THE INDUCTION AND DECAY OF SHORT-TERM HEAT ACCLIMATION

Abstract

Heat acclimation studies have focused mainly on its induction, such that there is very limited information on the decay of physiological and performance adaptations. PURPOSE To investigate adaptation to and decay from short-term heat acclimation. METHODS Ten moderately trained males (mean±SD age 27.7 ±7.1 yrs, mass 75.0 ±4.0 kg, maximal oxygen uptake 4.2 ±0.4 L.min −1 and peak power output 329 ±42 W) underwent heat acclimation for 90-min on five consecutive days (T a = 39.5°C, 60 % relative humidity) under controlled hyperthermia (rectal temperature 38.5 °C). Participants completed a standardised, exercising heat stress test (HST) one wk before acclimation (Acc), then on the 2 nd and 8 th day (1 wk) following Acc (Ta 35°C, 60 % relative humidity). Seven participants further completed HSTs at 2 and 3 wks after Acc. HST consisted of 90-min cycling at 40 % peak power output (PPO) before an incremental performance test (2 % PPO: 30 s intervals) to volitional exhaustion. RESULTS Rectal temperature (Tr) at rest (37.1 ±0.4 °C) was not lowered by Acc (95%CI: −0.3 to 0.2 °C), but after 90-min exercise (38.6 ±0.5 °C) it was reduced 0.3 °C (−0.5 to −0.1 °C) by Acc, and remained at this level one wk later (−0.5 to −0.05 °C), but not two (+0.1: −0.4 to 0.5 °C for n = 7) or three weeks. Similarly, heart rate after 90-min exercise (146 ±21 b.min −1) was reduced (−13: −6 to −20 b.min −1) and remained at this level after one wk (−13: −6 to −20 b.min −1) but not two (−9: 6 to −23 b.min −1 for n = 7) or three wks. Performance (746 s increased by 106 s: 59 to 152 s) after Acc and remained higher after one (76 s: 31 to 122 s) but not two (15 s: −88 to 142 s) or three (24 s: −23 to 97 s) wks. CONCLUSION Short-term (5-day) heat Acc induced adaptations permitting increased heat loss rather than lower resting body temperature. This adaptation persisted one wk but not two wks following Acc, as did the increased performance capacity. Supported by research grants from University of Otago and DSTO.