Rushall Thoughts, 1990.

Anaerobic metabolism may occur at the beginning of relatively high intensity exercise, when the oxygen transport mechanisms of the body have not had sufficient time to meet the energy requirements. Anaerobic metabolism may also occur when the aerobic metabolic pathways are over-stimulated so that certain key enzymes are over-taxed and not able to keep up with the required pace of converting one substance to another in the long chain of events that constitutes aerobic metabolism. This may happen even when there is sufficient molecular oxygen provided to the muscle cell. This latter feature is what is known as local fatigue, a feature that occurs in sports such as swimming, kayaking, and cycling, when specific large muscle groups work very intensely while the rest of the body works submaximally, that is, aerobically. In such situations, the athlete might not breathe very heavily, but at the localized site is working excessively. The lactic acid that is produced through the intensely working muscles is consumed by various organs (e.g., liver, heart) and submaximally working muscles. Thus, in sports where local fatigue is high it is possible to work those isolated muscle groups for a longer period more intensely than would be possible if the total body was working at a similar high intensity. Usually, higher levels of localized pain can be tolerated than can be endured with general fatigue.

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