Acclimation To Moderate Heat Adequately Prepares Individuals For Work In A Hotter Environment

Abstract

This work was funded by the Human Sciences Domain of the UK Ministry of Defence Scientific Research Programme. Optimising the preparation of individuals to perform exercise in a hot environment is typically achieved by implementing a programme of heat acclimation (HA). It is well-established that HA mitigates the problems associated with exercise-heat stress, including a reduction in heat strain and an improvement in physical performance. PURPOSE: The aim of this study was to establish whether HA in a moderately hot environment (wet bulb globe temperature (WBGT), 26 °C) would prepare individuals to undertake moderate exercise in a hotter environment (WBGT, 32 °C). METHODS: 15 men exercised on a cycle ergometer for up to 2 hours per day on 12 consecutive days in a hot chamber. The WBGT was 26 °C during the first 9 days (Days 1-9) and 32 °C during the last 3 days (Days 10-12). Rectal temperature (Tre), heart rate (HR), mean skin temperature (Tsk) and sweat rate (SR) were measured during the first 60 minutes of steady state exercise (~45% of maximum oxygen uptake). Thereafter, the tolerance time (TT) was measured during graded cycle exercise to volitional fatigue. RESULTS: During Days 1-9 (WBGT, 26 °C), Tre reduced to a plateau from 38.7 ± 0.1 to 38.2 ± 0.1 °C(p<0.05), asdidhr(156 ± 10 to 131 ± 11 b-mm-1, p<0.05) and tsk (34.2 ± 0.3 to 33.6 ± 0.2 °c, p<0.05). tt increased to a plateau from 37 ± 2 to 46 ± 1 min (p<0.05). sr was 1.0 ± 0.8 l.h-1 on day 1 and did not change thereafter. during days 10-12 (wbgt, 32 °c), tre, hr, and tsk increased (p<0.05), whereas tt decreased (p<0.05) compared to day 9. however, post-hoc testing showed no significant differences for any of the parameters within the 3 days in the hotter environment. CONCLUSION: Moderate exercise performed at a WBGT of 26 °C for ~2 hours per day on 9 consecutive days conferred full HA, and no further physiological adaptations were observed when the same exercise was undertaken at a WBGT of 32 °C. We conclude that individuals may be adequately prepared for working in a hotter environment by prior HA in a moderately hot environment. (NB: this is the complete text)