Validity of the CORE Wearable Sensor During Internal Cooling Induced by Hyperhydration with Cold Water at Rest

Pre-exercise internal cooling through cold-water-induced hyperhydration may attenuate increases in core body temperature (T_C) and improve endurance performance. Quantifying the extent of reduction in T_C induced by hyperhydration is important for optimizing its timing before exercise. We compared T_C values obtained from a wearable sensor, the CORE, with those of a gastrointestinal temperature telemetric sensor (G_TS) during hyperhydration. Eleven participants (two women; age: 24 ± 4 yrs) completed a 120 min seated period where they consumed, over the first 60 min, four boluses of 4 °C water (7.5 mL · kg fat-free mass [FFM]^-1), each containing 0.35 g · kg FFM^-1 of glycerol. Measures of T_C were taken every 20 min with both sensors. According to the G_TS, hyperhydration induced a peak T_C decline of -0.76 ± 0.31 °C at min 60; at this time, the change in T_C from baseline estimated by the CORE was -0.09 °C ± 0.20 °C. The greatest decline in T_C detected by the CORE was -0.11 ± 0.19 °C. Bland and Altman analyses revealed that average T_C declines of -0.1, -0.2, -0.3, -0.4, -0.5 and -0.6 °C from baseline were associated with T_C values estimated by the CORE that were respectively +0.29, +0.41, +0.57, +0.65, +0.77 and +0.89 °C higher than the G_TS. An intraclass correlation coefficient of 0.12 suggested poor agreement between instruments. These results raise concerns about the ability of the CORE to detect changes in T_C induced by a hyperhydration protocol generating a substantial heat sink.