Thermoregulation during Submaximal Exercise in the Heat

Abstract

PURPOSE: The thermoregulatory system is responsible for maintaining adequate heat production and sufficient heat dissipation. Children may be at an increased risk of developing hyperthermia (defined as Tc > 39.0°C) during exercise due to a higher surface area to mass ratio and reduced sweat rate compared to adults. In addition, children rely more on non-evaporative rather than evaporative routes of heat dissipation. However, in the heat, evaporation is the primary route. Whether children are at risk for hyperthermia during exercise is not well studied. Therefore, we designed a study in children to examine core body and skin temperature during submaximal exercise. METHODS: Twelve children (age 7-12 yrs) completed a standardized maximal treadmill test to determine peak aerobic power (VO2peak). On subsequent days, subjects completed 30 min of treadmill exercise at ~75% of VO2peak in a 24°C and a 33°C environment (RH=32.9±1.2%). Core intestinal temperature (Tc) and skin temperature (Tsk) were recorded at rest, every 2 min of exercise, and during recovery. Results are expressed as means±SEM; significance set at p<0.05. RESULTS: All children completed the exercise bout at 24°C. However, 2 of the 12 children were unable to complete the 30 min at 33°C. Tc was not significantly different between work environments at any timepoint, however, absolute Tsk was significantly different at all timepoints. Slopes were not significantly different for Tc (0.020±0.003 vs 0.021±0.002°C/min, p=0.89) or Tsk (0.05±0.01 vs 0.04±0.01°C/min, p=0.59) between 24°C or 33°C respectively. The peak change in temperature during exercise (relative to rest) was not different for Tc (0.56±0.09 vs 0.56±0.07°C, p=0.97) or Tsk (1.59±0.24 vs 1.47±0.24°C, p=0.66) between 24°C or 33°C. Finally, no child developed hyperthermia. CONCLUSIONS: The significantly warmer Tsk without an excessive rise in Tc during exercise in the heat demonstrates maintenance of Tc and heat dissipation, thus preventing hyperthermia. Our results provide support for the notion that during exercise in the heat, many children likely experience "heat intolerance" rather than hyperthermia. Additional testing should include more rigorous exercise and environmental conditions for further insight into the thermoregulatory capacity and safety of children during exercise.