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Physiological responses to work in cold water have been well studied but little is known about the effects of exercise in warm water; an overlooked but critical issue for certain military, scientific, recreational, and professional diving operations. This investigation examined core temperature responses to fatiguing, fully-immersed exercise in extremely warm waters. Twenty-one male U.S. Navy divers (body mass, 87.3 ± 12.3 kg) were monitored during rest and fatiguing exercise while fully-immersed in four different water temperatures (Tw): 34.4, 35.8, 37.2, and 38.6°C (Tw(34.4), Tw(35.8), Tw(37.2), and Tw(38.6) respectively). Participants exercised on an underwater cycle ergometer until volitional fatigue or core temperature limits were reached. Core body temperature and heart rate were monitored continuously. Trial performance time decreased significantly as water temperature increased (Tw(34.4), 174 ± 12 min; Tw(35.8), 115 ± 13 min; Tw(37.2), 50 ± 13 min; Tw(38.6), 34 ± 14 min). Peak core body temperature during work was significantly lower in Tw(34.4) water (38.31 ± 0.49°C) than in warmer temperatures (Tw(35.8), 38.60 ± 0.55°C; Tw(37.2), 38.82 ± 0.76°C; Tw(38.6), 38.97 ± 0.65°C). Core body temperature rate of change increased significantly with warmer water temperature (Tw(34.4), 0.39 ± 0.28°C·h(−1); Tw(35.8), 0.80 ± 0.19°C·h(−1); Tw(37.2), 2.02 ± 0.31°C·h(−1); Tw(38.6), 3.54 ± 0.41°C·h(−1)). Physically active divers risk severe hyperthermia in warmer waters. Increases in water temperature drastically increase the rate of core body temperature rise during work in warm water. New predictive models for core temperature based on workload and duration of warm water exposure are needed to ensure warm water diving safety.

Objectives: We aimed to determine the agreement between actual and predicted core body temperature, using the Heat Strain Decision Aid (HSDA), in non-Ground Close Combat (GCC) personnel wearing multi terrain pattern clothing during two stages of load carriage in temperate conditions. Design: Cross-sectional. Methods: Sixty participants (men = 49, women = 11, age 31 ± 8 years; height 171.1 ± 9.0 cm; body mass 78.1 ± 11.5 kg) completed two stages of load carriage, of increasing metabolic rate, as part of the development of new British Army physical employment standards (PES). An ingestible gastrointestinal sensor was used to measure core temperature. Testing was completed in wet bulb globe temperature conditions; 1.2-12.6°C. Predictive accuracy and precision were analysed using individual and group mean inputs. Assessments were evaluated by bias, limits of agreement (LoA), mean absolute error (MAE), and root mean square error (RMSE). Accuracy was evaluated using a prediction bias of ± 0.27°C and by comparing predictions to the standard deviation of the actual core temperature. Results: Modelling individual predictions provided an acceptable level of accuracy based on bias criterion; where the total of all trials bias ± LoA was 0.08 ± 0.82°C. Predicted values were in close agreement with the actual data: MAE 0.37°C and RMSE 0.46°C for the collective data. Modelling using group mean inputs were less accurate than using individual inputs, but within the mean observed. Conclusion: The HSDA acceptably predicts core temperature during load carriage to the new British Army non-GCC PES, in temperate conditions.