Principles
A motor unit consists of one alpha motoneuron and all the muscle fibres it innervates.
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.
MUNE uses a general formula of: Number of motor units = compound muscle action potential size divided by the mean surface-detected motor unit action potential size
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.
Read more about this topic: Motor Unit Number Estimation
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