Aerobic athletes produce physical work relatively slowly over long periods of time through the hydrolysis of ATP The demand for the re-synthesis of ATP to continue muscular work during prolonged exercise is met by the oxidation of fuel (carbohydrates, fats, and some protein) in the mitochondria. Under normal resting conditions the electron transport chain (ETC) of the mitochondria uses oxygen to produce ATP and during aerobic exercise this process is greatly accelerated. In fact, during aerobic exercise, oxygen processing occurs at rates fold above resting levels This accelerated oxygen processing contributes to increased free radical formation at the cytochrome level of the electron transport chain, with a two- to threefold increase in free radical levels.
Although ETC enzymes have evolved to efficiently process oxygen during the generation of ATP, even with this enzymatic efficiency, an estimated 2-5% of total oxygen flux through the mitochondria can form superoxide radicals at rest. It is speculated that, during exercise, the increased flow of oxygen through the ETC can lead to a significant increase in superoxide radicals beyond resting levels, In addition, at rest, endogenous antioxidants located in the mitochondria can effectively remove superoxide radicals but again, during exercise, the increase in oxygen radicals may be more than the endogenous antioxidants can neutralize.
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