Thermotolerance is the ability of the body and its cellular structures to withstand the destructive stresses of heat that exceeds the optimal core temperature range of human performance, from 96°F to 100°F (36°C-38.5°C). Thermotolerance is the end result of a successful program of heat acclimatization, where an athlete trains with the specific purpose of making the body functional in a warmer climate to which the athlete is accustomed.
When a person moves to a warmer climate than that in which he or she has previously lived, there will be a natural acclimatization to the warmer temperature; this process is known as passive acclimatization. To achieve a speedier result, an athlete will use exercise and a gradual buildup in both training volume and intensity over an approximate 14-day period to achieve a 100% heat acclimatization.
Exercise places additional heat stresses on the body during this acclimatization period. The function of the cardiovascular system generally, the ability of the body to maintain an ideal fluid/mineral balance, and the transmission of nerve impulses are all impacted by unaccustomed heat. In extreme circumstances, the stress of heat resulting in a core temperature that rises above the body's favorable operating range will initiate the destruction of cells, with organ damage and death the ultimate result. The process of thermotolreance that occurs from the heat training conducted by an athlete is related to a physiological response in the body. Heat shock proteins (HSPs) are chemicals produced by the body in response to prolonged heat stress. HSPs act to protect the cells from heat damage that would otherwise occur; scientific literature uses the expression "molecular chaperone" to describe the protective qualities of the HSPs with respect to heat. Exercise in warm weather accelerates the production of HSPs at a greater rate than simple passive acclimatization.
Conversely, thermotolerance is not a permanent protection against heat stress and resultant cell damage. Once thermotolerance is acquired, if the athlete is not exposed to warm weather conditions for a period of many months or years, the body will require another period in which to adapt to warm weather. In seasonal climates, such as those found in northern Europe and North America, athletes must adjust to summer training and competition to achieve the desired thermotolerance on an annual basis.