Is wearable technology the future of body core temperature measurement? An analysis of multiple commercial devices

Accurate measurement of body core temperature (Tcore) is central to understanding human thermoregulation. Non-invasive wearable and spot-check devices are purported to be practical alternatives to invasive techniques (e.g., rectal and oesophageal thermistors), but their accuracy and validity remain uncertain. This study investigated the accuracy of three wearable (two skin contact (CORE and Celsium), one in-ear (Bodytrak)) and three spot-check (two aural (Kinetik and Braun), one temporal infrared (Boots)) commercially available, non-invasive devices against rectal temperature (Trec). Tcore was measured in 30 participants (19 male) while they rested for 30 min in temperate conditions (25°C, 45% relative humidity) and then 60 min in hot conditions (50°C, 25% relative humidity). Twenty-one (15 male) of the participants then completed a 20 min bout of submaximal cycling (1.6 W.Kg-1) at 50°C. Device derived estimates of Tcore were compared with Trec using Bland-Altman plots, root mean square error (RMSE) and intra-class correlations (ICCs). Estimates were also compared with Tcore predicted from a heat storage model. Only Bodytrak showed acceptable agreement with Trec during both resting stages (ICC = 0.689; RMSE = 0.29°C) while only one of the aural spot-check devices met the criterion of ± 0.3°C during rest in the heat (RMSE = 0.27°C). No device demonstrated error within 0.3°C during exercise (RMSE = 0.57°C). Across the whole protocol, the heat storage model outperformed all devices except Bodytrak (RMSE = 0.35°C). The wearable and spot-check devices did not accurately estimate Trec during exercise, and only Bodytrak met the accuracy criterion during rest.