Role of Physical Fitness in Heat Acclimatisation, Decay and Reinduction

Abstract

Loss of heat acclimatization was investigated, in winter after decay periods of 3, 6, 12 and 18 days, by evaluation of the residual retention and the time necessary for reinduction. Twenty-four fit soldiers (mean .ovrhdot.VO2 max [maximal oxygen uptake] = 49.5 ml kg-1min-1) were concurrently heat acclimatized for 9 days, walking 1.34 m s-1 for 110 min each day at 49.degree. C, 20% R.H. [relative humidity] 1.4 m s-1 wind. Physiological measurements included heart rate (HR), rectal temperature (Tre), mean skin temperature (.hivin.Ts) and sweat rate. Final mean Tre at 110 min decreased from 39.1.degree. C (day 1) to 38.3.degree. C (day 9), and final mean HR from 160 beats min-1 to 124 beats min-1 (P < .01). An individuals .ovrhdot.VO2 max was significantly related (r = -068) to the number of days for his Tre to plateau during acclimization. For the decay and reacclimatization, 4 groups were formed (6 subjects each), homogeneous in .ovrhdot.VO2 max the mean .ovrhdot.VO2 max for each group was 49.5 .+-. 1 ml kg-1 min-1. Initial percent loss of acclimatization was small (P > 0.05) for all groups (3, 6, 12 and 18 days) ranging from group mean losses in Tre of 0-18% and in HR of 2-29%: using Tre and HR criteria, all groups reacclimatized within 2 days. Their physical fitness is hypothesized as the prime factor in the small decay and rapid reacclimatization of these subjects even after 18 days.