on how you look at it, the results were either utterly predictable or, from the perspective of textbook physiology, inexplicable. After the mentally draining computer game, the subjects gave up 15.1 percent sooner in the cycling test, stopping on average at 10 minutes and 40 seconds compared to 12 minutes and 34 seconds. It wasn’t because of any detectable physiological fatigue: heart rate, blood pressure, oxygen consumption, lactate levels, and a host of other metabolic measurements were identical during the two trials. Motivation levels, as measured by psychological questionnaires immediately before the cycling tests, were the same—helped along by a £50 prize for top performance. The only difference was that, right from the very first pedal stroke, the mentally fatigued subjects reported higher levels of perceived exertion. When their brains were tired, pedaling a bike simply felt harder.
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. A Borg score of 13 to 14, for example, corresponds to an effort you’d call “somewhat hard,” which would produce a heart rate of 130 to 140 beats per minute in most people. But Borg viewed the effort scale as far more than a convenient shortcut for researchers whose heart-rate monitor ran out of batteries. “In my opinion,” he wrote, “perceived exertion is the single best indicator of the degree of physical strain,” since it integrates information from muscles and joints, the cardiovascular and respiratory systems, and the central nervous system.
In his talk at the conference in Bathurst, Marcora took this argument a step further. Perceived exertion—what we’ll refer to in this book as your sense of effort—isn’t just a proxy for what’s going on in the rest of your body, he argued. It’s the final arbiter, the only thing that matters. If the effort feels easy, you can go faster; if it feels too hard, you stop. That may sound obvious, or even tautological, but it’s a profound statement—because, as we’ll discover, there are lots of ways you can alter your sense of effort, and thus your apparent physical limits, without altering what’s happening in your muscles. Case in point: getting mentally fatigued increases your sense of effort (by between one and two points on the Borg scale, in Marcora’s protocol) and thus reduces endurance. By definition, the cyclists always decided to quit as their perceived exertion approached the maximum of 20; they just reached that point sooner when they were mentally fatigued.
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. In his talk, Marcora offered a now-classic illustration of this, from a 1986 experiment by French researcher Michel Cabanac. Cabanac asked volunteers to sit bent-legged against a wall with no chair for as long as they could, offering varying rewards for each 20-second period they stayed in position. When the subjects were offered 0.2 francs per 20 seconds, their quads gave out after just over two minutes, on average; when they were offered 7.8 francs per 20 seconds, their endurance magically doubled. If the moment of collapse was dictated by a failure of the muscles, how did the muscles know about the richer payoff?
Marcora himself produced a similar mind-over-muscle demonstration with a group of elite rugby players who competed in a time-to-exhaustion cycling test. At an average target power of 242 watts, which corresponded to 80 percent of their peak power, the players lasted for about 10 minutes, with cash prizes to ensure they fully exhausted themselves. As soon as they gave up—within three to four seconds—they were asked to see how much power they could generate in a single 5-second burst of pedaling. Curiously, although they had just declared themselves incapable of producing 242 watts, they managed to average 731 watts during this five-second sprint. It follows that the subjects didn’t stop the test because their muscles were physically incapable of producing the required power; instead, the researchers argued, it was perception of effort that mattered.
At the exercise physiology conference in Bathurst, Marcora laid out his case with characteristic zeal. Amid the mostly uniform crowd of tracksuit-clad ex-athletes, he cut a swashbuckling figure, with untucked shirt, perma stubbled jaw, and casual asides about his plan to motorcycle along Australia’s Great Ocean Road after the conference. At one point, he showed a bewilderingly complex slide taken from a recent paper describing the conventional model of endurance fatigue—a flow chart with forty-four different boxes ranging from heart rate to “mitochondrial density/enzyme activity”—and then compared it to the equations for general relativity and quantum mechanics. “Physicists can explain the whole universe with two theories, and they’re not happy with that,” he said. “Endurance performance is complicated, but it’s not more complicated than the entire universe!”
The simple alternative, Marcora argued, is that anything that moves the “effort dial” in your head up or down affects how far or fast you can run. All the usual physical cues—dehydration, tired muscles, a pounding heart—contribute to how hard an effort feels. Athletes train their bodies to adapt to those cues, and over time the effort of running at a given pace gets lower. But less obvious factors, like mental fatigue, also contribute to how hard your run feels—and trying to hold marathon pace for hours and hours, for example, is pretty taxing on the brain. This, Marcora told the conference, leads to a radical idea: 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. “I have an eye for things that at a superficial level seem crazy,” he said. “If I tell somebody, okay, I’m going to improve your endurance performance by making you sit in front of a computer and do things on a keyboard, you will think I’m nuts. But 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. So my reasoning is simple: We should be able to get the same effect by using mental fatigue.”
This was an unexpectedly bold prediction, so I cornered Marcora during a break after his talk to find out more. He was designing a study to test whether “brain endurance training”—weeks of doing mentally fatiguing computer tasks—could, without any change in physical training, make people faster. I pestered him for details and asked if I could try it. He patiently answered my questions, then added a warning. “People who have done these mental fatigue studies—it’s not nice,” he said. “It’s really bad. They hate you at the end of the task.”
In June 1889, as the academic term at the University of Turin drew to a close, a physiologist named Angelo Mosso conducted a series of experiments on his fellow professors before and after they administered their year-end oral exams. He attached a two-kilogram weight to a string, and asked the professors to raise and lower the weight every two seconds by flexing their middle fingers, and then repeated the task using electric shocks to force the fingers to contract. The number of contractions they managed after three and a half hours of grilling their students was dramatically reduced compared to their baseline performance—a clear indication that “intellectual labor” had sapped their muscular endurance.
Mosso’s results, which were collected in an influential text called La Fatica (“Fatigue”) in
