Do greater rates of body heat storage precede the accelerated reduction of self-paced exercise intensity in the heat?

Eight trained volunteers cycled in 19 °C/1.8 kPa (COOL), 25 °C/1.2 kPa (NORM), and 34 °C/1.6 kPa (HOT), while maintaining an RPE of 16. Potential differences in S following the onset of exercise were assessed by comparing rates of esophageal temperature change (ΔT _es/Δt); and estimated S values using a traditional two-compartment thermometric model (S _therm) of changes in rectal (T _re) and skin (T _sk) temperature, and partitional calorimetry (S _cal).

After 15 min of exercise, workload decreased more in HOT vs. COOL (P = 0.03), resulting in a shorter time (HOT: 40.7 ± 14.9 min; COOL: 53.5 ± 18.7 min; P = 0.04) to 70 % of initial workload. However, there were no preceding differences in ΔT _es/Δt between conditions (P = 0.18). S _therm values were different between HOT and COOL during the first 5 min of exercise (P < 0.05), primarily due to negative S _therm values (−32 ± 15 kJ min⁻¹) in COOL, which according to partitional calorimetric measurements, required improbably high (~56 kJ min⁻¹) rates of evaporation when no sweating on the back and thigh was observed until after 7.6 ± 1.5 min and 4.8 ± 1.7 min of exercise, respectively. S _cal values in the first 5 min of exercise confirmed S was actually positive in COOL (+21 ± 8 kJ min⁻¹) and not negative. Different S _therm values following the onset of exercise at different environmental temperatures are simply due to transient differences in the rate of change in T _sk.

Reductions in self-paced exercise intensity in the heat are not mediated by early differences in S following the onset of exercise.