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 men cycled in hot conditions (40 degrees C, 39% R. H.) for 29 +/- 2 minutes. Subjects exercised at 60 +/- 9% VO2peak, 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 T-rec (degrees C.h(-1)), with stepwise multiple regressions performed on statistically significant variables (p <= 0.05). Linear regression identified the predicted intensity required to increase T-rec by 1.0-2.0 degrees C between 20-and 45-minute periods and the duration taken to increase T-rec by 1.5 degrees C in response to incremental intensities to guide prescription. Significant (p <= 0.05) relationships with the rate of change in T-rec were observed for prescriptions based on relative power (W.kg(-1); r = 0.764), power (% Power(max); r = 0.679), rating of perceived exertion (RPE) (r = 0.577), VO2 (% VO2peak; r = 0.562), heart rate (HR) (% HRmax; r = 0.534), and thermal sensation (r = 0.311). Stepwise multiple regressions observed relative power and RPE as variables to improve the model (r = 0.791), with no improvement after inclusion of any anthropometric variable. Prescription of exercise under heat stress using power (W.kg(-1) or % Powermax) has the strongest relationship with the rate of change in T-rec 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 T-rec efficiently and maximize adaptation.