Abstract
Background and objectives: The Australian Defence Force (ADF) engages in combat-related activities in hot climatic conditions, which exposes ADF members to the threat of exertional heat illness (EHI). After an episode of EHI, the heat tolerance test (HTT) is conducted to determine heat tolerance. Heat intolerance is the inability to maintain thermal balance while exercising in a hot environment. This study investigated the predictive roles of individual characteristics (age, gender, aerobic capacity (VO2max) and body composition) on physiological responses to the HTT in a group comprising ADF personnel and civilian volunteers. Materials and Methods: A quasi-experimental design was used and 52 (38 males and 14 females) participants were recruited from the ADF and the general population for the HTT. Heat intolerance was defined following the standard criteria for the HTT (temperature and heart rate). Data were analysed using inferential statistics. Results: The mean age of the participants was 31.1 ± 11.6 years, and 44% (23 people: 19 males and 4 females) of the participants were heat intolerant. Independent samples T-test showed that body mass index (p = 0.011) and body fat% (p = 0.034) of heat-intolerant participants were significantly higher than their heat-tolerant counterparts. Body surface area to mass ratio (p = 0.005) and aerobic capacity (p = 0.001) were significantly lower in heat-intolerant participants. Regression analyses showed that age, gender, aerobic capacity and body fat% were significant (p < 0.001) predictors of heat tolerance outcomes, with R2 values ranging from 0.505 to 0.636. Conclusions: This study showed that aerobic capacity, body fat%, age and gender are predictors of heat intolerance among military and non-military populations. However, there may be a need for future studies to consider identifying other indicators such as clinical biomarkers of heat intolerance, which could be used to develop a more reliable HTT protocol.
Highlights
Individuals vary in their ability to tolerate heat stress and the inability to maintain thermal balance while exercising in a hot environment is known as heat intolerance [5,6,7]
There was no significant difference between having a previous history of exertional heat stroke (EHS) and heat tolerance
The relationship between gender and each heat tolerance outcome was assessed, and the results showed that male participants had a significantly higher mean final Tc compared to their female counterparts (38.36 ◦ C ± 0.43 vs. 38.00 ◦ C ± 0.50; t = 2.59; p = 0.013)
Summary
The Australian Defence Force (ADF) is involved in combat-related activities in hot climatic conditions which exposes ADF members to the constant threat of exertional heat illness (EHI) [1]. Individuals who are recruited into the military should have the physical and physiological capacities to tolerate heat stress [4]. Individuals vary in their ability to tolerate heat stress and the inability to maintain thermal balance while exercising in a hot environment is known as heat intolerance [5,6,7]. Previous studies have reported that heat intolerance may be a by-product of inherent impaired thermoregulatory mechanisms or may occur as a direct result of exertional heat stroke (EHS) [8,9]
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