In vertebrates, the force of a muscle contraction is controlled by the number of activated motor units.
Larger motor units are typically composed of faster muscle fibers that generate higher forces.
Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fiber contractions.
This is tested by determining the recruitment threshold of a motor unit during isometric contraction in which the force is gradually increased.
When multiple MUAP’s are recorded within a short time interval, a motor unit action potential train (MUAPT) is then noted.
[9] In medical electrodiagnostic testing for a patient with weakness, careful analysis of the MUAP size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.
Motor units are generally categorized based upon the similarities between several factors: The typing of motor units has thus gone through many stages and reached a point where it is recognized that muscle fibers contain varying mixtures of several myosin types that can not easily be classified into specific groups of fibers.