Muscles differ in the number of motor units that they contain, and how many muscle fibres are within each unit (innervation ratio). In a general sense, muscles that require specificity of movement, such as muscles in charge of eye movement, have fewer fibres per unit, while those that are meant for less specific tasks, such as the calf muscles in charge of jumping, have more.
The compound muscle action potential (CMAP) size is found using supramaximal stimulation of the motor nerve to the muscle or muscle group (similar to a nerve conduction study). It is recorded using surface electrodes. This is representative of the sum of the surface detected motor unit action potentials from muscles innervated by that nerve.
Surface-detected motor unit action potential (SMUAP) size is the contribution of individual motor units. The way of finding the average size of these action potentials depends on the method used, as described below.
According to Henneman's size principle, motor unit recruitment is always in the same order from smallest to largest motor unit. Additionally, the motor unit action potential is an all-or-none phenomenon - once the recruitment threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully. Incremental stimulation starts giving electrical stimulation at a very low stimulus intensity and increases gradually to reach the recruitment threshold of successively larger motor units until the intensity of the CMAP is reached. A 'step' is noted when an increase in stimulus leads to an increase in recorded EMG (i.e. another motor unit's threshold is reached and it is recruited). The CMAP is then divided by the number of steps required to reach the intensity of the CMAP to get a mean SMUAP size. The number of steps does not correlate to the total number of motor units in the muscle. Instead, the CMAP size is then divided by the mean SMUAP size to get an estimation of the number of motor units in the muscle.