical science,” Burke says. “You’d be surprised at how many ‘new ideas’ are simply old ideas reimagined.
For now, Burke is betting on a “periodized” approach to carbohydrate and fat during training — that is, carefully selecting certain workouts to perform with full carbohydrate reserves and others to do on empty. The goal isn’t necessarily to boost fat usage in competition; instead, the carbohydrate-depleted workouts function as the nutritional equivalent of a weighted vest, forcing the body to work harder and triggering greater fitness gains in response.
Two Hours: March 6, 2017
Technology evolves, but when it evolves so quickly that it effectively picks winners, that’s a problem. The top three finishers in the men’s Olympic marathon in 2016, it turns out, were wearing disguised prototypes of the new shoe, which Nike has dubbed the Vaporfly. So was the women’s winner; so were the men’s winners of the 2016 London, Chicago, Berlin, and New York marathons.
Limit Breakers
Training the Brain
Long before you reach that point of extremity, you’ll be feeling the effects. At first you might not notice the subtle change, but gradually the effort required to sustain your pace will grow until you become conscious that you won’t be able to continue forever — that the unforgiving minute must eventually end. At this point, your core temperature is still within the normal range, your muscles still have all the fuel and oxygen they need, and the metabolic by-products of exercise haven’t yet accumulated to a level that interferes with your forward progress. Only your brain knows that trouble is coming. But the clock is still ticking.
There’s another physics analogy that this debate reminds me of: the dispute between various interpretations of quantum mechanics (Copenhagen, many-worlds, De Broglie-Bohm) that all converge on the same set of equations and predictions. They’re different ways of thinking about the same thing.
In 2009, one of Noakes’s former students, Ross Tucker, published a paper in the British Journal of Sports Medicine on the “anticipatory regulation of performance,” in which he tried to explain how, exactly, the brain knows in advance to slow you down before catastrophe strikes.
The answer, Tucker suggested, was Borg’s rating of perceived exertion, or RPE.
Pacing, in Tucker’s formulation, is the process of comparing the effort you feel at any given point in a race to the effort you expect at that stage — an internal template that you develop and fine-tune from experience.
Is this really an explanation of how endurance is regulated, or is it simply a description of how it feels?
Where Marcora disagrees with Tucker and Noakes is in the extent to which such decisions and computations take place consciously and voluntarily versus unconsciously and automatically.
Effort is no longer just a by-product of the physiological strain that causes you to slow down or stop; in the effort-centered view. Effort is what causes you to slow down or stop.
Marcora’s colleague Alexis Mauger had published a highly controversial study using a new effort-based protocol to measure VO2max. Instead of putting subjects through a “brainless” test where the speed increases in set increments, Mauger’s subjects ran or cycled at steadily increasing levels of self-determined effort. The results, which remain highly contentious, showed that subjects reached higher VO2max values in the effort-based test than in the traditional test — an impossible paradox if you believe that VO2max represents a physical ceiling on oxygen consumption.
How do you improve your response inhibition? By inhibiting your responses, over and over, in a systematic way. Marcora’s mental fatigue studies use a set of standard cognitive tasks that can be tailored to tax different aspects of cognitive control, including response inhibition.
“Being boring is an important characteristic for inducing mental fatigue and, therefore, a brain training effect,” he replied. “Just do a longer session of one test at a time.”
Anxious people, he found, tend to overreact to negative stimuli, producing a distinct pattern of brain activity. Elite endurance, athletes, on the other hand, display a completely opposite response pattern. Was there a way, he wondered, of training the brains of the former to look more like the latter?
Paulus and his colleagues have found that crucial differences show up in the activation of the insular cortex, a region of the brain that monitors sensory signals from within the body.
Zapping the Brain
Dylan Edwards and David Putrino, a pair of Australian-born neuroscientists.
People have been shocking their brains for fun and profit since long before anyone understood what electricity was. Scribonius Largus, the court physician for the Roman emperor Claudius more than two thousand years ago, recommended the application of a live torpedo fish — an electric ray capable of delivering up to 200 volts at a time — to the forehead for relief of headaches, and other cultures around the world prescribed electric fish for everything from epilepsy to exorcism.
