ACCURACY OF CORE TEMPERATURE ESTIMATION FROM WEARABLE DEVICES DURING EXERCISE

PURPOSE: Estimating core body temperature (Tc) from photoplethysmography heart rate (HR) measured using wearable technology may be preferable to more expensive and invasive measures of Tc, but its accuracy is unknown. METHODS: 29 healthy adults (55% female; mean ± SD, age = 25 ± 5 y, body mass = 74.8 ± 14.8 kg) completed 2 visits (TEMP, 22 °C; HOT, 35 °C). Participants inserted a flexible rectal thermistor for the measurement of rectal temperature (Tre), the criterion measure of Tc, and were outfitted with 2 wearable devices (SlateSafety BAND V2, Apple Watch Series 8). Data were recorded continuously during rest, 40 min of cycling (10 min each at 50%, 60%, 70%, and 80% of age-predicted maximal HR [HRmax]), and recovery. Tc was estimated directly from HR by the SlateSafety device (TcSS) and retrospectively from Apple Watch HR data (TcApple) using an open-source algorithm. A two-way (intensity × device) repeated measures analysis of variance was used to compare Tre, TcSS, and TcApple in TEMP and HOT. An acceptable mean difference between device Tc and Tre of ±0.25 °C was set a priori. RESULTS: There was a significant intensity × device interaction during TEMP (P < 0.001) and HOT (P < 0.001). At rest, Tc was not different during TEMP (Tre = 37.13 ± 0.37 °C, TcSS = 37.07 ± 0.30 °C, TcApple = 37.13 ± 0.37 °C, all P > 0.05) or HOT (Tre = 37.04 ± 0.31 °C, TcSS = 37.19 ± 0.39 °C, TcApple = 37.04 ± 0.31 °C, all P > 0.05) but estimated Tc was greater than Tre and exceeded the ±0.25 °C acceptable difference at vigorous intensity exercise (80% HRmax) during TEMP (Tre = 37.60 ± 0.22 °C, TcSS = 38.35 ± 0.15 °C, TcApple = 38.00 ± 0.16 °C, all P < 0.001) and HOT (Tre = 37.55 ± 0.32 °C, TcSS = 38.40 ± 0.15 °C, TcApple = 38.00 ± 0.14 °C, all P < 0.001). CONCLUSIONS: HR-estimated Tc agreed with criterion Tc under resting conditions but overestimated Tre at higher exercise intensities. Supported by: This study was supported by an equipment grant from SlateSafety (Atlanta, GA).