Individualized Mental Fatigue Does Not Impact Neuromuscular Function and Exercise Performance

Abstract

Introduction: Recent studies have questioned previous empirical evidence that mental fatigue negatively impacts physical performance. The purpose of this study was to investigate the critical role of individual differences in mental fatigue susceptibility by analyzing the neurophysiological and physical responses to an individualized mental fatigue task.

Methods: In a preregistered ( https://osf.io/xc8nr/ ), randomized, within-participant design experiment, 22 recreational athletes completed a time to failure test at 80% of their peak power output under mental fatigue (individual mental effort) or control (low mental effort). Before and after the cognitive tasks, subjective feeling of mental fatigue, neuromuscular function of the knee extensors, and corticospinal excitability were measured. Sequential Bayesian analysis until it reached strong evidence in favor of the alternative hypothesis (BF 10 > 6) or the null hypothesis (BF 10 < 1/6) were conducted.

Results: The individualized mental effort task resulted in a higher subjective feeling of mental fatigue in the mental fatigue condition (0.50 (95% confidence interval (CI), 0.39-0.62)) arbitrary units compared with control (0.19 (95% CI, 0.06-0.339)) arbitrary unit. However, exercise performance was similar in both conditions (control: 410 (95% CI, 357-463) s vs mental fatigue: 422 (95% CI, 367-477) s, BF 10 = 0.15). Likewise, mental fatigue did not impair knee extensor maximal force-generating capacity (BF 10 = 0.928) and did not change the extent of fatigability or its origin after the cycling exercise.

Conclusions: There is no evidence that mental fatigue adversely affects neuromuscular function or physical exercise; even if mental fatigue is individualized, computerized tasks seem not to affect physical performance.

FIGURE 1

Experimental procedure for the experiment. Created with BioRender.com.

FIGURE 2

Bayes sequential analysis for the alternative hypothesis that individualized mental effort negatively influences performance in the cycling task. The figure clearly illustrates the increasing evidence for the null hypothesis as sample size increased.

FIGURE 3

Manipulation check. Panels A and B depict a raincloud plot for the VAS questions. The raincloud plot (27) shows the cloud of points (i.e., individual raw data) connected by lines between each condition, a box plot and a one-sided violin plot (showing the probability density of the data at different values). A, Subjective level of mental fatigue. The shape of distribution indicates that, in both conditions, the observed values were located around the median, but the subjective level of mental fatigue was twice higher after the individualized mental fatigue task compared with the control condition. Furthermore, individual data show that most of the participants were more mentally fatigued after completing the mental fatigue task. B, Subjective level of arousal. Subjective arousal level was similar across conditions, which shows that performing a task with low cognitive load (control task) did not reduce the arousal level. C, Performance in the cognitive task for each condition and across time. Shaded areas represent the 95% CI.

FIGURE 4

Raincloud plot for (A) physical performance in the cycling time-to-exhaustion test at 80% of peak power output and (B) average RPE during the time-to-exhaustion test. The shape of distribution indicates that, in both conditions, values were located around the median and there were no differences between conditions. Individual data show that 12 of 22 participants performed better in the mental fatigue condition, 9 performed better in the control condition, and 1 performed equally. Similarly, the distribution of the RPE values was around the median, and there were no differences between both conditions.

FIGURE 5

A, Original recordings of knee extensor neuromuscular function evaluation in a representative participant. The MVC was conducted with a superimposed 100 Hz doublet, followed by supramaximal stimulation evoked at intervals of 2 s with paired pulses at 100 and 10 Hz, and a single pulse. Raincloud plot for knee extensors evaluation outcome. B, MVC. C, Voluntary activation level. D, Potentiated doublet amplitude. E, Ratio of 10:100 Hz.

FIGURE 6

Raincloud for the FDI MEP amplitude (average of 20 MEP) for each condition and time point.

FIGURE 7

Raincloud plots with changes in oxyhemoglobin (A), deoxyhemoglobin (B), and total hemoglobin (C) during the performance of the cognitive tasks.