Mental Fatigue Negatively Impacts Cognitive and Physical Performance Outcomes: A Large-Scale Randomized Crossover Trial.

Although many studies have demonstrated that prolonged cognitive load can impair subsequent cognitive and physical performance, commonly described as mental fatigue, more recent findings suggest that the relationship is more nuanced. Moreover, significant heterogeneity between different smaller studies make it difficult to summarize definitive conclusions. The aim of the present study was to provide a more robust examination of the detrimental effects of sustained cognitive demands on cognitive and physical performance fatigability, using a large sample within a controlled environment. 117 participants (57 ♀; 32 ± 9 y) were included in this randomized counterbalanced crossover experiment (NCT05576935), consisting of a familiarization, intervention (45 min individualized Stroop task) and control (45 min self-chosen documentary) session. Performance was evaluated using a 6-min GoNoGo task and a 20-min cycling time trial. Multiple secondary outcomes, such as rate of perceived exertion (RPE), feelings of fatigue and motivation, were incorporated to explore their relationship to possible behavioural effects. There was a significant worsening of GoNoGo reaction time between conditions (p < 0.001; ηp2 = 0.07). A trend towards significance was found regarding the negative influence of the Stroop task on time trial distance (p = 0.059; d = 0.20), which was linked to a significant decrease in cadence (p = 0.042; d = 0.22) in the intervention condition. Moreover, the feelings of physical fatigue (p < 0.001; ηp2 = 0.01), mental fatigue (p < 0.001; r = 0.37-0.47) and RPE (p = 0.002; ηp2 = 0.01) remained higher during the time trial in the intervention condition compared to the control condition. In the present study, prolonged cognitive effort impaired specific cognitive and physical performance outcomes, consistent with the characteristics of mental fatigue. Secondary outcomes show an important role for the feeling of fatigue in the determination of physical performance.