How are muscle fiber recruitment patterns determined in electromyography, and how might muscle recruitment act prognostically?

Updated: Jan 27, 2020
  • Author: Adam B Agranoff, MD; Chief Editor: Stephen Kishner, MD, MHA  more...
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Answer

Answer

With minimal volitional activity, the recruitment patterns of muscle are normal. An individual muscle fiber begins to fire, reaching a threshold of 15-20 Hz, at which point the original fiber recruits a second fiber, which fires up to 15-20 Hz, and so on until there is full recruitment of the muscle.

Determination of recruitment patterns is difficult and requires more modern, computerized equipment; however, recruitment patterns provide critical information for determining the degree of injury and prognosis. Furthermore, this information is helpful in identifying individuals who are malingering. A patient may not be fully cooperative in manual muscle testing for weakness. Determination of the recruitment pattern, however, is not dependent on patient cooperation.

A study by Impastato et al indicated that in patients with traumatic brachial plexus injuries, needle EMG results are prognostic for spontaneous motor recovery. With manual muscle testing for strength conducted at least 1 year postinjury, the investigators found that patients in whom voluntary motor unit potential recruitment was absent at 1-9 months had a poor prognosis for spontaneous recovery. Moreover, as measured on the Medical Research Council scale at mean 1.4-year postinjury follow-up, a strength score of over 3/5 was reached in only 25% of muscles with discrete or severely reduced recruitment. [7]


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