Rushall notes, (1989).

The selection of the muscle fiber bundles which are stimulated in exercise occurs according to the following principle. The smallest alpha-motoneurons (the nerve axon serving the muscle unit) have the lowest functional threshold. In any exertion they are contracted first. With increasing muscular forces, successively larger motoneurons are recruited. Thus, the force of contraction will determine which fiber bundles contract. The recruitment is according to motoneuron size and fiber type, the order being:

• slow-twitch oxidative;
• fast-twitch oxidative; and
• fast-twitch glycolytic fibers.

The relationship between motoneuron size and excitability is known as the "size principle". Small motoneurons innervate slow-twitch fibers while the largest innervate fast-twitch fibers. In between the two extremes are bundles of fibers that are served by graded sizes of neurons. At some point the bundles change from slow-twitch to fast-twitch fibers. Where the change occurs to fast-twitch and the servicing neuron is of moderate size, the fibers often contract in response to the need for exertion in endurance type activities. Those fast-twitch fibers adapt and become fast-twitch oxidative fibers, that is, they have the contractile properties of fast-twitch fibers but can use oxygen for energy purposes. At the end of the continuum of distribution of fiber bundles are the pure fast-twitch glycolytic fibers that never take on any of the properties of "endurance-adapted" fibers.

Several implications of these adaptation and recruitment phenomena are:

1. Continuous low level workloads usually excite only slow-twitch fibers resulting in endurance adaptation. Some minor speed improvements occur but only when training is initiated from an untrained or marginally trained state. Thus, continuous training of this type is limiting with regard to its potential to make athletes complete events faster.
2. For speed to improve, higher forces have to be developed in the muscles. This is best done through interval training where volumes of higher forces can be tolerated because of the intermittent rests that are involved. The increased forces recruit slow and fast-twitch fibers and with moderate training intensities adapt both fiber types aerobically. Higher intensities increase the threat of lactic acid accumulation.
3. Interval training that requires very high levels of force (possibly 80%+) also recruits pure fast-twitch glycolytic fibers which produce lactic acid and could cause fatigue to occur rapidly if work intervals are too long. To develop speed through repetitive training it is necessary to employ fast-interval training (high exertion levels and long rest periods to allow the resynthesis of glucose from lactate) or ultra-short training (very high exertion levels of sufficiently short duration to avoid the accumulation of lactic acid and short rest periods).

There is no option for improving speed and endurance in training other than to design training items that stimulate both types of muscle fiber bundles. That is best accommodated through interval work rather than continuous training (Le Rossignol, 1985).

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