Purpose: This study systematically reviewed the fatigue mechanisms that constrain trail running performance and examined the influence of sex and event distance on these responses. PRISMA guidelines were followed. PubMed, Web of Science, and Dimensions AI databases were searched up to August 10, 2025 for studies involving adult-trained or experienced trail runners that compared prerace and postrace fatigue markers after a standard trail race or laboratory race simulation. Risk of bias was assessed using the ROBINS-I. Twenty-four studies met the inclusion criteria (n = 674 subjects; 506 men and 168 women). Most studies (n = 21) examined only or predominantly male runners, and only four studies specifically assessed sex differences. Participants were trained adults (mean age = 39.4 [8.5] y), with sample sizes ranging from 6 to 72 individuals. Regarding race distance, 4 studies investigated short races (=60 km), 4 medium races (60-100 km), 8 long races (=100 km), and 8 compared multiple distances within the same study. Fatigue assessment primarily relied on knee extensor and plantar flexor torque testing to evaluate central and peripheral fatigue, complemented by alternative methods such as tensiomyography, soft tissue vibration, strength, and jump tests. Ten of the studies analyzed blood biomarkers related to muscle damage, inflammation, and renal function. Conclusions: Neuromuscular fatigue in trail running arises from both central and peripheral mechanisms, depending on race duration and testing methods. While central fatigue predominates in shorter races, peripheral fatigue and muscle damage become more evident in longer events (=100 km). Moreover, sex differences are minimal, although methodological heterogeneity limits the strength and comparability of the findings.
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