As Marcora and his fellow motorcyclists rumbled across Europe and Central Asia, they were gradually becoming fitter: losing weight, increasing grip strength, gaining aerobic fitness. But they were also getting increasingly tired. Before and after each day’s ride, Marcora administered a Psychomotor Vigilance Test to his subjects, who had to tap a button as quickly as possible on a small handheld device in response to an irregular series of flashing lights. On average, their reaction time slowed from about 300 milliseconds in the morning to 350 milliseconds after nine or more hours in the saddle—a significant decrease if you’re whipping around a blind corner on a mountain road or swerving to avoid a wandering goat. The decline was most pronounced as they crossed the Tibetan plateau, where the thin air magnified the effects of mental fatigue: average end-of-ride scores on the Psychomotor Vigilance Test ballooned to 450 milliseconds.
Fortunately, Marcora had a potent countermeasure. Tucked into his pannier of lab equipment was a stash of Military Energy Gum, a chewing gum containing 100 milligrams of caffeine that is quickly absorbed through the inner lining of your mouth. Half of the gums were the standard-issue rocket fuel; the other half were specially prepared caffeine-free placebos. Starting after lunch each day, Marcora chewed six pieces of gum, having organized and disguised them so that even he didn’t know if he was getting caffeine or not that day. When he crunched the data after the trip, the results were striking: the slowdown in reaction time between the beginning and end of the day was completely eliminated on the days his gum contained caffeine.
Caffeine’s perk-up powers aren’t exactly a secret—without even considering coffee, caffeine pills are already one of the most widely used legal supplements among athletes—but the results illustrate how, in Marcora’s view, everything comes down to the perception of effort. 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. Warding off mental fatigue, in turn, keeps your sense of effort lower, allowing you to exert yourself harder and longer.
The demands of riding a motorcycle may seem far removed from typical tests of endurance, but in fact they closely mimic the demands encountered by soldiers, Marcora points out. In both cases, you have to maintain high levels of focus and concentration for hours at a time while doing moderate physical activity in bulky, poorly ventilated gear. And in both cases, even a brief lapse can be fatal. As a result, much of the funding for Marcora’s research, from caffeine gum to “brain endurance training,” comes from Britain’s Ministry of Defence, who are interested in ways of fighting both mental and physical fatigue.
Closely linked to the sustained attention required by adventure motorcyclists and soldiers is another 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. The experimenters offered preschoolers a choice between one treat right away, or two treats if they waited for fifteen minutes. Over decades of follow-up, the children who resisted temptation the longest ended up with better test scores, more education, and lower body-mass index. Other studies have linked low response inhibition to higher risk of outcomes like divorce and even crack cocaine addiction.
No one has checked whether the kids who aced the marshmallow test were more likely to become champion endurance athletes—but they should. For motorcyclists and soldiers, impulse inhibition matters because you have to suppress the urge to let your mind wander, and a similar challenge faces marathoners and other endurance athletes. Think of it this way: If you stick your finger in a candle flame, your natural response will be to yank it out as soon as you start feeling heat. The essence of pushing to your limits in endurance sports is learning to override that instinct so that you can hold your finger a little closer to the flame—and keep it there, not for seconds but for minutes or even hours.
Marcora and his colleagues tested this idea in an experiment in 2014, using a technique called the Stroop task to tax their subjects’ response inhibition. The task involves words flashing on a screen in various colors; you have to press a particular button in response to each color. What’s tricky is that the words themselves are colors: you might see the word green in blue letters, and you have to overcome your initial impulse to press the button corresponding to green instead of blue. In the study, subjects performed the task twice: once with the words and colors mismatched, requiring response inhibition, and once with the words and colors matched, as a control. In both cases, after 30 minutes of the cognitive task, they ran a 5K as fast as possible on a treadmill.
The results were clear. Even though the subjects weren’t aware of any mental fatigue, they started their 5K slower after the response inhibition version of the task, rated their level of effort higher throughout the run, and finished with times 6 percent slower. That suggests that 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. Holding your finger to the flame (or simply focusing on a tricky computer task) takes mental effort, and that effort is just as real as the effort of moving your legs.
It has long been a cliché that the best athletes are defined as much by their superior minds as by their muscle. With response inhibition, we have a way of testing this, which is what a team based at the University of Canberra and the neighboring Australian Institute of Sport, working with Marcora, decided to do. They recruited eleven elite professional cyclists and compared them with nine trained amateur cyclists. All the volunteers completed two 20-minute time trials, one preceded by a 30-minute Stroop task to deplete their response inhibition, the other preceded by a control task of simply gazing at a black cross on a white screen for 10 minutes.
The first interesting finding was that the professionals were significantly better at the Stroop task, amassing an average of 705 correct responses during the 30-minute test compared to 576 for the amateurs. In other words, to the list of measurable traits that distinguish the pros from the rest of us—the size of their heart, the number of capillaries feeding their muscles, their lactate threshold, and so on—we can now add response inhibition.
The second interesting finding was how the cyclists performed in the time trial after completing the response-inhibiting Stroop task. The amateurs, depleted by the mental effort of focusing on all those flashing letters, produced 4.4 percent less power than in their control ride. The pros, on the other hand, didn’t slow down at all. They were able to resist the effects of mental fatigue, at least in the doses produced by a 30-minute Stroop task, and cycle just as fast as when they were fresh.
There are two ways to explain these findings. One is that the pros were born with superior response inhibition and resistance to mental fatigue, and that’s one of the reasons they’ve ended up as elite athletes. The other is that long years of training help the mind adapt to resist mental fatigue, just as the body adapts to resist physical fatigue. Which is it? I suspect a bit of both, and the smattering of evidence that exists supports the idea that these traits are partly inherited but also can