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sports with an emphasis on overall endurance rather than on individual muscles, the transport capacities of the blood and oxygen exchange into the cell become the limiting factors. In both cases, transient oxygen deficiency can occur, despite the high oxygen turnover (Berg et al., 1987).

Classical sports physiology has concerned itself intensively with the phenomena of availability, turnover, and regeneration of substrates for obtaining energy. In addition, the effects on the structured components of the cell are of essential relevance, a fact that has as yet hardly been acknowledged. It is no coincidence that compartmentalization of the cell is a major feature of higher organisms. The cell membrane contains important switching points for transport processes as well as for reactive processes. Indeed this is the basis of cell function. Membranes participate in some form in the vast majority of metabolic processes. The main objective, even in the case of intense physical performance, is always to maintain the integrity of the membrane structures or at least to ensure that changes are reversible (Berg et al., 1987).

The major cause of membrane damage is the formation of free radicals that can arise from various processes during metabolism. In the aerobic energy supply, most of the adenosine triphosphate (ATP) is formed during endoxidation, when electrons of a substrate (e.g., pyruvate or succinate) are transformed via a so-called redox chain to oxygen. The end product formed is water (reduction of the oxygen to water), where:

It is known that free radicals can arise in the case of incomplete oxygen reduction. If less than four electrons are made available, the following activated oxygen forms are created:

In the resting state, 250 to 300 ml of oxygen per minute are usually taken up. Under physical exertion, oxygen uptake can increase to 4,700 ml/min, or even more, depending on training conditions. Around 3 to 10 percent of the metabolized oxygen is not completely reduced to water but to these different radicals (Demopoulos et al., 1986). The increased formation of oxygen radicals is also termed oxidative stress (Demopoulos et al., 1986).

Other metabolic processes also lead to the generation of free radicals. Physically strenuous activity induces certain inflammatory-like reactions that are associated with the increased formation of radicals (e.g., leukocyte activation with phagocytosis, leukotriene synthesis). In addition, radicals from outside are taken up into the body, for example, from air pollution, ultraviolet radiation, and cigarette smoke. These sources can also be of great relevance

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