The impact of prior heat stress on subsequent aerobic exercise-heat performance has not been studied.
Purpose: To determine whether prior heat stress degrades subsequent aerobic exercise performance in the heat.
Methods: Eighteen nonheat acclimated males were trained (four practice trials) on an aerobic exercise performance test in 22°C and then divided into two (n = 8) groups. One group (EUHPH; V˙O2peak = 44 ± 7 mL·kg−1·min−1) was tested after 90 min of recovery (in 22°C) from 3 h of intermittent light-intensity (<30% V˙O2peak) exercise-heat (50°C) stress, where sweat losses were matched with fluid intake (3.5 ± 0.5 L) to maintain euhydration. The other group (EUH; V˙O2peak = 45 ± 5 mL·kg−1·min−1) was tested while euhydrated without prior exercise-heat stress. Aerobic performance was determined from a 30-min cycling preload (50% V˙O2peak) 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 percent changes from the best practice trials.
Results: Volunteers were euhydrated (plasma osmolality <290 mOsm·kg−1) and normothermic before each exercise-heat trial. 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 ± 28.3 kJ; 2.0 ± 0.3 kJ·kg−1) and EUHPH (160.3 ± 24.0 kJ; 1.8 ± 0.2 kJ·kg−1). 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 stress does not result in a greater degradation in aerobic time trial performance in the heat compared with heat exposure alone.
US Army Research Institute of Environmental Medicine, Natick, MA
Address for correspondence: Robert W. Kenefick, Ph.D., FACSM, Thermal and Mountain Medicine Division, US Army Research Institute of Environmental Medicine, Kansas Street, Natick, MA 01760; E-mail: Robert.Kenefick@us.army.mil.
Submitted for publication November 2008.
Accepted for publication December 2008.