MUSCLE FIBER RECRUITMENT AND THE TRAINING RESPONSE
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:
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:
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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