What is motor unit recruitment?

Updated: Oct 16, 2019
  • Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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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 (firing frequency) 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, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, increase in firing frequency becomes the predominating mechanism to increase motor strength. At this level and beyond, motor units may be driven to fire in their secondary range to rates greater than 50 Hz.

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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