Motor Units

Muscles are composed of muscle fibers. The muscle fibers within a muscle group are innervated by several motor neurons. A motor neuron and the muscle fibers it innervates form what is called a motor unit.¹

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The number of muscle fibers in a motor unit varies. Generally, the larger the number of muscle fibers in a motor unit, the less precise the movement it allows.¹


It is impossible to activate some but not all of the muscle fibers within a motor unit. Thus, the motor unit is the basic unit of motor control.¹

However, it is possible to voluntarily recruit some motor units but not others. The first activated motor units are usually the ones whose muscle fibers are smallest and least forceful. As recruitment continues, the motor units that turn on have larger and more forceful muscle fibers. This relationship is called the size principle.¹