Mental fatigue induced by prolonged self-regulation does not exacerbate central fatigue during subsequent whole-body endurance exercise

Abstract

It has been shown that the mental fatigue induced by prolonged self-regulation increases perception of effort and reduces performance during subsequent endurance exercise. However, the physiological mechanisms underlying these negative effects of mental fatigue are unclear. The primary aim of this study was to test the hypothesis that mental fatigue exacerbates central fatigue induced by whole-body endurance exercise. Twelve subjects performed 30 min of either an incongruent Stroop task to induce a condition of mental fatigue or a congruent Stroop task (control condition) in a random and counterbalanced order. Both cognitive tasks (CTs) were followed by a whole-body endurance task (ET) consisting of 6 min of cycling exercise at 80% of peak power output measured during a preliminary incremental test. Neuromuscular function of the knee extensors was assessed before and after CT, and after ET. Rating of perceived exertion (RPE) was measured during ET. Both CTs did not induce any decrease in maximal voluntary contraction (MVC) torque (p = 0.194). During ET, mentally fatigued subjects reported higher RPE (mental fatigue 13.9 ± 3.0, control 13.3 ± 3.2, p = 0.044). ET induced a similar decrease in MVC torque (mental fatigue -17 ± 15%, control -15 ± 11%, p = 0.001), maximal voluntary activation level (mental fatigue -6 ± 9%, control -6 ± 7%, p = 0.013) and resting twitch (mental fatigue -30 ± 14%, control -32 ± 10%, p < 0.001) in both conditions. These findings reject our hypothesis and confirm previous findings that mental fatigue does not reduce the capacity of the central nervous system to recruit the working muscles. The negative effect of mental fatigue on perception of effort does not reflect a greater development of either central or peripheral fatigue. Consequently, mentally fatigued subjects are still able to perform maximal exercise, but they are experiencing an altered performance during submaximal exercise due to higher-than-normal perception of effort.

Keywords: Stroop task; mental exertion; muscle fatigue; neuromuscular fatigue; perceived exertion; perception of effort; response inhibition; sense of effort.

FIGURE 1

Graphical overview of the experimental protocol. Order and timing were the same for each subject and each visit. Q, psychological questionnaires; PPO, peak power output; MVC, maximal voluntary contraction; ET, whole-body endurance task.

FIGURE 2

Subjective workload of the cognitive tasks (CTs, A) and of the whole-body endurance task (ET, B). National Aeronautics and Space Administration Task Load Index (NASA-TLX) subscales. $ Significant main effect of condition (p < 0.05). Data are presented as mean ± SEM.

FIGURE 3

Effects of mental fatigue on heart rate and electromyogram (EMG) amplitude of the knee extensors during the whole-body endurance task (ET). Heart rate (HR) during ET (A). EMG root mean square (RMS) for the vastus lateralis (VL) muscle normalized by the first minute of ET (baseline; B). EMG RMS of the VL muscle during ET (C). EMG RMS for the rectus femoris (RF) muscle normalized by the first minute of ET (baseline; D). $ Significant main effect of condition (p < 0.05). ## Significant main effect of time (p < 0.01). ### Significant main effect of time (p < 0.001). Data are presented as mean ± SEM.

FIGURE 4

Effects of mental fatigue on perception of effort during the whole-body ET. Overall rating of perceived exertion (RPE) during ET (A). Individual effects of condition on the mean overall RPE during ET (B). ### Significant main effect of time (p < 0.001). $ Significant main effect of condition (p < 0.05). Data are presented as mean ± SEM.

FIGURE 5

Effects of mental fatigue on maximal voluntary contraction (MVC) of the knee extensors and central parameters of neuromuscular function. MVC torque of the knee extensors (KE, A). Maximal voluntary activation level (VAL, B). RMS/Mmax (M-wave) ratio of the VL muscle (C). CT, cognitive tasks; ET, whole-body endurance task; baseline, pre CT. # Significant main effect of time (p < 0.05). ## Significant main effect of time (p < 0.01). * Significant difference between conditions for the same time (p < 0.05). Data are presented as mean ± SEM.