A new study says maybe.
A recent study published this summer looked at the impact mental fatigue – and social media use – can have on exercise performance. And while the study found that mental fatigue did not impair exercise performance in terms of force applied in a single rep, other studies have shown that similar mental fatigue can reduce performance in tasks involving muscular endurance and whole-body endurance. This may be due to the depletion of cognitive resources and a higher perceived effort during exercise. Tasks that require decision-making and self-regulation, such as endurance tasks, are more likely to be influenced by mental fatigue compared to short-duration, high-intensity exercises.
How did they study this?
Researchers conducted a randomized controlled trial to determine the effects that mental fatigue might have on bench press force-velocity in one repetition, as well as jump performance. To do this they collected data from 25 men with an average age of 26, who could bench press about 1.4 times their body weight.
The study participants either engaged in intense activity on social media, including reading texts, writing messages and posting; performed a “Stroop” task, which is a neuropsychological test that measures a person’s attention capacity and skills, as well as their processing speed, etc.; or were part of the control, which had participants watching a documentary for 30 minutes. All the men then completed visual testing to determine their mental fatigue, followed by exercise tests.
The physical portion of the study had each man performing two countermovement jumps with 60 seconds of rest in between, followed by three sets of bench pressing on the Smith machine, separated by two minutes of rest each time. Each set followed the 2, 2, and 1 repetition at maximum velocity of 50%, 70% and 90%. Bench press 1RM was estimated from calculated force output during the 5 completed repetitions.
What did they find?
Notably, the study did NOT find that mental fatigue seemed to play a role in the force output or the countermovement jump performance. But, the impact of this mental fatigue may be different for muscular endurance and whole-body endurance activities. These studies seem to suggest that mental fatigue does not impair short duration anaerobic exercises, but it does seem to affect performance during resistance exercise with light loads and high reps, and when the sets are designed to have the person go to muscle failure. .
Mental fatigue before exercise has also been shown to hinder endurance performance in the whole body , manifesting either in a lowered time to muscle exhaustion, a longer time during time trials, a lower distance covered in a given timeframe, a decrease in self-paced velocity, or a lowered power output. This impairment in performance does not appear to be due to alterations in neuromuscular function [but rather a higher than normal perceived effort during exercise. 
The studies may show that cognitive resources are limited, and completing mentally demanding tasks can deplete these resources, which would then affect physical movememnt that alsoafter. Tasks requiring decision-making and self-regulation are more likely to be influenced by mental fatigue, as they involve constant oversight of factors such as pain, fatigue, breathing, and effort. In contrast, near-maximal and maximal-intensity exercises rely less on cognitive functioning and decision-making processes and are therefore less affected by mental fatigue.
So, what do you think? Will you limit screen time before big training days or blocks? Before race day?
- Pageaux B, Marcora SM, Lepers RProlonged mental exertion does not alter neuromuscular function of the knee extensors.Med Sci Sports Exerc.(2013-Dec)
- Martin K, Thompson KG, Keegan R, Ball N, Rattray BMental fatigue does not affect maximal anaerobic exercise performance.Eur J Appl Physiol.(2015-Apr)
- Van Cutsem J, Marcora S, De Pauw K, Bailey S, Meeusen R, Roelands BThe Effects of Mental Fatigue on Physical Performance: A Systematic Review.Sports Med.(2017-Aug)
- Brown DMY, Graham JD, Innes KI, Harris S, Flemington A, Bray SREffects of Prior Cognitive Exertion on Physical Performance: A Systematic Review and Meta-analysis.Sports Med.(2020-Mar)
- Pageaux B, Marcora SM, Rozand V, Lepers RMental fatigue induced by prolonged self-regulation does not exacerbate central fatigue during subsequent whole-body endurance exercise.Front Hum Neurosci.(2015)