Mental fatigue has emerged as a key factor influencing endurance performance, yet its physiological mechanisms remain unclear. This study examined the associations between mental fatigue, physiological tolerance, and performance in endurance athletes. Sixty elite athletes (32 males, 28 females) from running, cycling, and triathlon disciplines completed a randomized, counterbalanced crossover design with two sessions: a 90-min cognitively demanding task (mental fatigue) and passive rest (control), separated by one week. Before and after each condition, participants underwent physiological and neurophysiological assessments followed by sport-specific tests, including treadmill time-to-exhaustion runs, 20-km cycling time trials, and multi-stage triathlon simulations. Measurements included electroencephalography (EEG), functional near-infrared spectroscopy (fNIRS), electromyography (EMG), and blood lactate analysis. Compared with the control, mental fatigue significantly reduced running endurance, cycling power output, and triathlon performance. Prefrontal oxygenation changes strongly correlated with performance decrements (r = 0.81, p < 0.001), and EMG data indicated altered neuromuscular activation. Both central and peripheral physiological markers differed between conditions. These findings demonstrate that mental fatigue is associated with measurable neurophysiological and metabolic alterations leading to reduced endurance performance, highlighting the importance of cognitive load management in endurance training and competition.
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