Effect of mental fatigue on hand force production capacities.

Mental fatigue is common in society, but its effects on force production capacities remain unclear. This study aimed to investigate the impact of mental fatigue on maximal force production, rate of force development-scaling factor (RFD-SF), and force steadiness during handgrip contractions. Fourteen participants performed two randomized sessions, during which they either carried out a cognitively demanding task (i.e., a visual attention task) or a cognitively nondemanding task (i.e., documentary watching for 62 min). The mental fatigue was evaluated subjectively and objectively (performances and electroencephalography). Maximal voluntary contraction (MVC) force, RFD-SF, and force steadiness (i.e., force coefficient of variation at submaximal intensities; 25, 50, and 75% of MVC) were recorded before and after both tasks. The feeling of mental fatigue was much higher after completing the cognitively demanding task than after documentary watching (p < .001). During the cognitively demanding task, mental fatigue was evidenced by increased errors, missed trials, and decreased N100 amplitude over time. While no effect was reported on force steadiness, both tasks induced a decrease in MVC (p = .040), a force RFD-SF lower slope (p = .011), and a reduction in the coefficient of determination (p = .011). Nevertheless, these effects were not explicitly linked to mental fatigue since they appeared both after the mentally fatiguing task and after watching the documentary. The study highlights the importance of considering cognitive engagement and mental load when optimizing motor performance to mitigate adverse effects and improve force production capacities.

Acute smartphone use impairs vigilance and inhibition capacities.

Smartphones are now in very widespread use, and concerns have arisen about potential detrimental effects, even with acute use. These adverse consequences are often linked to the emergence of mental fatigue. While the cognitive implications of fatigue are well-documented, knowledge about the specific influence of acute smartphone use on cognitive performance remains scarce. The aim of this study was therefore to investigate the impact of acute smartphone use on cognitive performance. It included two experiments: one designed to assess the impact of smartphone use on vigilance, and the other focusing on evaluating inhibition capacities. In Experiment 1, two groups of 40 participants completed a Psychomotor Vigilance Task (PVT) before and after using a smartphone for 45 min (experimental group), or before and after watching a documentary (control group). In Experiment 2, two groups of 40 participants were subjected to a similar experimental design but had to perform a Go/NoGo task instead of a PVT. Mental fatigue and drowsiness were evaluated with visual analog scales before and after smartphone use and watching a documentary. Results suggested that both watching a documentary and using a smartphone for 45 min increased subjective mental fatigue and drowsiness. Watching the documentary did not impair cognitive performance. Reaction times on the PVT and number of errors on NoGo trials in the Go/NoGo task were higher among the participants in the smartphone condition. These results indicate reduced vigilance and impaired inhibition capacities only after smartphone use. We conclude that acute smartphone use induces mental fatigue and decreases cognitive performance. Further research is needed to understand the mechanisms underlying this decline in cognitive performance.

Physical Activity and Music to Counteract Mental Fatigue.

Mental fatigue impairs both cognitive and physical performance. Bioactive substances (e.g., caffeine) have been used to counteract mental fatigue but could have side effects. The present study aimed to test two non-bioactive strategies to counteract mental fatigue: physical activity and listening to music. The participants first performed an arm-pointing task, then carried out a 32-min cognitively demanding task to induce mental fatigue (TLDB task), followed by another arm-pointing task at the end of the experiment. Between the end of the cognitively demanding task and the last arm-pointing task, 20 min went during which participants performed either 15 min of physical activity, of listening to music or of discussion (control). The subjective feeling of mental fatigue was assessed before each arm-pointing task and after the cognitively demanding task. For "physical activity" and "listening to music" groups, EEG was recorded at rest after each evaluation of subjective feeling of mental fatigue and during the cognitively demanding task. An increase in alpha power during the cognitively demanding task evidenced the presence of mental fatigue, without recovery during the following 20-min period. In the control condition, the arm-pointing task performance was deteriorated 20-min after the cognitively demanding task, while it remained stable after both physical activity and listening to music. Furthermore, recovery on the subjective feeling of mental fatigue was similar for both groups. The present results suggested that practicing physical activity and listening to music could be efficient strategies to counteract the negative effects of mental fatigue on motor performances.

