Impact Of Prior Heat Stress On Subsequent Aerobic Exercise Performance

Abstract

The impact of prior heat exposure on subsequent aerobic exercise-heat performance has not been studied. However, recent research shows that despite returning to normothermia, residual effects persisted during an orthostatic challenge. PURPOSE: To determine if prior heat exposure degrades subsequent aerobic exercise performance in the heat. METHODS: Eighteen non-heat acclimated males established their baseline aerobic exercise performance in temperate conditions (4 practice trials; 22°C) and then were divided into two (n=8) groups. One group (EUHPH; VO2peak, 44.4 ± 6 ml·kg-1·min-1) was tested after 90 minutes of recovery (22°C) from 3 hr of intermittent light intensity (< 30% VO2peak) exercise-heat (50°C) exposure where sweat losses were matched with fluid intake (3.5 ± 0.5 L) to maintain euhydration. The other group (EUH; VO2peak, 46 ± 5 ml·kg-1·min-1) was tested while euhydrated without prior exercise-heat exposure. Aerobic performance was determined from a 30-min cycling pre-load (50% VO2peak) followed by a 15 min time trial in 40°C. Total work during the 15-min performance time trial in EUH and EUHPH was compared, as were the percentage changes from their best practice trial. RESULTS: Volunteers were euhydrated (plasma osmolality <290 mOsm·kg-1) and normothermic prior to each aerobic exercise performance trial. Therefore, the 90 rest was sufficient to recover from hyperthermia (core temperature elevation of 1.4 ± 0.5°C). Heart rate and core temperature were not different (p>0.05) between groups at any time point during exercise. Total work was not different (p>0.05) at baseline or between EUH (150.5 ± 28.3kJ; 2.0 ± 0.3 kj·kg-1) and EUHPH (160.3 ± 24.0 kJ; 1.8 ± 0.2 kj·kg-1) exercise-heat trials. The percent change in total work relative to baseline was not different (p>0.05) between EUH (-18.7 ± 9.2%) and EUHPH (-15.0 ± 7.8%). CONCLUSIONS: If hydration and body temperatures recover, prior exercise-heat exposure does not result in a greater degradation in aerobic exercise performance in the heat compared to heat exposure alone. Author's opinion, not government policy.