How does EMG assess motor unit recruitment in myogenic conditions?

Updated: Oct 16, 2019
  • Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Answer

In muscle diseases such as polymyositis or muscular dystrophies, muscle fibers are damaged. A number of motor units are unaffected but the muscle fiber content of each motor unit is reduced; therefore, the force output of each unit is diminished. The number of units required to maintain a given force increases in proportion to the inefficiency of the individual motor unit discharge. Compensation occurs by having multiple motor units begin firing simultaneously. See the image below.

Early recruitment in myopathic conditions. In this Early recruitment in myopathic conditions. In this electromyographic (EMG) study of a patient with inclusion body myositis, many motor units are activated simultaneously at a low level of muscle contraction. Note the low amplitude and short duration of individual units.

In a myopathy, isolating a single firing motor unit often is impossible. Even with minimal muscular effort, typically 2 or more units may be activated. This recruitment pattern in myopathic conditions is called "early recruitment" or "increased recruitment." The recruitment frequency is decreased.


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