Acclimatization is an athletic training system whereby the body is forced to compensate for the stresses of a new or different climatic condition. Through compensation, the body is able to tolerate such physical stresses in a more efficient fashion, and the athlete will usually achieve better physical performance. The tolerance developed to the particular training condition will generally result in better competitive results, in competitions where the training climatic conditions exist, as well as in the athlete's accustomed environment.

Acclimatization methods consist of two types, heat and altitude acclimitazation. Heat acclimatization, sometimes referred to generically as heat training, is a technique directed to improving athletic performance in warm climates. The process of acclimatization is distinct from heat acclimation. Heat acclimation is the process by which an athlete becomes accustomed to increased heat, over the course of a 4- to 14-day period. Heat acclimatization is the entire spectrum of heat training, including both the initial acclimation period as well as the timeframe leading to the competition where the heat training practices will be employed.

Heat training is a common technique employed in such sports as marathon running. The improved ability of an athlete to combat heat and accompanying humidity will bear a vital relationship to performance. As an example, where the athlete is accustomed to living and training in a temperate climate such as the northwestern United States or England, acclimatization to hot weather will be essential if the athlete competes in an environment such as a September race in southern California. Heat training will be tied closely to the corresponding factor of dehydration, and the ability of an athlete to replenish fluids.

As there are few athletic activities that are contested at high altitudes (the exceptions include mountain climbing, alpine skiing, and events contested in high altitude locales such as Mexico City, Mexico, and Denver, Colorado), altitude acclimatization, or altitude training, does not prepare an athlete for high altitude competition, so much as develop the ability of an athlete to better utilize oxygen, which makes the athlete more effective in sea level competitions.

At sea level, the atmosphere is 20.93% oxygen; this percentage becomes progressively less at greater altitudes, due to the combined effects of decreased gravity and temperature. At higher altitudes, the body compensates for the decrease in available oxygen by increasing its production of erythrocytes, commonly known as red blood cells, which transport oxygen through the body. This increased production in red blood cells begins in the kidneys, through the production of the hormone erythropoietin (EPO).

The athlete who trains at altitude will develop—over a period of one to three months—a greater physical ability to utilize oxygen for performance in the thinner, less oxygen-rich air of high altitude; once trained, the capacity to produce greater numbers of red blood cells will remain a factor for a number of weeks, in an ever-decreasing amount. Altitude training is used by a wide variety of athletes, including those in sports where aerobic capacity is of prime importance, such as swimmers and cyclists, as well

Altitude acclimatization, or altitude training, helps the athlete's body to better utilize oxygen.
as athletes in sports where the body's anaerobic system is the focus, including sprinters and team sport athletes.

Altitude training is broken further into three types: "live high/train high," whereby the athlete both lives and trains at altitude; "live high/train low," a regime where the athlete lives at altitude but trains at sea level; and sea-level training, where the reduced oxygen environment of higher altitudes may be replicated through an artificially configured house or training "tent."

The extensive scientific research regarding altitude training confirms that all three methods will enhance sea level performance. At altitude training conducted at 8,000 ft (2,500 m), the reduced level of oxygen compels the body to increase production of red blood cells, the agents for the transport of oxygen in the bloodstream. Increased red blood cell production is triggered by the releases in the kidneys of the hormone erythropoietin (EPO). Altitude training will increase oxygen capacity by between 2% and 3% within three months of commencement, a significant factor in many sports. This benefit will be lost to the athlete within three months of the completion of altitude training.

The principle common to the theories underlying heat and altitude acclimatization is that of passive stress versus active stress on the bodily systems. Passive stresses are those features of a different training environment that will impact an athlete without special effort. For example, day-to-day living at a higher altitude or in a warmer climate requires the body to adapt to change. Active stresses are the deliberate introduction of training factors in the new environment, such as workouts in the new environment. While both types of stresses will contribute to the acclimatization process, heat without exercise will not be as effective.

The human body is very adaptable to heat, and to corresponding humidity. The major physiological adjustments will be made by a trained athlete within 10 to 14 days of the commencement of heat training; most athletes will reach an acclimatization of approximately 75% (defined as an ability to perform to 75% of their top level) within five days of their exposure to a warmer climate.

The most successful heat training programs will follow a progression:

  1. Training volume and training intensity are at first reduced on the athlete's first exposure to the hot environment.
  2. Both volume and intensity are increased as the athlete begins to adapt.
  3. The body mass, hydration rates, and other physical indicators must be monitored through the heat training phase.
  4. Extreme care to ensure the proper hydration of athletes must be maintained.

SEE ALSO Heat exhaustion; Hormonal response to exercise; Running strength training and exercises.