Motor unit recruitment
The size principle states that slow fatigue resistant motor units are recruited first. This theory serves to demonstrate the relationship between the motor unit twitch force and the recruitment threshold of the fibers. For example these first muscle fibers are not able to sustain a lasting power output and give out rapidly. They do not have the ability to produce great force. Thus after lifting a certain number of repetitions the original fibers are fatigued and not producing the necessary force to continue. New fibers are then called upon to lift the weight. These new fibers are faster and much more powerful but also fatigue much quicker than the original fibers. These soon become exhausted and of limited use in the lifting process. But the advantage of this recruitment process is the majority of the muscles’ fibers have contributed all they can to the lift. In other words the fibers have all been exhausted and will have to repair the damage caused by the lift in order to become stronger and better able to tolerate the resistance. The next time this load is placed upon them, they will have accommodated and grown stronger.
Training in such a manner as to inhibit the weaker fibers and going straight to the fast acting powerful ones is the key to instant and explosive force and power. Conditioning the CNS to bypass the non-power fibers occurs in some of the elite strength athletes. This takes the body a long time to make this adaptation and requires a deep dedication to the strength sport-more so than many people have at the lower levels of participation.
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