Heat flux systems for body core temperature assessment during exercise

Heat flux systems are increasingly used to assess core body temperature. However, validation of multiple systems is scarce. Therefore, an experiment was performed in which three commercially available heat flux systems (3 M, Medisim and Core) were compared to rectal temperature (T_re). Five females and four males performed exercise in a climate chamber set at 18 °C/50% relative humidity until exhaustion. Exercise duration was 36.3 ± 5.6 min (mean ± standard deviation). T_re in rest was 37.2 ± 0.3 °C. Medisim's-values were lower than T_re (36.9 ± 0.4 °C, p < 0.05); 3 M (37.2 ± 0.1 °C) and Core's (37.4 ± 0.3 °C) did not differ from T_re. Maximal temperatures after exercise were 38.4 ± 0.2 °C (T_re), 38.0 ± 0.4 °C (3 M), 38.8 ± 0.3 °C (Medisim) and 38.6 ± 0.3 °C (Core); Medisim was significantly higher than T_re (p < 0.05). The temperature profiles of the heat flux systems during exercise differed to varying degree from the rectal profiles; the Medisim system showed a faster increase during exercise than T_re (0.48 ± 0.25 °C in 20 min, p < 0.05), the Core system tended to show a systematic overestimation during the entire exercise period and the 3 M system showed large errors at the end of exercise, likely due to sweat entering the sensor. Therefore, the interpretation of heat flux sensor values as core body temperature estimates should be done with care; more research is required to elucidate the physiological significance of the generated temperature values.