Background: Fatigue is a major factor influencing athletic performance, yet findings across studies remain inconsistent. This inconsistency is likely related to substantial heterogeneity in fatigue-induction protocols, fatigue verification methods, and performance outcomes. This systematic review aimed to synthesize current evidence on the effects of sports fatigue on athletic performance and to discuss the ecological relevance of existing experimental fatigue models. Methods: A systematic search was conducted in PubMed, Web of Science, Embase, and Scopus up to November 2025. Randomized controlled trials with crossover or parallel-group designs investigating athletic performance under fatigue in trained athletes were eligible for inclusion. Results: A total of 56 experimental studies were included. Physical fatigue generally impaired physical performance, whereas its effects on perceptual-cognitive performance were heterogeneous across protocols, tasks, and outcome measures. Mental fatigue generally impaired endurance, motor skill, and perceptual-cognitive performance, with less consistent evidence for strength- and power-related outcomes. Evidence on dual fatigue was limited but suggested negative effects on endurance and perceptual-cognitive performance. Conclusion: Current evidence is characterized by considerable heterogeneity in fatigue-induction protocols, fatigue verification methods, and performance outcomes. Single-fatigue paradigms remain valuable for mechanistic isolation, whereas dual-fatigue paradigms may offer complementary ecological relevance by approximating the concurrent physical and cognitive demands of sport. Future research should develop more standardized and ecologically valid fatigue models and further examine potential countermeasures, including neuroenhancement tools and targeted training strategies.
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