Could spending too much time on Twitter, Facebook or Instagram affect a footballer’s performance? Manchester United appear to think so. One report before their trip to Anfield claimed the club were “consulting leading experts” about how to help their stars deal with the negative mental effects of social media and also suggested there were concerns that players were suffering dips in form as a result of being abused online.

Some may suggest the players should just “man up”. But scientists are increasingly scrutinising how mental fatigue – the lack of energy we feel from long periods of purely cognitive activity – can influence sporting performance, often with startling results.

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Only last week a study by academics on Brazilian professional footballers, published in the prestigious Journal of Sports Sciences, came to an eye-opening conclusion. Being on “social networks on smartphones and/or playing video games for at least 30 minutes before official matches impairs the passing decision-making performance in professional soccer athletes”. The researchers’ advice for managers was also as crisp as a Zico free-kick. “Coaches should evaluate the use of social networks on smartphones and/or playing video games before official matches in soccer athletes.”

It is easy to imagine many old-school managers rolling their eyes and saying “tell us something we didn’t know 10 years ago”.

However, the snappily titled article, “The effect of smartphones and playing video games on decision‑making in soccer players: A crossover and randomised study”, is significant because it backs up that instinct with hard science.

The academics’ research was based on a series of pre‑season matches played by professional players from two teams in Brazil – only with a twist. Before each game they split players into three groups. The first control group watched videos for 30 minutes. The second looked at social media on their phone for the same period. The third played Fifa 18. The footballers also did the Stroop task, a well-established psychological test, before and after these activities to assess their mental fatigue. They then warmed up and played a 90-minute match which was filmed and analysed.

Three or four days later the players switched their pre-match activity and played another 90 minutes. Six matches were played in total and to keep the players motivated they were told the games would be used to select the team once the season started.

The due diligence did not end there. The academics asked the participants to rate their levels of energy before each game to assess recovery from the previous match. Afterwards players had their blood lactate levels analysed and were required to give their perceived rate of exertion during the game.

As well as all that, two experienced analysts watched each game. Their task was simple: assess every pass made by every player and assess whether it was a good decision. After the numbers had been crunched the scientists found that, while each of the three groups – video watchers, social media viewers and Fifa 18 players – made a similar number of passes overall, the quality of passing decision-making decreased in the players who had used social media or played Fifa beforehand when compared with the passive video watchers.

So what might be going on? We all know football is an extremely dynamic and tactical sport, which places high mental demands on decision-making. However, the academics suggest being on social media and playing Fifa 18 leads to a prolonged period of electrical impulses in certain areas of the brain, reducing the speed of information processing – thus leading to less successful decision‑making accuracy.

“When mentally fatigued, athletes usually demonstrate longer fixation time on opponents and shorter fixation time on their teammates,” they point out. “Mentally fatigued athletes may not adequately interpret (sports perception) or anticipate (frontal cortex) opponent’s actions, which affects the decision-making accuracy. Thus, it seems that mental fatigue negatively affects the way players choose their tactical actions during matches.”

This is a small study, one of the veryfew that have looked at concrete evidence of the link between smartphones and video game use on elite performance on the pitch. However, other studies based on non-elite athletes have found mental fatigue has led to slower and less accurate reactions, poorer visual perception and even a reduction in the collective movement of a team in small‑sided games.

Elsewhere laboratory studies have also found some evidence of a decline in endurance performance following a mentally challenging task. One study, for instance, found a significant decline in power output in a 20-minute cycling time trial when athletes are already mentally fatigued.

Other research, based on interviews with elite Australian athletes, found mental fatigue can negatively influence player performance in other ways – including decreased motivation and enthusiasm and less attention to details.

Perhaps most of this comes as no surprise. Even so, after years when academics have focused largely on the physical nature of fatigue as opposed to its mental aspects, the focus on potential mental stressors and their relation to mental fatigue in elite sport is surely overdue.

At any rate the message from the Brazilian academics seems clear enough: staying off social media immediately before a match will not just help you to avoid the trolls; it might help you to perform better.