HUMAN ACCLIMATIZATION AND ADAPTATION TO STRESSESRushall Thoughts (1994).
To better understand the mechanisms and dispositions of humans when reacting to altitude it is worthwhile to consider some general principles of human acclimatization and adaptation. Acclimatization is appropriate for a relatively short period of exposure, such as when athletes are taken to an "altitude" camp for a month. Adaptation refers mainly to changes that occur over generations under constant exposure to a stress. Many of the features of acclimatization and adaptation are similar and in most researches there is no distinction drawn between the two.
When individuals habitually live at sea-level, their energy and muscular systems function optimally to accommodate the existing atmospheric conditions. Climate moderates that functioning. In hot-humid environments, the adaptations are different to those of dry-cool climates. The flexibility of responses of the human body to various combinations of altitude and climates has made it possible for humans to populate many parts of the earth.
Generally, the body acclimatizes in a variety of ways depending upon the environmental stresses to which it is exposed. Passive stresses are those which are persistent and relatively invariant (e.g., altitude, climate) and active stresses are those which are occasional/variable (e.g., exercise, emotions, diet, vocations). Reactions to excessive stresses are modified by the individual attributes of each person.
The adaptations and acclimatizations to passive environmental stresses are specific and do not generalize between environments. This is in accord with the Principle of Specific Adaptation. The specificity of adaptations is often illustrated in the physical characteristics of indigenous peoples. For example, the Inuit of the northern hemisphere have a general physical structure that minimizes body surface area which in turn reduces heat loss so that survival in the cold is facilitated. This is an example of the exclusive specificity of passive environment adaptation. Since altitude acclimatization is a different stress to exercise stimulation, one should expect little transfer of the effects of altitude acclimatization.
As an example of the specific demands of active stresses, it has long been recognized that in serious athletes oxygen transport enhancements developed in one form of exercise rarely, and if then only marginally, influence similar characteristics in other activities. For example, VO2max improvements in cycling do not show any substantial changes in VO2max of running. Similarly, VO2max improvements in swimming do not influence any change in VO2max of running. It should be noted though, that VO2max improvements in running do "transfer" some very minor effects to more specific activities, such as swimming, kayaking, and cycling, in moderately-trained persons. However, with elite athletes, training effects are so specific that there is no beneficial carry-over of circulatory improvements in one activity to another. If the active stress adaptations of specific exercises are exclusive, it is difficult to comprehend why the benefits of a passive stress acclimatization would transfer to a specific sporting activity such as swimming. There is no justification for asserting that a specific trained state would be enhanced by acclimatization to altitude stress any more than would be expected from any other stress or activity that produced circulatory alterations.
The length of exposure to passive stresses modifies the nature of changes and the resiliency of those changes when the stress is removed. When a person is exposed to moderate altitude, some initial responses occur to accommodate the changed conditions and in time others follow. As the duration of the exposure increases, the repertoire of acclimatization changes becomes complete. The greater the length of time that the body is in a fully acclimatized condition, the more habitual become the changes. Thus, upon exposure to a passive stress, the body undergoes a hierarchy of responsive changes, and eventually becomes fully acclimatized to the point that the changes become constant as well as permanent while residing in the environment. This full acclimatization is compromised by shorter periods of exposure. Generally, the shorter the time spent at altitude, the less dramatic are the acclimatizations and those which do have an opportunity to occur are quite transient.
There are four main features that modify acclimatization responses.
When an organism is stressed excessively by the cumulative pressures of more than one stress, it is possible to recover from one or more stresses while succumbing to another. It is common to observe hard training athletes fall ill and have to take time off from training. Upon returning to the practice environment, not only has the athlete's health improved but so has performance. The illness has allowed recovery from an overtrained or excessively fatigued state. A similar recovery phenomenon can be expected upon exposure to altitude. Performance capacity is reduced as the new stress is incurred. Acclimatization to altitude occurs slowly and exercise quantity, and often quality, has to be reintroduced gradually. Upon returning to sea-level, performances could well be enhanced due to the lightened exercise demands associated with the initial acclimatization to an altitude training program. That improved performance capacity could result from an "unintentional taper" rather than from any benefits due to altitude training. The same or possibly an even more impressive performance change could be achieved by reducing the work load at sea-level, as is the practice in an intentional taper. A similar phenomenon occurs when hard training athletes are exposed to noticeably different hot or cold conditions. The acclimatization and coping associated with the changed environmental conditions usually is accompanied by reduced physical strain. This recovery of physical performance capacity is embraced by the principle: when athletes are exposed to passive stresses and continue to perform physically, but at a reduced level of effort, recovery from previously accumulated physical stress will occur.
Changes in training locations often are accompanied by a reduction in the total effects of all life stresses. For example, when college students leave campus for an altitude training site, that site often offers a simpler life-style than that of the campus. Attending classes, studying, being involved in serious interpersonal relationships, etc., all events which tax the stress-coping capacity of the individual, do not exist at the altitude camp site. Consequently, the new stress of altitude acclimatization could be associated with a significant reduction in life-style stresses. That change in living demands (the "camp" effect) could also foster recovery from previous "hard" sea-level training. Simplified living of this nature at an altitude camp could provoke an unintentional taper. In many situations, this could be the factor responsible for performance improvements upon returning to sea-level.
If athletes go to altitude "tired" and spend less than a month on a reduced workload and often with a simplified (less stressful) life-style, then an unintentional taper could be expected. When those athletes return to sea-level they may perform as well as or even slightly better than comparable sea-level athletes, not because of beneficial altitude effects, but because they are more rested. This is the most common explanation when sea-level improvements are found after altitude training. The same or a better effect could be achieved by staying at sea-level, reducing the strain of training, and/or simplifying athletes' life-styles.
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