Muscles involved in fine, precise movements tend to have which characteristic regarding motor unit size?

• A. Fewer muscle fibers per motor unit.
• B. More muscle fibers per motor unit.
• C. The same as large muscle groups.
• D. No motor units.

Answer: (A)

Fine, precise movements rely on small motor units. When there are fewer muscle fibers controlled by a single motor neuron, each contraction adds only a small amount of force, allowing very fine gradations in movement. This enables smooth, accurate control required for tasks like eye movements or fingertip dexterity.

This fits with the size principle: the nervous system tends to recruit smaller motor units first, giving you fine control at lower forces, and only brings in larger units as more force is needed. If a motor unit had many fibers, a single recruitment would produce a larger, less controllable change in length and force, reducing precision. Muscles used for delicate work generally have small motor units, while those used for powerful gross movements have larger ones. Therefore, fewer fibers per motor unit best explains the increased precision seen in fine motor control.