Results

Principal characteristics of the two groups of participants were compared to check whether they were homogenous or not (see Table 1). ANOVA with Group (Experimental vs. Control) as the between-subjects factor was performed on age, body mass index (BMI), trait self-control, maximal voluntary contraction (MVC), and average time to exhaustion (TTE) on the handgrip task performed at the beginning of the session (TTE-1). Table 1 shows that the two groups significantly differed in BMI and TTE-1 only. The control group had significantly lower BMI and TTE-1 than the experimental group. In addition, BMI and TTE1 significantly correlated (r = .330; p = .033). Consequently, BMI was used as a covariate in ANOVA leading to a significant effect and including the factor Group as a between-subjects factor. In the same way, the difference between TTE-1 and TTE-2 was used as the main outcome to test the ego-depletion effect, instead of TTE-2 only. The chi-square statistic on the percentage of men and women showed no significant difference between the groups.
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Maximal voluntary contraction

ANOVA with Moment (before the mental task vs. after the mental task) and Repetition (HG1 vs. HG2) as within-subjects factors and Group (Experimental vs. Control) as a between-subjects factor was performed on MVC performance. The results of this test showed that the interaction Moment X Repetition X Group did not reach significance: F (1, 30) = .004, p = .946, η ²p = .000. In the same way, the interactions Moment X Repetition, Moment X Group and Repetition X Group were not significant: F (1, 30) = 1.791, p = .191,η ²p = .056; F (1, 30) = .037, p = .849, η ²p = .001; F (1, 30) = 2.925,p = .098, η ²p = .089, respectively. Simple main effect analysis showed that the effect of Moment was significant:F (1, 30) = 83.254, p < .001,η ²p = .735. The mean value of MVC was significantly lower after the time-to-exhaustion handgrip task (M= 13.3 kg, SE = .738) than before it (M = 17.5 kg, SE = .862). In addition, we observed a significant effect of Repetition:F (1, 30) = 4.948, p = .034, η ²p = .142. The mean value of MVC was significantly higher during the first handgrip task (M = 15.7 kg, SE = .826) compared to the second handgrip task (M = 15.0 kg, SE = .745). On average, the participants’ MVC was reduced by 23.33% after the time-to-exhaustion handgrip task. This decrease in MVC reflects a lower capacity to produce a muscular force. Finally, we did not observe a significant effect of Group: F (1, 30) = .321, p = .575,η ²p = .011.

Ego depletion effect

Since the initial performance of the two groups in the handgrip task was not homogeneous, the time-to-exhaustion (TTE) was calculated separately for each subject as follows: TTE Delta (min) = time-to-exhaustion in the second handgrip task minus time-to-exhaustion in the first handgrip task. Finally, to highlight the ego-depletion effect, the TTE delta of the two groups was compared with the two-sample t test.
The results indicated that the TTE delta was significantly lower for the experimental group (M = -3.45 min, SD = 3.17) than for the control group (M = -1.47 min, SD = 1.31): t (30) = 2.31, p = .028, d = 0.818. However, the results of the Shapiro-Wilk test for the TTE delta also showed a violation of the assumption of normality: W = .933, p = .046. Therefore, we used the nonparametric Mann-Whitney U test to avoid any violation of normality, and the results showed a significant difference as well:U (NVideo = 16, NStroop = 16) = 71, p = .032, r = .455; (see Figure 3). The decrease in performance in the experimental group (M = 33.5%, SD = 30.2%) was larger than the decrease in performance observed in the control group (M = 18.2%, SD = 25.2%).
Because the replication of the ego depletion effect is the main objective of this paper, we also conducted ANCOVA with Group as a between-subjects factor and BMI as a covariate. In this case, the effect of Group no longer reached the significance level: F (1, 29) = 3.939, p = .057, η²p = .120. We also performed the analyses while excluding participants who had a decrease or an increase more than 2 standard deviations from the mean. One participant was removed (point 26 on Figure 4). The results of the t test without the outlier showed a significantly lower TTE delta for the experimental group (M = -2.92 min, SD = 2.44) compared to the control group (M = -1.47 min, SD = 1.31): t(29) = 2.08, p = .046, d = 0.749. While conducting ANCOVA with BMI as a covariate and removing the outlier, the effect of Group reached significance: F (1, 28) = 4.464, p = .044,η²p = .137.
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Perceptions of effort and muscle pain during the handgrip endurance task

