What is the role of interference pattern analysis in motor unit recruitment EMG?

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

Answer

With increasing effort, the firing frequency of individual motor units increases and progressively more and larger units are activated. In a healthy subject providing maximal voluntary effort of the muscle under investigation, the action potentials of individual motor units no longer can be separated from each other but are mixed with the signals of other units. The recruitment pattern with maximal voluntary contraction is called "interference pattern" because of the increasing degree of superimposition of action potentials from different units. With increasing force, the EMG becomes continuously denser and the maximal peaks in the signal have a higher amplitude.

American Association of Electrodiagnostic Medicine defines the interference pattern as "electric activity recorded from the muscle with a needle electrode during maximal voluntary effort."

During a maximal voluntary muscle contraction of a healthy individual, a "full" or "complete" interference pattern is present. No individual MUAPs can be identified clearly (this is normal). The baseline is obscured completely by motor unit activity.

Incomplete interference pattern may be divided as follows:

  • Reduced interference pattern (ie, intermediate interference pattern): Some of the individual MUAPs may be identified, while other individual MUAPs cannot be identified because of overlap.

  • Discrete activity: Each of several different MUAPs can be identified. See the image below.

    Incomplete interference pattern. This example show Incomplete interference pattern. This example shows a discrete interference pattern in a patient with amyotrophic lateral sclerosis. Despite maximal voluntary effort, individual MUAPs can be identified and the baseline is partly visible.
  • Single unit pattern: A single motor unit fires at rapid rate during maximum voluntary effort.

An incomplete interference pattern typically signifies a decreased number of MUAPs being activated with maximal effort. This may be suggestive of a neurogenic lesion resulting in a decreased number of functional motor units. It may, however, occur with incomplete effort of muscle contraction, possibly as a result of poor cooperation or pain. In myopathic conditions, the interference pattern is typically complete, even though low-amplitude MUAPs may be noted on the recording of the interference pattern; however, in very advanced stages of muscle disorders, the interference pattern may be incomplete because of marked loss of muscle fibers.


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