Motor Units

A motor unit consists of the motor neuron and the grouping of muscle fibers innervated by the neuron. The number of muscle fibers within a motor unit varies, and is a function of the muscle's ability for accurate and refined motion. Precision is inversely proportional to the size of the motor unit. Thus, small motor units can exercise greater precision of movement compared to larger motor units. For instance, thigh muscles, responsible for large powerful movements, can have a thousand fibers in each unit, while eye muscles, requiring small precise movements, might only have ten. Groups of motor units are innervated to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are considered a motor pool. 

Motor Pools and Force

There are often multiple sizes of motor unit within a motor pool as a means of modulating the precision and force produced by a single muscle. For example, a small motor unit in the biceps can be activated for small precise movements, while a larger motor unit can be activated to facilitate more forceful actions.

These multiple motor units of different sizes within a motor pool allow for very fine control of force either spatially or temporally.

• Spatial – The recruitment of more or larger motor units to increase force.
• Temporal – The repeated stimulation of a motor unit so that the force of contraction generated is summed.