Motor units are composed of a single motor neuron and all the muscle fibers it innervates. The size of a motor unit and the number of muscle fibers it controls vary significantly between different muscles, depending on the function and required precision of that muscle.

Motor Unit Size and Muscle Function

• Muscles requiring fine motor control, such as the muscles of the hand and extraocular muscles, have small motor units. For example, extraocular muscles may have motor units with as few as 3 muscle fibers per neuron. This allows for very precise, delicate movements.

• Muscles responsible for powerful, gross movements, such as the muscles of the leg, have large motor units, innervating many muscle fibers per neuron to generate strong contractions.

• Muscles of the face and hand occupy an intermediate position but generally have smaller motor units than leg muscles, reflecting the need for finer control.

Highest Number of Fibers per Motor Unit in Humans

Among the listed muscles:

• Muscles of the leg have the highest number of muscle fibers per motor unit among the options. For example, the gastrocnemius muscle can have motor units innervating from 1,000 to 2,000 muscle fibers per neuron, allowing for powerful contractions needed in activities like running and jumping.

• In contrast, hand muscles have smaller motor units (around several dozen fibers), and extraocular muscles have the smallest.

Conclusion

The skeletal muscle with the highest number of muscle fibers in a motor unit in the human body is:

(3) Muscles of leg