Power Relative to Body Mass Best Predicts Change in Core Temperature During Exercise-Heat Stress

Abstract Gibson, OR, Willmott, AGB, James, CA, Hayes, M, and Maxwell, NS. Power relative to body mass best predicts change in core temperature during exercise-heat stress. J Strength Cond Res 31(2): 403–414, 2017—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% V̇ o 2peak , 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), V̇ o 2 (%V̇ o 2peak ; 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.