These days, talk of electricity and the brain still tends to provoke comments about either Frankenstein (a book that was reputedly inspired, in part, by Aldini’s public demonstrations) or One Flew Over the Cuckoo’s Nest.
In tDCS, the current is 500 to 1,000 times smaller — too small to directly cause the neurons to fire. Instead, sustaining this small trickle of current for ten to twenty minutes alters the sensitivity of the neurons, making them slightly more likely to fire (or, if you run the current in the opposite direction, slightly less likely to fire).
Functional MRI scans showed that two regions of the brain, the insular cortex and the thalamus, were more active during the failed contractions.
The insular cortex monitors incoming signals from the rest of the body. “It’s not just muscle signals,” Lutz notes. “The insular cortex is also involved in the emotional response of hearing your heart pound and so on.”
You can use electroencephalography — better known as EEG — to listen directly to the electrical activity in the brain in real time.
With the heightened time sensitivity of the EEG data, a telltale pattern emerged in the data. Shortly before the cyclists gave up, there was an increase in communication between the insular cortex, which was monitoring their internal condition, and the motor cortex, which issued the final commands to their leg muscles. The brain, in other words, knew that the cyclists were about to reach their limits before their legs actually failed, seemingly demonstrating Noakes’s anticipatory regulation in action.
Suppress the excitability of neurons in the insular cortex — the site of Lutz’s central governor — and you might turn down the insular cortex’s brake signal, allowing the motor cortex to keep driving the muscles for a little longer.
Red Bull’s ethos of extreme adventure and boundary-pushing applies as much to its high-performance research program as it does to its athletes and advertising (all of which are, of course, intertwined).
“Our brains are sending signals to our muscles; as we fatigue, those signals are getting weaker and weaker,” Putrino explained. “The brain is making a choice. But the brain’s opinion isn’t always right.”
Brain stimulation may or may not turn out to be an effective way of accessing your hidden reserves, but there was little doubt that the athletes at the camp came away from the experience convinced that these reserves exist. In the end, when it comes down to two guys on a bike, maybe that’s the real secret weapon: believing that you have another gear.
Belief
In a sense, every stride you take during a race is a microdecision: will you speed up, slow down, or maintain your current pace? But some decisions are more consequential than others.
The Kenyan up-and-comers would simply run with the leaders — often international champions — for as long as possible, then drop out or start jogging when they could no longer keep up. Coolsaet and other foreigners, meanwhile, would maintain a steady but sustainable pace. At one point, he took some friends to watch the famous weekly fartlek workout in the hills around the town of Iten. More than two hundred runners streamed past them, raising a cloud of red dust from the dirt roads; about a third of them had dropped out of the workout before the halfway mark.
Trent Stellingwerff. At a conference in 2013, Stellingwerff noted the wide variety of supplements and training methods that have been shown to produce a 1 – 3 percent boost in performance, from caffeine to beet juice to altitude training. In theory, combining all these approaches should create a superathlete; in practice, studies that combine multiple interventions in elite athletes tend to see overall improvements of … 1 to 3 percent. If 1 + 1 + 1 = 1, the implication is that many different “proven” training aids act, at least in part, on the same target: the brain.
Believing you can run a 2:05 marathon isn’t the same as running it. Philosophers make a distinction between justified beliefs and true beliefs. You can have a good reason for believing something (that your car is in the garage, for example) even if it turns out not to be true (because someone has stolen it). Conversely, you can believe something that turns out to be true (that you will draw an ace) for no good reason. Knowledge, according to some philosophical accounts, requires justified true belief.
Training is the cake and belief is the icing — but sometimes that thin smear of frosting makes all the difference.
Two Hours: May 6, 2017
The thing about a two-hour marathon, under the heavily scripted conditions that Nike has orchestrated, is that it should be almost comically boring. If all goes well, there will be no surges, no breaks, no comebacks, and not even the slightest variation in pace: just three men, an arrowhead, and a clock.
The wheels don’t fall off; Kipchoge doesn’t hit the wall like Tadese and Desisa, who are now six and fourteen minutes behind, respectively. But he doesn’t manage to reaccelerate. Fighting all the way to the finish, he crosses the line in 2: 00: 25.
“The world now,” he says, “is just twenty-five seconds away.”
