Responsiveness of the CALERA Research Sensor to heat-acclimation-induced reductions in rectal temperature

Military personnel and athletes increasingly use non-invasive methods to estimate body core temperature during heat acclimation. This study determined the responsiveness of the CALERA Research Sensor (CRS) to changes in resting and end-exercise body core temperature induced by heat acclimation as well as the criterion validity for determining the thermal dose, defined as time spent with a body core temperature ≥38.5 ^°C. Fifteen participants completed a nine-day controlled-hyperthermia heat acclimation protocol. Body core temperature was continuously monitored prior to and during heat exposures using both the CRS (T_crs) and a rectal temperature (T_re) probe. This study assessed responsiveness and criterion validity using Bland-Altman analysis and intraclass correlation coefficients (ICC). Additionally, the analyses included a linear mixed model to further analyze responsiveness. T_crs showed limited responsiveness to heat acclimation-induced changes in resting and end-exercise T_re with wide limits of agreement (±0.32 ^°C), proportional bias (indicating increasing underestimation with larger adaptations), and low ICC (≤0.18). Additionally, resting and end-exercise T_re decreased to a greater extent than T_crs, which showed a downward trend during end-exercise over the nine-day heat acclimation (P < 0.001). The CRS showed limited criterion validity for estimating thermal dose, with wide limits of agreement (±20 min), proportional bias (indicating increasing underestimation at higher thermal doses), and low ICC of 0.35. These results indicate that the current version of the CRS underestimates both heat acclimation-induced changes in T_re and the thermal dose during controlled hyperthermia.