Methods
There are at least six techniques that are currently in use to estimate motor unit numbers. These include incremental stimulation, multi-point stimulation method, F-response method, spike-triggered averaging method and the statistical method. Incremental stimulation is the most illustrative of the concept, and so will be discussed here.
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.
Read more about this topic: Motor Unit Number Estimation
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