Effect of aerobic fitness on the validity of the Calera Research™ sensor to estimate core temperature during exercise

Abstract

The Calera Research™ is a heat flux device advertised to estimate core temperature (Tc) during physical activity and is widely used by athletes in various sport disciplines. The device estimates Tc from skin temperature, heart rate, and heat flux, outcomes that can be affected by aerobic fitness. However, there is a relatively small body of literature exploring its validity and, specifically, how the device bias may be influenced by aerobic fitness. The objective of this study was to assess the validity of the Calera Research™ sensor compared with telemetric ingestible pills for estimating Tc and to determine whether aerobic fitness impacts accuracy. Twenty participants (10 females) performed a cycling-based ramp-incremental exercise test to volitional exhaustion in a temperature controlled environment (22 °C) during which Tc was measured directly from ingested pills (eCelsius performance system) and by the Calera device. Compared to the ingestible telemetric pills, the Calera device exhibited lower Tc values (95%CI[-0.2, -0.3 °C], p < 0.001) with an intraclass correlation of 0.47 and a bias of −0.3 ± 0.2 °C. Participants were divided into two groups (10 participants at each group) based on peak oxygen uptake ($\dot{V}$ O_2peak) (HL: high aerobic; LL: lower aerobic fitness). Throughout ramp-incremental exercise, HL had a lower heart rate (p < 0.001 and η² = 0.10; 95%CI[0.1, 17.2 bits/min]) and Tc measured by the ingestible pills (p < 0.001 and η² < 0.01, 95%CI[0.1, 0.3 °C]). However, there were no-between group differences for skin temperature and intraclass correlation values and bias were also similar. In conclusion, the estimation of Tc by Calera Research™ underestimated Tc by approximately 0.3 °C in a moderate thermal environment, but the intensity-dependent profile was similar to that of the ingestible pill method and was unaffected by aerobic fitness differences.