Does mental fatigue impair exercise performance? Original paper

In this randomized controlled trial, pre-exercise mental fatigue did not influence maximal power or force output during jumping and bench pressing tests, though it may affect other kinds of exercise.

The effects of mental fatigue on bench press force-velocity profile and one-repetition maximum (1RM) strength, as well as countermovement jump performance.

25 resistance-trained men (average age of 26) who could bench press 1.4 times their body mass, on average.

In a randomized crossover design, the participants either performed the Stroop task (a neuropsychological test of executive function), performed intense activity on social media apps (i.e., reading texts, writing messages, and posting information), or watched a documentary (the control) for 30 minutes. The participants then completed visual analog scales to assess mental fatigue and motivation, followed by exercise tests.

The participants performed two maximal countermovement jumps (separated by 60 seconds of rest), followed by 3 sets of Smith machine bench press (separated by 2 minutes of rest) for 2, 2, and 1 repetition(s) at the maximum possible velocity using 50%, 70%, and 90% of their bench press 1RM, respectively. Bench press 1RM was estimated from calculated force output during the 5 completed repetitions.

Overview of the experimental protocol

Adapted from ^[1]. CMJ = countermovement jump. F-V = force-velocity.

Perceived mental fatigue was higher after the Stroop task and social media activity than the control, and perceived mental fatigue was higher after the Stroop task than the social media activity. There were no differences between interventions for perceived motivation.

There were no differences in exercise performance between groups.

Voluntary movement depends on muscle force, which is determined by the behavior of motor units (i.e., a motor neuron and all of the muscle fibers it innervates). For the body to perform a movement, a signal must originate in the primary motor cortex of the brain.^[2] Motor neurons then integrate information from the brain and translate it into a signal that will initiate movement in the stimulated (effector) muscle.^[2]

Neural drive to the muscles is ultimately the script for movement. It contains the information of the motor task, controls which motor units are recruited and their discharge rates, and thus determines force output. Because the nervous system plays an instrumental role in movement, mental fatigue caused by cognitive tasks has the potential to affect physical performance.^[3]

Notably, the summarized study found that mental fatigue did not affect exercise performance. The participants were able to apply the same amount of force against each submaximal load (50%, 70%, 90% 1RM), and there was no difference in countermovement jump performance between conditions. However, it’s important to keep in mind that the participants were performing a specific type of exercise.

Other studies that assessed performance during near-maximal exercise or during maximal-intensity, short-duration anaerobic exercise have reported equivalent results: mental fatigue did not affect maximal voluntary contraction of the knee extensor muscles,^[4][5] countermovement jump performance,^[5][6] power output during a single or multiple bouts of 30 seconds of all-out cycling,^[7][5] or 100–200-meter sprinting performance.^[6]

The question remains, however, whether these findings are generalizable to other performance outcomes, namely, muscular endurance (i.e., the ability of a single muscle or muscle group to sustain prolonged dynamic exercise) and whole-body endurance (i.e., the ability to sustain prolonged dynamic exercise that uses multiple large muscle groups,^[3] such as running, cycling, or rowing).

In terms of muscular endurance, one study found that 30 minutes of intensive smartphone use (the same protocol used in the summarized study) before exercise reduced the total number of repetitions performed across three sets of back squats performed to muscular failure using a load of 80% of the participants’ 15RM.^[8] Similarly, a study found that performing a 90-minute cognitive task before exercise reduced time to exhaustion during submaximal (20% of maximal voluntary contraction) isometric contraction of the knee extensors muscles by 13%.^[4]

In a third study that used a slightly different design,^[9] the participants performed mental tasks between sets of machine bench press at 60% of their 1RM and leg extensions at 40% of their 1RM. The researchers found that a lower number of total repetitions were performed across the exercises in the group that performed the more mentally fatiguing task.

In contrast to these findings, a 2022 study reported that 30 minutes of intensive smartphone use before exercise did not affect the total number of repetitions performed across 5 sets each of the back squat and bench press exercises.^[10] Notably, each set was performed using 80% of the participants’ 15RM, and the set was stopped when a velocity loss of 20% was reached, meaning that when the velocity of the repetition was 80% of its peak at the beginning of the set. The primary difference between this study and the others is that each set was performed well short of muscular failure.

In sum, these studies suggest that mental fatigue does not impair performance in the context of near maximal-intensity exercise or maximal-intensity, short-duration anaerobic exercise, or during submaximal resistance exercise involving relatively light loads and a high number of repetitions. However, mental fatigue appears to impair performance during resistance exercise involving relatively light loads and high repetitions when each set is performed to muscular failure.

By the same token, pre-exercise mental fatigue has been consistently found to impair whole-body endurance performance,^[3][11] as evidenced by either a decrease in time to exhaustion, an increase in completion time during a time trial, a decrease in distance covered in a given time, a decrease in self-paced velocity, or a decrease in self-selected power output. This impairment in performance does not appear to be due to alterations in neuromuscular function,^[12][13] but rather a higher than normal perceived effort during exercise.^[3]

The underlying theory, for which there is good supporting evidence, is that cognitive resources are limited, and the completion of a cognitively demanding task can deplete cognitive resources that a subsequent physical task is dependent on.^[14] This results in a higher than normal perceived exertion during exercise. When exercise feels more difficult, it’s enticing to stop sooner or reduce average power output.

Physical tasks that require greater amounts of decision-making are more likely to be influenced by mental fatigue. During endurance tasks, the person exercising must continuously make decisions about how and when to invest their energy to avoid early exhaustion and maximize performance outcomes.^[15] The process of making these decisions involves constant oversight of factors like pain and muscular fatigue, breathing, body movements and form, and overall effort.^[16] These factors, in combination with the individual’s goals and the demands of the task, determine the pacing strategy.^[15]

In contrast, near maximal-intensity exercise or maximal-intensity, short-duration anaerobic exercise is characterized by a lack of decision-making and self-regulatory processes. Instead, it “only” requires all-out effort. Consequently, it is less reliant on cognitive functioning than endurance tasks. Essentially, the shorter duration of the task and the higher intensity of effort required, the less likely performance will be impaired by mental fatigue.^[14][3]