Ashley, C. D., Bernard, T. E., & Ferron, J. (2006). Re-acclimation to heat after a 2-week and 4-week absence from the heat. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 2051.

This study attempted to establish the time for re-acclimation after periods of two and four weeks absence from the heat. Healthy participants (M = 4; F = 6) acclimated for approximately two weeks by walking on a treadmill in a climatic chamber (50ºC, 20% RH). The treadmill speed and grade was set to elicit a moderate metabolic rate of approximately 40% VO2max. Trials lasted 120 minutes unless one of the termination criteria was met. Weight was recorded pre- and post-trial. Fluid intake was measured. Heart rate and rectal temperature were monitored continuously and recorded every five minutes during the trials. In addition, VO2 was taken at 30 minute intervals. A three day plateau in rectal temperature was used to indicate acclimation. After acclimation, Ss were randomly assigned to one of two re-acclimation groups and refrained from physical activity in the heat for a period equal to 1/3 (2 weeks) and 2/3 (4 weeks) complete loss of acclimation (6 weeks).

The average time for acclimation was 8.2 + 1.5 days. The average time for re-acclimation for the 2-week and 4-week groups was 6.2 + 0.8 and 6.6 + 2.0 respectively. There was no significant trial by re-acclimation interaction. End-time was less and end heart-rate greater for the beginning of acclimation and re-acclimation than for the end of acclimation and re-acclimation. End rectal-temperature was less for the end of re-acclimation (38.06ºC) than for the beginning of acclimation (38.63ºC).

Implication. Absence from the heat did not affect the time for re-acclimation or related physiological variables. Ss reached acclimation in approximately seven days in both acclimation and re-acclimation.

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