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° 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 (°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°C between 20- and 45-minute periods and the duration taken to increase T rec by 1.5°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) (%HR max; 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 %Power max) 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.