A Quantitative Investigation of Mental Fatigue Elicited during Motor Imagery Practice: Selective Effects on Maximal Force Performance and Imagery Ability

Abstract

In the present study, we examined the development of mental fatigue during the kinesthetic motor imagery (MI) of isometric force contractions performed with the dominant upper limb. Participants (n = 24) underwent four blocks of 20 MI trials of isometric contractions at 20% of the maximal voluntary contraction threshold (20% MVC_MI) and 20 MI trials of maximal isometric contractions (100% MVC_MI). Mental fatigue was assessed after each block using a visual analogue scale (VAS). We assessed maximal isometric force before, during and after MI sessions. We also assessed MI ability from self-report ratings and skin conductance recordings. Results showed a logarithmic pattern of increase in mental fatigue over the course of MI, which was superior during 100% MVC_MI. Unexpectedly, maximal force improved during 100% MVC_MI between the 1st and 2nd evaluations but remained unchanged during 20% MVC_MI. MI ease and vividness improved during 100% MVC_MI, with a positive association between phasic skin conductance and VAS mental fatigue scores. Conversely, subjective measures revealed decreased MI ability during 20% MVC_MI. Mental fatigue did not hamper the priming effects of MI on maximal force performance, nor MI's ability for tasks involving high physical demands. By contrast, mental fatigue impaired MI vividness and elicited boredom effects in the case of motor tasks with low physical demands.

Keywords: mental fatigue; mental practice; motor performance; recuperation.

Figure 1

Flowchart of the experimental design.

Figure 2

Stripchart depicting the loglinear increase in mental fatigue VAS scores from the 1st to the 4th block of the MI practice session for both experimental conditions. The increase was more pronounced during 100% MVC_MI compared to 20% MVC_MI, particularly over the course of the first 3 blocks of MI practice. VAS = Visual Analogue Scale. MI = Motor Imagery. Regression slopes are shown with 95% confidence intervals (dotted lines). * p < 0.05.

Figure 3

(a) Total force values expressed as percentage of the Pretest maximal isometric trials recorded during performance assessments throughout the MI practice session, for both experimental conditions. (b) EMG_RATIO between agonist and antagonist activation, expressed as percentage of the Pretest maximal isometric trials, recorded during performance assessments throughout MI practice session, for both experimental conditions. Both variables displayed a very comparable pattern of change across assessments, i.e., greater improvement from the pretest to the MVC trial performed between blocks 2 and 3 during 100% MVC_MI compared to 20% MVC_MI. Improvements appeared maintained from the MVC trial performed between blocks 2 and 3 and MVC trials performed after block 4. * p < 0.05, ** p < 0.01, NS = Not statistically significant.

Figure 4

(a) Regression slope describing the relationship between MI trials blocs and the perceived difficulty of MI under 100% MVC_MI and 20% MVC_MI. Regression slopes are shown with 95% confidence intervals (dotted lines). (b) Regression slope describing the relationship between blocs of MI trials and the perceived MI vividness under 100% MVC_MI and 20% MVC_MI. A reverse pattern of change emerged, for both subjective indexes of MI ability, across the successive blocks of MI practice between 100% MVC_MI and 20% MVC_MI. * p < 0.05, ** p < 0.01.

Figure 5

(a). Barplot of mean heart rate values recorded during MI trials, under 100% MVC_MI and 20% MVC_MI conditions (b) Electrodermal amplitudes during MI trials, under 100% MVC_MI and 20% MVC_MI conditions. A decrease in HR across the successive blocks of the MI practice session was found only during 20% MVC_MI. * p < 0.05.