Motor Unit Recruitment
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Motor Unit Recruitment is very dependent on FORCE production needs of the muscle and proceeds from the ‘smallest‘ motor units in size (Type I) to ‘largest’ motor units (Type IIa and then to Type IIx).Can You Explain Motor Unit Recruitment? Read more below from a former student and see if you can describe as well.
Motor unit recruitment depends on the force/resistance of the exercise. With light intensity exercise the Type I (slow twitch) motor units are recruited. When the load is increased, the Type IIa (fast twitch) will be recruited with the help of the Type I fibers. When the load becomes even greater, the Type IIx will be recruited with the help of the Type IIa and Type I motor units. Therefore Type I motor units are always firing no matter what the intensity.

Jennifer Hill, Exercise Physiology Student, Spring 2010

Learning Tip: Some texts help student learn the recruitment ORDER of motor units using the 'size' principle analogy. Recruitment starts with the smaller (in diameter) Type I, progressing to the larger Type IIa, and then to the largest Type IIx. However, most importantly I want you to understand that physiologically, it is the FORCE production needs of muscle that truly dictate recruitment.
The Motor Unit: What two different anatomical constituents compose a motor unit?
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