We conducted ANOVA with Group (Experimental vs. Control) as the between-subjects factor and Time of measurement (before the mental task vs. after the mental task) and Individual isotime (0%, 33%, 66% and 100%) as within-subjects factors on the subjective effort perception during the two time-to-exhaustion handgrip tasks. When testing an effect involving individual isotime, we applied a Greenhouse-Geisser correction to consider any violation of the sphericity assumption. The interaction Group X Time of measurement X Individual isotime did not reach significance: F (2.11, 63.30) = 1.364, p = .263,η 2p = .043. In the same way, the interactions Group X Time of measurement and Group X Individual isotime did not reach significance: F (1, 30) = 2.081, p = .160, η 2p = .065) and F(2.21, 66.41) = .887; p = .426,η 2p = .029, respectively. In contrast, the interaction between Time of measurement and Individual isotime reached significance: F (2.11, 63.30) = 11.825, p< .001, η 2p = .283 (see Figure 4). The perception of effort increased more sharply during the second time-to-exhaustion task (TTE-2) than during the first time-to-exhaustion task (TTE-1). The effect of Group was not significant: F (1, 30) = .321, p = .575,η 2p = .011.
The same ANOVA was conducted on the subjective perception of muscle pain during the two time-to-exhaustion handgrip tasks. The second-order interaction between among Group, Time of measurement and Individual isotime did not reach significance: F (1.696, 50.869) = .793,p = .439, η 2p = .026. In the same way, the interactions Group X Time of measurement and Group X Individual isotime did not reach significance: F (1, 30) = 1.195,p = .283, η 2p = .038 andF (2.053, 61.601) = .303, p = .745,η 2p = .010, respectively. In contrast, the interaction between Time of measurement and Individual isotime reached significance: F (1.696, 50.8869) = 6.302,p = .005, η 2p = .174 (see Figure 4). The perception of pain increased more sharply during the second time-to-exhaustion task. Finally, the effect of Group was not significant: F (1, 30) = 1.54, p = .224,η 2p = .049.
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Performance in the mental tasks

To assess the performance in the Stroop task, two separate ANOVA with Time on task (TOT; T1, T2, T3 & T4) and Type of trial (reading vs. naming ink color) as within-subjects factors were conducted on the mean reaction time (RT) and the error rate. A Greenhouse-Geisser correction was applied to the degrees of freedom to avoid any problem of sphericity. The interaction Time on task X Type of trial reached significance for mean RT (see Figure 5): F (2.282, 34.227) = 4.881; p < .05;η 2p = .245. This interaction is explained by the difference between ‘reading’ trials and ‘naming ink color’ trials that increased with time on task. The interaction Time on task X Type of trial and the simple effect of Time on task did not reach significance for error rate: F (2.344, 35.154) = 0.125; p = .910;η 2p = .000 and F (1.024, 15.367) = 1.102; p = .312; η 2p= .053, respectively.
In addition, the simple effect of Type of trial is significant for mean RT and error rate: F (1, 15) = 155.904; p < .0001;η 2p = .912 and F (1, 15) = 156.565; p < .0001;η 2p = .913, respectively. As expected, reading trials leaded to shorter mean RT (629.44 ms vs. 741.19 ms) and smaller error rate (6.66% vs. 15.42%) than ‘naming ink color’ trials.
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To verify whether the participants paid attention to the content of the documentary, the percentage of their correct responses was compared to the level of chance (i.e., 50%) via the one-sample t test. According to the results, the participants answered significantly better than chance (M = 78.1%, SD = 9.11%): t (16) = 12.4, p< .001, d = 3.09.

Subjective measurements using visual analog scales

Subjective feeling of fatigue

ANOVA with Group (Experimental vs. Control) as the between-subjects factor and Time of measurement (T1 = Baseline, T2 = Pre-Mental Task, T3 = Post-Mental Task, T4 = Post-Handgrip Task) as the within-subjects factor was performed on the subjective feeling of fatigue. We applied a Greenhouse-Geisser correction to consider any violation of the sphericity assumption. The results showed that the interaction between Group and Time of measurement did not reach significance: F (2.774, 83.212) = 2.24, p = .095,η 2p = .069. In the same way, the effect of Group was not significant: F (1, 30) = 2.629,p = .115, η 2p = .081. In contrast, the effect of Time of measurement reached significance:F (2.774, 83.212) = 22.299, p < .001,η 2p = .426. According to a planned comparison conducted to explain the effect of time of measurement, we observed a significant, linear increase in fatigue level as time progressed, regardless of the first Mental task: t (90) = 7.637, p < .001.

Motivation

Repeated-measures ANOVA with Group (Experimental vs. Control) as a between-subjects factor and Time of measurement (T1 = Pre-Mental Task, T2 = Post-Mental Task) as a within-subjects factor was conducted on the motivation to perform the time-to exhaustion handgrip task. According to the results, the interaction between Group and Time of measurement did not reach significance: F (1, 30) = 0.030, p = .862,η 2p = .001. The effects of Time of measurement and Group did not reach significance either: F(2.57, 77.12) = 3.216, p = .083,η 2p = .097 and F (1, 30) = 2.14, p = .154, η 2p = .067, respectively.

