Humans increasingly participate in endurance sports in extreme environments. In order to prepare for such events, the athletes try to adopt not only to endurance exercise by physical training, but also to the expected extreme climate. Heat Acclimation Humans can effectively adapt to heat. Decreased heart rate, rectal temperature and perceived exertion are observed in repeated fixed-intensity exercise as well as increased plasma volume and sweat rate. In order to acquire the physiological adaptations, the core temperature should be elevated above 38°C daily for at least an hour and preferably two hours. De-acclimation and Re-acclimation to heat Since athletes generally taper prior to the match, it is important that the acclimation status is maintained and does not decline during the tapering period. It was generally assumed that the decline of acclimation status took twice as long as the acquisition of acclimation. More recent studies show that the acclimation status is maintained for a much longer period, even two months, in particular for Rating of Perceived Exertion and rectal temperature adjustments (Weller et al. 2007). These results suggest that, once acclimation has been attained, the time that individuals may spend in cooler conditions before returning to a hot environment could be as long as a month, without the need for extensive re-acclimation to heat. When insufficient time for recovery is provided, the adaptations can be postponed to after the acclimation period (Daanen et al. 2011). Cold acclimation Repeated whole body exposure to cold leads to minor adaptations only compared to heat acclimation. The metabolic heat production increases, insulation by skin vasoconstriction improves and cold-acclimated subjects are better able to tolerate the cold. However, the major physiological textbooks describe that repeated local cold exposure of the extremities leads to considerably improved peripheral blood flow. A recent review (Cheung and Daanen, 2012) and experimental work (Daanen et al. 2012) showed that the extremities also do not adapt to cold.
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