BACKGROUND: Open water swimming (OWS) is conducted in natural aquatic environments, where external factors such as waves, currents, water temperature, and limited visibility substantially influence the characteristics of the sport. These environmental conditions impose loading patterns that differ from those of pool-based competitive swimming and are thought to increase the accumulation of fatigue around the shoulder and hip joints during prolonged swimming. The purpose of this study was to investigate acute changes in quadriceps tightness following a 10-km OWS race and to explore potential biomechanical contributors to anterior hip loading in elite OWS swimmers. METHODS: The subject was a Japanese male elite open-water swimmer (19 years old; 2024 national champion) who competed in a 10-km race at the 2025 World Aquatics Championships. Quadriceps tightness was assessed using heel-to-buttock distance (HBD) measured bilaterally before and immediately after the race. RESULTS: HBD increased bilaterally following the race (right: 17 to 22 cm; left: 18 to 23 cm), indicating an acute reduction in quadriceps tightness. CONCLUSIONS: A 10-km OWS race was associated with an acute increase in HBD, reflecting reduced quadriceps tightness. The postural and technical demands unique to OWS, particularly frequent "head-up" swimming, may increase anterior hip muscle tension and mechanical loading
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