Background: Muscle fatigue during open-water swimming (OWS) remains poorly characterized under real-world conditions despite its relevance to performance and injury risk. Aim: To analyse neuromuscular adaptations and fatigue progression during a 5-km OWS trial. Methods: Nine male swimmers (25.4 ± 11.9 years; 75.9 ± 9.0 kg; 180.7 ± 6.7 cm) completed five consecutive 1000-m laps at maximal sustainable intensity in a lake, with 30 s recovery between laps. Swimmers wore their own long-sleeved swimming suit. Surface electromyography (EMG) was recorded from seven upper-body muscles: upper trapezius (UT), latissimus dorsi (LD), pectoralis major (PM), posterior deltoid (PD), anterior deltoid (AD), triceps brachii (TB), and biceps brachii (BB). Muscle activation was analysed using a mixed-model approach considering Lap, Muscle, and Lap × Muscle interaction effects. Means, 95% confidence intervals, and post-hoc comparisons were calculated. Results: PM exhibited the highest activation (2.34; 95% CI: 2.30-2.38), whereas AD showed the lowest activation (0.72; 95% CI: 0.68-0.76). UT and TB maintained relatively stable activation throughout the trial, suggesting greater fatigue resistance. In contrast, PM demonstrated a progressive increase in activation from 1000 to 5000 m (p < 0.001), indicating increased neural drive to maintain force production under fatigue. Similar increases were observed in LD and BB (p < 0.05), whereas PD showed a progressive decline in activation across laps. AD exhibited a non-linear response, peaking during intermediate laps. A marked reduction in activation between 4500 and 5000 m was evident across muscles, consistent with fatigue accumulation. Conclusions: Fatigue during OWS is characterized by muscle-specific neuromuscular adaptations rather than a uniform decline in activation. Significant Lap × Muscle interactions indicate a dynamic neuromuscular reorganization, defined as the progressive redistribution of motor output among synergistic muscles to preserve swimming performance as fatigue develops. These findings highlight distinct compensatory strategies adopted by upper-body muscles during prolonged OWS.
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