Motor unit recruitment

[5] Burke later demonstrated that there was a graded decrease of both EPSP and inhibitory postsynaptic potential (IPSP) amplitudes from small to large motoneurons.

In their 1982 paper, Burke and colleagues propose that the small cell size and high surface-to-volume ratio of S motor units allows for greater metabolic activity, optimized for the "highest duty cycles" of motoneurons, while other motor unit types may be involved in "lower duty cycles."

However, they state that the evidence is not conclusive "to support or deny the intuitively appealing notion that there is a  correlation between metabolic activity, motoneuron size, and motor unit type.

Another topic of controversy resides in the way in which Burke and colleagues categorized motor unit types.

However, it is important to note that Burke himself recognized the risk in classifying motor units:My friend the late Elwood Henneman told me several times in conversation that he thought classifying motor units into distinct categories was probably a bad idea because, unless used with care, classifications tend to distort reality.

The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction.

[4] In medical electrodiagnostic testing for a patient with weakness, careful analysis of the "motor unit action potential" (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.

General depiction of a motor unit, consisting of a motor neuron innervating a group of muscle fibers.
Graph depicting increasing muscle tension by recruitment of subsequent muscle fiber types (i.e. Slow-Oxidative Fibers, Fast-Oxidative Fibers, and Fast-Glycolytic Fibers)