Power relative to body mass best predicts change in core temperature during exercise-heat stress

Controlling internal temperature is crucial when prescribing exercise-heat stress, particularly during interventions designed to induce thermoregulatory adaptations. This study aimed to determine the relationship between the rate of rectal temperature (T ­rec) increase, and various methods for prescribing exercise-heat stress, to identify the most efficient method of prescribing isothermic heat acclimation (HA) training. Thirty-five males cycled in hot conditions (40°C, 39%R.H.) for 29±2 min. Subjects exercised at 60±9%\s\up8(.O2peak, with methods for prescribing exercise retrospectively observed for each participant. Pearson product moment correlations were calculated for each prescriptive variable against the rate of change in T­rec (°C.hr-1), with stepwise multiple regressions performed on statistically significant variables (p<0.05). Linear regression identified the predicted intensity required to increase Trec by 1.0-2.0°C between 20-45 min periods, and the duration taken to increase Trec by 1.5°C in response to incremental intensities to guide prescription. Significant (p<0.05) relationships with the rate of change in Trec were observed for prescriptions based upon relative power (W.kg-1; r=0.764), power (%Powermax; r=0.679), RPE (r=0.577), \s\up8(.O2 (%\s\up8(.O2peak; r=0.562), HR (%HRmax; r=0.534), and TS (r=0.311). Stepwise multiple regressions observed relative power and RPE as variables to improve the model (r=0.791), with no improvement following inclusion of any anthropometric variable. Prescription of exercise under heat stress utilizing power (W.kg-1 or %Powermax), has the strongest relationship with the rate of change in Trec with no additional requirement to correct for body composition within a normal range. Practitioners should therefore prescribe exercise intensity using relative power during isothermic HA training to increase Trec efficiently and maximize adaptation.