Purpose This study aimed to characterize core-temperature (Tc) responses of swimmers competing at the 2024 3-km World Aquatics Masters Open Water Swimming Championships. Methods Within a cross-sectional observational study design, swimmers (n = 48 [23 male, 25 female]) ingested a telemetric Tc pill 4 hours before their race. Water temperatures were 20.9 °C (male; mean wet-bulb-globe temperature 25.9 °C) and 20.5 °C (female; mean wet-bulb-globe temperature: 25.4 °C). Linear mixed models determined differences between males and females for race-start, mean in-race, and peak Tc. Random coefficient models determined relationships between (1) sex, race time, body mass index (BMI), body-mass change or age, and Tc peak; (2) sex, race time, BMI, body-mass change or age, and Tc nadir; (3) sex, Tc peak, BMI, body-mass change or age, and race time, and (4) sex, Tc peak, BMI, body-mass change, or age and race ranking. Results The mean in-race Tc was 37.8 °C (36.1-38.8 °C); however, there were highly variable individual Tc responses (-3.1 to +2.7 °C). One swimmer experienced mild hypothermia (Tc = 35.0 °C), and 5 exceeded 39.0 °C, a risk factor for heat-related illness. There were no statistical differences between males and females for race-start, mean, and peak Tc (P = .243). Higher peak Tc was associated with faster race time (P = .05), higher race rank (P = .027), and higher BMI (P = .001). Conclusions Highly variable Tc responses (-3.1 to +2.7 °C) were observed, demonstrating that organizers of mass-participation open-water swimming events should always be prepared for, and provide facilities to treat, swimmers experiencing cold-/heat-related health incidents during competition.
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