Excerpt from Motor Unit Recruitment in EMG

Synonyms, Key Words, and Related Terms: motor units, muscle contraction, firing rate, firing frequency, electromyography, EMG, motor unit recruitment, strength of muscle contraction, EMG study

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Motor unit recruitment may be defined as "the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction."¹

The central nervous system can increase the strength of muscle contraction by the following:

Increasing the number of active motor units (ie, spatial recruitment)
Increasing the firing rate at which individual motor units fire to optimize the summated tension generated (ie, temporal recruitment)

Both mechanisms occur concurrently. The primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, even though this increases the firing rate of the initially recruited motor units. The recruitment of different units takes precedence over increase in firing rate until nearly all motor units are recruited. At this level and beyond, motor units may be driven to fire in their secondary range to rates greater than 50 Hz.

The terms firing rate and firing frequency are used interchangeably.

The next section of this article discusses the physiology of motor unit recruitment in detail. Subsequent sections look at ways of examining recruitment during an electromyography (EMG) study. Assessment is made at different levels of innervation—minimal muscle contraction to determine the onset and recruitment firing rates (ie, recruitment pattern); maximal voluntary contraction to provide information about the interference pattern; and moderate voluntary contraction at various levels for assessment of the turns/amplitude analysis.

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