Rushall Thoughts, 1988.

The anaerobic threshold is the state where the work of exercise can no longer be supported through aerobic energy supply alone. Above that state lactic acid begins to accumulate.

In an untrained state, the anaerobic threshold is a measure of the aerobic capacity of one's system. With training and consequent adaptation to exercise stress, the nature of the slow-twitch fibers changes. There is an increase in the capillarization within the muscle fiber bundles as well as the number of mitochondria in each fiber. That means that after training the amount of oxygen that can be used by the body is increased. Consequently, more work than in the untrained state can be performed before lactic acid starts to accumulate because of aerobic insufficiency. This results in one of the effects of aerobic training being an improvement in the anaerobic threshold.

However, the anaerobic threshold change that results from aerobic training is not only due to slow-twitch fiber adaptations. If the level of exercise strain is high enough some fast-twitch fibers will also adapt and become fast-twitch oxidative fibers. They will be recruited when the level of exercise approaches the anaerobic threshold. Thus, aerobic adaptation results in an anaerobic threshold shift because of principally slow-twitch fiber adaptations and, to a lesser extent, fast-twitch fiber adaptations.

This means that for maximum aerobic adaptation it is necessary to perform work of sufficient intensity to stimulate the use of fast-twitch fibers while almost the maximum aerobic capacity is being taxed. This is best achieved in interval work, or high-intensity fartlek work where the athlete periodically exceeds the existing anaerobic threshold in the one training session. Without the high intensity of work that requires the use of fast-twitch fibers, a maximum aerobically trained state and level of anaerobic threshold will not be attained.

For events that require both aerobic and anaerobic components in performance it is best to perform the greatest amount of work possible using both capacities at the pace of the intended performance. Any slower pace will not stimulate the full capacity and any faster pace will not produce sufficient volume of work to maximally adapt the aerobic system. Thus, a central theme of much training for many sports will be to perform training stimuli at such an intensity that fast-twitch fibers will be recruited and "converted" to perform aerobic work.

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