In perhaps a defining moment in his career, Lionel Messi missed his penalty kick at the end of the 2016 Copa America Final. The soccer world asked how this iconic player, voted to be the best in the world five times, could blast the ball over the goal in such a crucial moment at a major tournament? Certainly, Messi had played in overtime games before and was able to handle the physical toll. However, the mental stress of the moment may have been too much for his world-class skill to take over.

Coaches and players talk about it, complain about it and even blame results on it but it's been difficult to measure mental fatigue. Physical endurance is easily tracked and managed through several physiological metrics. But during a strenuous game in the middle of a long season, how does the mental grind affect technical sports performance? Dr. Samuele Marcora, professor and director of research at the School of Sport and Exercise Sciences at the University of Kent, found a lack of research evidence on how the two are related so he designed an intriguing study that found a direct correlation between cognitive load and decreased physical and technical performance in soccer players.

While it seems obvious that a player’s performance will decline when physically tired, so much of their skill on the field or court is directly related to their brain directing those athletic movements. Attention, visual perception, reaction time and skill execution is only as sharp as the neural instructions given.

“To date however, only one study has examined the effects of mental fatigue on a factor directly associated with team sport performance,” wrote Dr. Marcora in a paper recently published in Medicine & Science in Sports & Exercise.

In that research, intermittent running was shown to suffer after a mentally draining task. Still, the actual sport-specific performance (i.e. passing and shooting in soccer) is what matters to coaches.

For this latest research, Dr. Marcora chose to conduct two separate studies on a group of trained soccer players, one to measure the effect of mental stress on physical performance and the second to measure how mental stress challenges technical soccer performance. Both studies also looked at fatigue perception after a cognitively demanding task.

He and his team combined three measurement tools, the Yo-Yo Intermittent Recovery Test (to test the athlete’s physical performance), a paper version of the Stroop test (to inflict some mental stress), and the Loughborough Soccer Passing and Shooting Tests (to measure their soccer-specific technical skills).

As Dr. Marcora has found in his extensive research on fatigue, an athlete’s perception is often more powerful than reality as his brain sends out warning signals before the body is actually out of energy. So, in this study, he also asked the athletes for their pre and post workout assessment of their fatigue measured as their Rate of Perceived Exertion (RPE) on the standard Borg scale from 6 to 20 (where 20 indicates being completely exhausted.)

In the first study, twelve players were divided into two groups with different mental tasks. Half of the participants simply read some magazines for 30 minutes. However, the other half were assigned to complete a half hour of the Stroop test, a cognitively grueling task that gives your brain a tough workout.

You’ve probably heard about the Stroop test. It is a common neuropsychological test of selective attention, cognitive flexibility and processing speed. You are shown a list of words that spell different colors but with letters that are of a different color. So the word “blue” may be shown with red letters. Your job is to call out the color of the ink rather than the meaning of the word. So you would call out “red” instead of what the word actually spells, “blue”. Still confused? Here’s a good overview of the Stroop test.

After the two groups completed their 30 min mental exercise, they were asked to perform the Yo-Yo IR1 (aka the “beep test”), where they repeat an intermittent running task of 20 meters to a timed rhythm until they can’t keep up with the prompts. Their performance, their heart rate and their RPE were recorded at the completion of the exercise.

As hypothesized, both perceived exertion and running performance were significantly worse for the Stroop group over the controls, with RPE scores going up while distance covered dropped. Interestingly, heart rate averages were similar between the groups.

So, the first study confirmed previous research that mental fatigue can directly affect physical performance. In soccer, stressed out players will most likely cover less ground at a slower pace late in the game.

But what about their technical performance? Will pass completion rates and shot accuracy suffer if the brain is tired? That was the focus of Dr. Marcora’s second study.

Using 14 soccer players, the experimental set-up was similar. Two groups were asked to either read magazines or do the Stroop test for 30 minutes, followed by recording their RPEs.

However, instead of testing their physical response, they were asked to complete two soccer-specific skill tests, the Loughborough soccer passing test (LSPT) and the Loughborough soccer shooting test (LSST). The easiest way to describe the tests is by watching them with these links; LSPT and LSST.

Sure enough, the stressed-out Stroop group committed more passing and ball control errors and were slower and less accurate with their shots on goal during the tests compared to the control group. In addition, just like with the first study, RPE scores went up for the Stroop group showing their perceived exertion of the soccer tasks was higher.

What can players and coaches take away from this research? Dr. Marcora has some final thoughts, “This multifaceted effect of mental fatigue on physical and technical performance of soccer players suggests that mental fatigue may have a very negative impact on performance during a soccer match. Therefore, we suggest that coaches and players assess their pre-match activities to ensure players do not engage in tasks requiring sustained attention for 30 min or more before competition, as this may induce mental fatigue and impair performance.”

“Furthermore, sport scientists and coaches should identify and implement strategies, such as half-time caffeine supplementation that reduce the mental fatigue induced by the high cognitive demands of playing soccer for up to 90 min.”

As for Mr. Messi, his very rare miss may be a one-off mistake. But with the immense pressure put on him, it is understandable and helpful for fans to admit that he too is human.