Mental fatigue induced by prolonged motor imagery increases perception of effort and the activity of motor areas.

Recent literature suggests that when prolonged, motor imagery (MI) induces mental fatigue and negatively impacts subsequent physical exercise. The aim of this study was to confirm this possibility with neurophysiological and self-reported measures. Thirteen participants performed 200 imagined isometric knee extension contractions (Prolonged MI condition) or watched a documentary (Control condition), and then performed 150 actual isometric knee extensions. Electroencephalography was continuously recorded to obtain motor-related cortical potential amplitude at Cz electrode (MRCP, index of motor area activity) for each imagined and actual contraction. Electromyography of the vastus lateralis muscle as well as the perceived effort required to perform prolonged MI, watch the documentary, and perform the actual contractions were measured. During prolonged MI, mental fatigue level, the effort required to imagine the contractions and MRCP amplitude increased over time. The increase in the effort required to imagine the contractions was significantly correlated with the MRCP amplitude. During the physical exercise, a significant condition × time interaction revealed a greater increase over time in perceived effort in the prolonged MI condition compared to the control condition, as well as a specific alteration in EMG RMS of the vastus lateralis muscle. These alterations observed in the presence of mental fatigue during actual contractions, combined with those observed during prolonged MI, suggest that prolonged MI may impair the motor command required to perform imagined or actual contractions. While the observed effect of mental fatigue on MRCP amplitude was clear during MI, future studies should tailor the physical exercise to minimize the exercise-induced decrease in force production capacity and control for its confounding effects on MRCP amplitude in the presence of mental fatigue.

Persistence of Mental Fatigue on Motor Control.

The effects of mental fatigue on both cognitive and physical performance are well described in the literature, but the recovery aspects of mental fatigue have been less investigated. The present study aimed to fill this gap by examining the persistence of mental fatigue on behavior and electrophysiological mechanisms. Fifteen participants performed an arm-pointing task consisting of reaching a target as fast as possible, before carrying out a 32-min cognitively demanding task [Time Load Dual Back (TLDB) task], and immediately, 10 and 20 min after completion of the TLDB task. During the experiment, electroencephalography was continuously recorded. The significant increase in mental fatigue feeling after the TLDB task was followed by a decrease during the 20 min of recovery without returning to premeasurement values. Brain oscillations recorded at rest during the recovery period showed an increase in both theta and alpha power over time, suggesting a persistence of mental fatigue. Arm-pointing movement duration increased gradually over time during the recovery period, indicating that behavioral performance remained impaired 20 min after the end of the cognitively demanding task. To conclude, subjective measurements indicated a partial recovery of mental fatigue following a cognitively demanding task, whereas electrophysiological and behavioral markers suggested that the effects of mental fatigue persisted for at least 20 min. While the subjective evaluation of mental fatigue is a very practical way to attest the presence of mental fatigue, electrophysiological and behavioral measures seem more relevant to evaluate the time course of mental fatigue effects.

Neural mechanisms of strength increase after one-week motor imagery training.

The neural mechanisms explaining strength increase following mental training by motor imagery (MI) are not clearly understood. While gains are mostly attributed to cortical reorganization, the sub-cortical adaptations have never been investigated. The present study investigated the effects of MI training on muscle force capacity and the related spinal and supraspinal mechanisms. Eighteen young healthy participants (mean age: 22.5 ± 2.6) took part in the experiment. They were distributed into two groups: a control group (n = 9) and an MI training group (n = 9). The MI group performed seven consecutive sessions (one per day) of imagined maximal isometric plantar flexion (4 blocks of 25 trials per session). The control group did not engage in any physical or mental training. Both groups were tested for the isometric maximal plantar flexion torque (MVC) and the rate of torque development (RTD) before and after the training session. In addition, soleus and medial gastrocnemius spinal and supraspinal adaptations were assessed through the recording of H-reflexes and V-waves, with electrical stimulations of the posterior tibial nerve evoked at rest and during MVC, respectively. After one week, only the MI training group increased both plantar flexion MVC and RTD. The enhancement of muscle torque capacity was accompanied by significant increase of electromyographic activity and V-wave during MVC and of H-reflex at rest. The increased cortical descending neural drive and the excitability of spinal networks at rest could explain the greater RTD and MVC after one week of MI training.