Figuring out fatigue: A tired brain can hinder performance as much as a tired muscle

Once considered only a physical phenomenon, new research suggests fatigue can arise from the brain. Brain Endurance Training is just one way athletes are trying to beat mental fatigue.

Physical skill transcends sport and can be recognized in its absolute form—speed, power, coordination—across a variety of athletic endeavors. But less visible, and perhaps less considered, is the power of the elite athlete’s mind. In fact, what often differentiates the greatest athletes has more to do with mental strength than physical. 

It’s because sport not only demands a lot of the body, but the brain as well. 

“With the exception of military combat, it has been suggested that team sports such as football [soccer] place more stress on the brain than any other activity,” writes Dr. Andrew Coutts in recent Journal of Sports Sciences article. “Indeed, football players are required to remain vigilant for long periods before and during matches, adhering to tactical strategies, constantly adjusting to changes in the opposition and their teammates.”

Those demanding game situations—in combination with factors like training, sleep and stress—challenge an athlete physically and mentally and lead to the development of fatigue. But while physical fatigue has long been considered a factor in performance, diminishing an athlete’s capacity to react, run faster and jump higher, researchers are beginning to understand that a tired brain can negatively affect performance as much as a tired muscle.

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Researchers have suggested that the sensation of fatigue, once considered solely a physical phenomenon, might also arise from the brain. Meaning that the brain is responsible for collecting the physical sensations of the body—the burning legs and heaving lungs—and deciding how much is too much. This research has demonstrated that mental fatigue—produced by sustained periods of demanding cognitive activity, and described by feelings of “tiredness” and “lack of energy”—can reduce the time it takes to reach exhaustion during exercise. 

Dr. Samuele Marcora has studied the effects of mental fatigue on soccer performance and discovered that mentally tired athletes don’t perform as well. After inducing mental fatigue with a demanding cognitive test, Marcora and his team of researchers found that the mentally fatigued soccer players couldn’t run as far or kick a ball as skillfully as their mentally-fresh counterparts.

It is important to note that even though the mentally fatigued athletes were performing at an equal level of physical exertion as a control group, those mentally fatigued players perceived the effort as more difficult than those not asked to take a mentally demanding test. Meaning their effort wasn’t physically harder, it just felt harder. 

“Physiologically you may be fine but mentally fatigued athletes find the same task much more effortful,” says Marcora.

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Similarly, a recent study by Coutts and Dr. Mitchell Smith from the University of Technology Sydney found that mental fatigue impaired the accuracy and speed of soccer-specific decision-making.

But for soccer players, and all team sport athletes, prevention of mental fatigue is more than just avoiding a math exam before activity. While research requires that mental fatigue be artificially produced with a test, mental fatigue can be developed through a variety of natural activities. 

“Though studies have found that mental fatigue can develop from sleep deprivation, video games, and having to perform an activity deemed unfamiliar or difficult, such as an interview, any sport that is mentally demanding can induce mental fatigue,” Marcora says.

In that context, it’s easy to see how mental fatigue may develop before a game or practice and subsequently decrease performance.  

“I’d always been told by coaches that when the legs are too tired, the mind will take over,” says Brad Evans, a defender for MLS’ Seattle Sounders of MLS. “But later on I realized that it doesn’t work that way.” Evans, also a veteran of the U.S. men’s National Soccer Team, is well aware of the role of mental fatigue, “Mistakes happen when you are mentally and physically fatigued—like missing a tackle I would usually make.”

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Some of soccer’s most successful teams have invested in research on tracking fatigue and recovery, realizing the importance of the subject. Andrea Azzalin, sports scientist for Premier League champion Leicester City and a former student of Marcora, continually monitors the team for signs of mental fatigue after every training session and game. For Azzalin, it comes down to knowing his athletes.

“Monitoring how the players feel—their perceived effort—is often enough for me to understand that they are more fatigued than usual,” he says. 

Leicester City’s sports science staff uses markers of mental fatigue, along with GPS and heart rate information from each player, as tools to detect situations of overload. That information is relayed to the club’s manager, Claudio Ranieri, who decides if the training schedule or other player demands need to be adjusted.

Golden State Warriors’ head of physical performance and sports medicine Lachlan Penfold, who comes from a background in rugby and soccer, is also aware of the research and absolutely believes that mental fatigue can impair performance in any sport.   

“A tremendous amount of fatigue—mental and physical—develops over the course of a long NBA season,” Penfold says. “Players can be more or less resistant to its effects.”

While Penfold believes that an awareness of how to avoid mental fatigue has the potential to help many professional athletes, he says that getting players to buy into avoiding using social media or video games—potentially mentally fatiguing activities—before games may be a tough sell. 

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“Since the effects of mental fatigue sometimes aren’t obvious—manifesting as an opening not capitalized upon or a move not made—it can be difficult to quantify or present its effects,” maintains Penfold.

With its long 162-game season, it should be no surprise that mental fatigue is also evident among Major League Baseball players. A study by researcher Dr. Scott Kuscher found that plate discipline—as measured by a hitter's tendency to swing at pitches outside of the strike zone—got progressively worse over the course of a MLB season, contradicting the belief that plate discipline should improve over time due to frequent practice. Kutscher believes that sleep, or a lack of it, is behind the decline in performance. 

“We theorize that this decline is tied to fatigue that develops over the course of the season due to a combination of frequency of travel and paucity of days off,” concludes Kutscher.

Avoiding mental fatigue before competition is one thing, but because an athlete can tire over the course of game or training week, building resilience to mental fatigue is also important. Andrea Bosio, a sports scientist working with Serie A soccer teams Sassuolo and Juventus, believes that much like taxing the heart, lungs and muscles with physical conditioning, a combination of small sided soccer games (somewhat like a half-court basketball game) and mental training can be used to make the brain more resistant to mental fatigue. Call it brain training—or more specifically, Brain Endurance Training, the term Bosio and other researchers at Mapei Sport in Milan, Italy, have coined during their developing and testing of this mental conditioning for the brain.

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The BET program, developed by Marcora, aims to improve mental performance. Athletes are given repeated mental to perform during the rest periods of intense small-sided games. Similar to "normal" physical training, these brain-training sessions can be systematically carried out over a period of weeks and months.

The adaptations occurring in the brain after repeated sessions of BET (the scientists are still trying to understand the biology of what occurs) seem to positively influence the perception of effort during endurance activity. Early results suggest that at the same intensity of training, players perceive less effort, and at the same perceived effort, players are able to sustain a higher intensity.

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Seattle Sounders sports science and performance manager David Tenney uses yoga in his attempts to reduce mental fatigue in his players. 

“I’ve found that the mental aspects of yoga, with its focus on breathing and relaxation, have been helpful in reducing the mental fatigue of practice and competition,” he says.

No matter the sport or proposed solution, mental fatigue and findings have proven one thing for certain: for an athlete, a tired mind is just as meaningful as a tired muscle.