Prior heat stress: effect on subsequent 15-min time trial performance in the heat.
The impact of prior heat stress on subsequent aerobic exercise-heat performance has not been studied.


OBJECTIVE
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 degrees C and then divided into two (n = 8) groups. One group (EUHPH; (.)VO2peak = 44 +/- 7 mL x kg x min(-1)) was tested after 90 min of recovery (in 22 degrees C) from 3 h of intermittent light-intensity (<30% (.)VO2peak) exercise-heat (50 degrees C) stress, where sweat losses were matched with fluid intake (3.5 +/- 0.5 L) to maintain euhydration. The other group (EUH; (.)VO2peak = 45 +/- 5 mL x kg x min(-1)) was tested while euhydrated without prior exercise-heat stress. Aerobic performance was determined from a 30-min cycling preload (50% (.)VO2peak) followed by a 15-min time trial in 40 degrees 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 x 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 x kg(-1)) and EUHPH (160.3 +/- 24.0 kJ; 1.8 +/- 0.2 kJ x 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.
Kenefick
Robert W
RW
US Army Research Institute of Environmental Medicine, Natick, MA 01760, USA. Robert.Kenefick@us.army.mil
Ely
Brett R
BR
Cheuvront
Samuel N
SN
Palombo
Laura J
LJ
Goodman
Daniel A
DA
Sawka
Michael N
MN
D000222:Adaptation, Physiological; D000328:Adult; D001642:Bicycling; D001831:Body Temperature; D001833:Body Temperature Regulation; D003681:Dehydration; D016552:Ergometry; D015444:Exercise; D005080:Exercise Test; D017079:Exercise Tolerance; D018882:Heat Stress Disorders; D006801:Humans; D008297:Male; D018763:Muscle Fatigue; D010101:Oxygen Consumption; D013312:Stress, Physiological; D013997:Time Factors