Task difficulty

The results of the two-sample t test demonstrated that the participants in the experimental group perceived the Stroop task (M = 74.9,SD = 19.7) to be more difficult than the control group did for the documentary viewing task (M = 20.1, SD = 22.2):t (30) = − 7.39, p < .001, d = -2.61.

Boredom

The two-sample t test was performed to compare perceived boredom during the depleting versus control tasks. The effect of Group (Experimental vs. Control) did not reach significance: t (39) = − 1.11,p = .274, d = -0.394. Unexpectedly, the Stroop task performed by the experimental group was not perceived as boring more than the documentary viewing task performed by the control group.

EEG indices

Power spectral analysis

We used the unpaired t test with FDR correction for multiple comparisons to compare the theta wave band power spectral density during the Stroop task compared to the documentary viewing task. A statistically significant higher theta power spectral density was observed mainly in the frontal, prefrontal and central areas during the Stroop task compared to the documentary viewing task, more specifically in the AF7, AF3, FC5, FC3, AF4, AF8, F8, F6, FC6, C6, C4, C2 and P4 electrodes. However, we also detected significantly higher theta power during the documentary viewing task, mainly in occipital areas, more precisely the POz, O1, Iz, Oz, PO7 and PO8 electrodes (see Figure 6).
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Later, to assess the effect of time on task on the stimulus-locked theta power density recorded during the modified Stroop task, the EEG recording was divided into 4 equal time periods of 7 min and 30 s. Then, we performed a repeated-measures ANOVA with Time on task (T1, T2, T3 & T4) and Type of trial (reading vs naming ink color) as within-subjects factors on the 64 electrodes. The results showed that the effect of Time on task on the stimulus-locked theta power density reached significance only for 4 electrodes: FC1, P2, CP2, P4. The effect is identical for the 4 electrodes: stimulus-locked theta power decreased with time on task. We reported hereafter the results of the ANOVA only for FC1: F(2.684, 40.258) = 11.008, p < .001,η 2p = .423 (see Figure 7). The effect of Type of trial did not reach significance for the 4 electrodes. For the electrode FC1, we obtained the following results: F (1, 15) = 0.004, p = .952, η 2p = .000.
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Source localization analysis

A K-means clustering (K = 33) algorithm using EEGLAB Study Statistics was performed only on the components that represented higher theta power during the Stroop task in the same regions, as previously observed based on the results of our power spectral analysis. To identify the best clusters, first, the outlier clusters recognized automatically by EEGLAB were excluded, and then the clusters including sufficient numbers of participants and components (i.e., at least 8 participants representing more than 50% of the data) were considered. From the results of kmean clustering, 6 of 35 clusters were identified and contained more than 50% of the data. As shown in Figure 9, the sources of cerebral activation originated from the prefrontal, frontal and central areas, more precisely regions close to the ACC, thalamus and posterior cingulate cortex.
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ECG indices

We conducted 3 separate repeated-measures ANOVAs with Time on task (T1, T2, T3 and T4) as the within-subjects factor and Group as the between-subjects factor on the HRV parameters (SDNN, HF & LF). When testing an effect involving Time on task as a repeated-measure factor, we applied a Greenhouse-Geisser correction to consider any violation of the sphericity assumption.
The results for the SDNN index showed a significant interaction of Time on task X Group: F (2.073, 60.130) = 3.889, p = .025,ƞ 2p = .118 (see Figure 9, panel A). This interaction disappeared when conducting ANCOVA with BMI as a covariate: F (2.09, 58.62) = 2.198; p = 0.118;ƞ 2p = .073.
For the HF, the interaction of Time on task X Group and the simple effect of Group did not reach significance: F(2.746, 79.639) =.589, p = .609, ƞ 2p = .020 and F(1 29) = .516, p = .478,ƞ 2p = .017, respectively. However, the effect of Time on task was significant: F (2.746, 79.639) = 2.964, p =.041,ƞ 2p = .093. HF increased throughout the mental task, regardless of the task.
Finally, concerning the results for LF, the interaction of Time on task X Group reached significance: F (2.477, 70.959) = 5.104, p= .005, ƞ 2p = .150 (see Figure 9, panel B). This interaction was still significant when conducting ANOVA with BMI as a covariate: F (2.58, 72.24) = 3.40; p = .028; ƞ 2p = .108. A breakdown of this interaction showed that LF for the control group decreased, whereas LF for the experimental group increased between T1 and T2:F (1, 29) = 5.915; p = .021;ƞ 2p = .169.
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