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Training hot for performance gains

Athletes know all too well that sudden exposure to heat or altitude can severely impact performance, so acclimation is a good idea. And it turns out that exposure to one of these stressors may actually help the other one.

Study under review: Cross Acclimation between Heat and Hypoxia: Heat Acclimation Improves Cellular Tolerance and Exercise Performance in Acute Normobaric Hypoxia

Introduction

All exercise causes stress, which is followed by adaptations that allow the body to better handle that stress in the future. Environmental conditions, such as extreme temperatures or changes in altitude, or even humidity, can also cause stress and adaptation. Recent research has shown that adaptation to one type of stressor can induce protective responses upon exposure to a different stressor, presuming they share common adaptive responses. When improved cellular protection or reduced physiological strain is observed in an organism in response to a different type of stressor, it is referred to as cross-tolerance. Some examples of possible cross-tolerance from rat experiments are shown in Figure 1.

Figure 1: Some cross-adaptations found in rats

Adapted from: Fregly MJ.Compr Physiol. 2011.

Changes associated with heat acclimation[1] include an increased cardiac output and plasma volume, an earlier onset of sweating with an increased sweat rate (and more diluted sweat), lower core temperature at rest and during exercise (reduced thermoregulatory strain) and a lower heart rate for a given workload during exercise (reduced cardiovascular strain). Hypoxic exposure refers to a state of reduced oxygen availability, experienced when traveling to high altitudes or simulated in a lab. Responses to hypoxia include an increase in red blood cells and oxygen-carrying capacity, as well as adaptations for improving carbohydrate metabolism[2].

At the cellular level, both heat and hypoxic stress stimulate the heat shock response[3]. Heat shock proteins[4] (HSPs) refer to a large family of proteins that aid in a cell’s response to physical stress. These proteins are important because they are observed in species across the animal kingdom. The most commonly studied HSPs expressed in response to heat or hypoxia are the HSP70 and HSP90 families, which are named after their molecular weight. In addition to HSPs, increases in hypoxia-inducible factor 1-α (HIF1-α), the master regulator of oxygen-regulated genes, have also been observed[5] in animals after heat stress and heat acclimation. This suggests an association, and potential interaction, between HIF-1α and HSPs during heat acclimation.

Previous[6] research[7] using exposure to heat and hypoxia (alone and in combination) has shown reductions in physiological strain (measured by the heat shock response) during subsequent hypoxic exercise. Human studies have yet to examine the HIF-1α response to heat acclimation, which may play a role in the improved performance[8] seen during hypoxic exercise after heat acclimation. Accordingly, the goal of this new study was to examine the effects of heat or hypoxic acclimation on physiological, cellular, and performance responses to hypoxic exercise in humans.

Who and what was studied?

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