Limitations
truck, fatigue
The current study may be limited in several ways and the findings should therefore be interpreted in the context of these limitations. Firstly, we did not assess participant’s level of motivation at the end of the conditioning period (i.e., Stroop/documentary) to test for task (dis)engagement. This in a way limited our conclusion regarding the observed different types of mental fatigue due to underload/overload and should thus be investigated in future studies to provide further evidence for the difference between them. Regarding the lack of effect of mental fatigue on static balance, individual differences in personality and other traits may affect the participants’ response to mentally fatiguing tasks and the subsequent effect on balance. Thus, an analysis of the influence of these characteristics may reveal a sub-group of participants displaying a negative effect of mental fatigue on static balance, which was not investigated in this study. The gender of the participants may also play a role in how they respond to prolonged performance of cognitive tasks both in terms of drop in cognitive performance and balance control. But we did not look at gender difference in the effect of mental fatigue in our participants which is another limitation. Another potential drawback of this study was the limited task duration (45 min) which may be small both from the perspective of inducing objective performance decrement and the typical hours spent on mental tasks in real world situations such as at places of work (for example air traffic controllers and truck drivers on long haul trips) or school. Therefore, it may be interesting to investigate the effect of mental fatigue induced by longer time-on-task on static balance in healthy young adults in future studies. The remaining limitations are related to our balance outcome assessment. We assessed balance using a mobile phone-based balance assessment tool that measured postural sway. However, this might not be as sensitive as other tools of measuring postural sway such as force platforms which give various measures of sway (e.g., sway velocity, sway amplitude, total sway area et cetera) in different directions (antero-posterior and mediolateral). Therefore, we might have missed subtle changes in sway due to mental fatigue that can only be detected in the different parameters obtained from force platform output. Also, the mobile phone-based balance assessment tool we used allowed each balance trial to be conducted for 10 s only. As longer trials and more repeats have been shown to improve the reliability of static balance assessment, the short duration of the balance trials in this study may affect the consistency of the measurement. Last, we did not measure the other component of postural control, i.e., postural muscle tone which may be affected by mental fatigue independent of changes in balance (equilibrium) control. Therefore, we might have missed a potential effect of mental fatigue on this important component of postural control too.