Aerobic Fitness, Thermoregulation, and Exercise in the Heat
Aerobic Fitness, Thermoregulation, and Exercise in the Heat
It is believed that the adaptations induced by aerobic training would prevent athletes from overheating while retaining body electrolytes during exercise in the heat. Data from our laboratory suggest that aerobic training is not enough to induce these adaptations. This review considers the factors that can lead to hyperthermia in trained and untrained populations when exercising in the heat.
The majority of current literature regarding the thermoregulatory responses to exercise in a hot environment is based on studies conducted on aerobically trained individuals (military recruits and endurance athletes) during prolonged exercise. In the recent ACSM position stand on exercise and fluid replacement, it is advised that fluid and electrolyte replacement should vary depending on the individual degree of heat acclimatization and training status. Yet, the article includes tables for predicting sweat rates and dehydration when running a marathon. Many people will never run a marathon but will exercise at moderate levels for health benefits or for leisure throughout their lifespan. Furthermore, in many countries, industrial workers labor under heat stress, although their labor is not of enough intensity to elicit the cardiovascular adaptations that would increase their maximal aerobic capacity (i.e., V̇O2max). Despite being a much larger sector of the population, there is less information on the thermoregulatory responses of healthy but not endurance-trained people during exercise in a hot environment.
The objective of this review is to gain an understanding of the factors that may lead to excessive hyperthermia (core temperature, >38.5°C) during exercise in the heat, particularly in the healthy but unfit population. Excessive hyperthermia during exercise in the heat induces fatigue and could lead to heat injury. Furthermore, heat exhaustion episodes may deter some individuals from engaging in exercise and thereby prevent them from receiving the associated health benefits. By understanding the factors that can lead to hyperthermia, recommendations can be made to attenuate their impact. This review will focus on situations of submaximal exercise during environmental conditions where heat dissipation is dependent greatly on sweat evaporation (>33°C dry bulb; <60% relative humidity). Our work has shown that during prolonged exercise in that environment, unfit individuals do not reach higher levels of hyperthermia than their aerobically fit counterparts. In fact, during intense exercise, fit individuals are the ones at a higher risk of developing excessive hyperthermia. However, aerobic fitness enhances the thermal benefits of rehydrating during exercise. Our hypothesis is that the thermal adaptations induced by aerobic fitness are not enough to prevent overheating, which will be determined by the interaction between exercise intensity, hydration, and environmental heat load.
Abstract and Introduction
Abstract
It is believed that the adaptations induced by aerobic training would prevent athletes from overheating while retaining body electrolytes during exercise in the heat. Data from our laboratory suggest that aerobic training is not enough to induce these adaptations. This review considers the factors that can lead to hyperthermia in trained and untrained populations when exercising in the heat.
Introduction
The majority of current literature regarding the thermoregulatory responses to exercise in a hot environment is based on studies conducted on aerobically trained individuals (military recruits and endurance athletes) during prolonged exercise. In the recent ACSM position stand on exercise and fluid replacement, it is advised that fluid and electrolyte replacement should vary depending on the individual degree of heat acclimatization and training status. Yet, the article includes tables for predicting sweat rates and dehydration when running a marathon. Many people will never run a marathon but will exercise at moderate levels for health benefits or for leisure throughout their lifespan. Furthermore, in many countries, industrial workers labor under heat stress, although their labor is not of enough intensity to elicit the cardiovascular adaptations that would increase their maximal aerobic capacity (i.e., V̇O2max). Despite being a much larger sector of the population, there is less information on the thermoregulatory responses of healthy but not endurance-trained people during exercise in a hot environment.
The objective of this review is to gain an understanding of the factors that may lead to excessive hyperthermia (core temperature, >38.5°C) during exercise in the heat, particularly in the healthy but unfit population. Excessive hyperthermia during exercise in the heat induces fatigue and could lead to heat injury. Furthermore, heat exhaustion episodes may deter some individuals from engaging in exercise and thereby prevent them from receiving the associated health benefits. By understanding the factors that can lead to hyperthermia, recommendations can be made to attenuate their impact. This review will focus on situations of submaximal exercise during environmental conditions where heat dissipation is dependent greatly on sweat evaporation (>33°C dry bulb; <60% relative humidity). Our work has shown that during prolonged exercise in that environment, unfit individuals do not reach higher levels of hyperthermia than their aerobically fit counterparts. In fact, during intense exercise, fit individuals are the ones at a higher risk of developing excessive hyperthermia. However, aerobic fitness enhances the thermal benefits of rehydrating during exercise. Our hypothesis is that the thermal adaptations induced by aerobic fitness are not enough to prevent overheating, which will be determined by the interaction between exercise intensity, hydration, and environmental heat load.
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