Acclimation to exercise in the heat

How can athletes prepare for prolonged activity in the heat? Does repeated exercise in the heat make us better able to tolerate thermal stress? Many studies have addressed these questions and have concluded that repeated exercise in the heat causes a relatively fast adjustment that enables us to perform better in hot conditions. A similar but much more gradual set of adaptations occurs in people who adapt to hot conditions by living in hot environments for months to years. This is known as acclimatization(note that the word “climate” is part of this latter term).

Effects of heat acclimation

Repeated bouts of prolonged, low-intensity exercise in the heat cause a relatively rapid improvement in the ability to eliminate excess body heat, which reduces psychological strain. This process, termed heat acclimation, results in psychological adjustments in plasma volume, cardiovascular function, sweating, and skin blood flow that allow for subsequent exercise bouts in the heat to be performed with a lower core temperature and heart rate response. Because the body’s heat loss capacity at a given rate of work is enhanced by acclimation, core temperatures during exercise increase less than before acclimation(picture a), and heart rate increases less in response to standardized submaximal exercise after heat acclimation(picture b). In addition, after heat acclimation, more work can be done before the onset of fatique or exhaustion.

The sequence of positive adaptations takes a period of 9 to 14 days of exercise in the heat to fully occur, as shown in the figure below. Well-trained individuals need fewer exposures than untrained individuals to fully acclimate. A critical physiological adjustment that occurs over the first one to three days of acclimation is the expansion of plasma volume. The exact mechanism by which plasma volume expands after these initial exercise-heat exposures is not universally agreed upon. The process likely involves: 1) proteins being forced out of the circulation as muscles contract, 2) these same proteins then being returned to the blood through the lymph, and 3) fluid moving into the blood because of the oncotic pressure exerted by the increased protein content. However, this change is temporary, and blood volume usually returns to original levels within 10 days. This early expansion of blood volume is important because it supports stroke volume, allowing the body to maintain cardiac output while additional physiological adjustments are made.

As shown in the figure above, as gradual acclimation allows heart rate and core temperature to decrease, sweat rate during exercise in the heat increases with heat acclimation. Additionally, the amount of sweat produced often increases on the most exposed body areas such as the arms and legs, the areas that are most effective in dissipating body heat. At the beginning of exercise, sweating starts earlier in an acclimated person, which improves heat tolerance; and the sweat that is produced becomes more dilute, conserving sodium. This latter effect occurs in part because the eccrine sweat glands become more sensitive to the effects of circulating aldosterone.

Achieving heat acclimation

Heat acclimation requires more than merely resting in a hot environment. The benefits of acclimation, as well as the rate at which we acclimate, depend on:

• The environmental conditions during each exercise session;
• The duration of exercise-heat exposure, and
• The rate of internal heat production(exercise intensity).

An athlete must exercise in a hot environment to attain acclimation that carries over to exercise in the heat. Simply sitting in a hot environment, such as a sauna, for long periods each day will not fully or adequately prepare the individual for physical exertion in the heat, at least not to the same extent as will exercising in the heat.

How can athlete maximize heat acclimation? Because body temperature is elevated and sweating occurs, athletes gain partial heat tolerance simply by training, even in a cooler environment. Therefore, athletes are “preacclimated” to heat and need fewer exercise-heat exposures to fully acclimate. To gain maximal benefits, athletes who train in environments cooler than those in which they will compete must achieve heat acclimation before the contest or event. Heat acclimation will improve their performance and reduce the associated physiological stress and risk of heat injury.