The Mental Fatigue Induced by Physical, Cognitive and Combined Effort in Amateur Soccer Players: A Comparative Study Using EEG

<b>Objective:</b> Mental fatigue (MF) worsens soccer performance. Further knowledge is needed to understand MF's effects on soccer players and its underlying mechanisms. Our aim was to analyze the subjective, objective, and neural MF-related outcomes induced by different type of tasks. <b>Methods:</b> A randomized crossover experimental design with repeated measures was used. Thirteen amateur soccer players (<i>M_age</i> = 23 ± 5.43) completed three conditions: cognitive (30 min. Stroop.), physical (30 min. cycling), or combined (30 min. Stroop while cycling). Ratings of mental fatigue (measured via the Visual Analogue Scale), electroencephalographical signals (electroencephalography), and psychomotor performance (Brief-Psychomotor Vigilance Test) were measured pre- and post-condition. Soccer-related decision-making (TacticUP^® test) was assessed post-condition. <b>Results:</b> Linear Mixed Models analysis revealed increments in perceived mental fatigue in all conditions, especially cognitive (<i>p</i> = 0.004) and combined (<i>p</i> < 0.0001) conditions. Psychomotor performance worsened, especially for cognitive (<i>p</i> = 0.039) and combined (<i>p</i> = 0.009) conditions. The Individual Alpha Peak Frequency was lower after the cognitive task (<i>p</i> = 0.040) and compared with the physical task (<i>p</i> = 0.021). The Alpha midline power increased after the cognitive task in the central-frontal (<i>p</i> = 0.047) and central-posterior brain regions (<i>p</i> = 0.043). <b>Conclusions:</b> Cognitive and combined conditions were found to be more mentally demanding and fatiguing than single physical tasks. This was also reflected by an impaired reaction time. Based on the neural activity recorded, the performance impairments caused by mental fatigue were caused by reduced brain readiness (i.e., a lower Alpha Peak Frequency). However, non-significant changes were found in soccer-related decision-making. Coaches should consider the type of training tasks they recommend in light of their different effects on mental fatigue and performance.