What is the role of turns/amplitude 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

The interference pattern is dependent on the shape of the individual motor unit potentials (eg, amplitude, duration, polyphasia), the firing rates, and the number of active units. In general, myopathies have low amplitude and short duration components, resulting in relatively large high-frequency content. In contrast, neurogenic conditions have low-frequency components due to the large amplitude and long-duration motor unit potentials. Attempts have been made to quantify the characteristics of the interference pattern.

Willison in 1964 described a technique to analyze "turns" and "amplitudes" within the EMG trace. [6] The EMG is converted into 2 trains of pulses that are counted to characterize the signal in terms of amplitude and turns. A turn was defined as a change in the direction of the signal of at least 100 microvolts (µV). An amplitude count is produced for a fixed voltage change, usually 100 µV, between successive turns. In neurogenic conditions, the turns count is normal or low and the amplitude high, and in myopathic lesions, the turns count is high and the amplitude low. Both the amplitudes/second and the turns/second increase with strength of muscle contraction. Thus, the force exerted by the muscle is critical for the analysis and needs to be kept constant at a defined level. This is a major limitation of this method.

In 1983, Stalberg introduced a new version of the method, making it less dependent on force, by plotting the turns/second against the mean amplitude change/turn in an XY-diagram, the so-called turns/amplitude (T/A) analysis (see images below). [7] The recordings are made during an epoch of steady contraction (typically 1 s, but 250-300 ms may be sufficient) at various levels of muscle contraction. Typically, 20 recordings are collected. The position of the needle should be such that it is recording relatively sharp action potentials.

Turns/amplitude analysis. In this patient with Cha Turns/amplitude analysis. In this patient with Charcot-Marie-Tooth disease type 1 (also called hereditary motor and sensory neuropathy type 1), an inherited neuropathy, the turns/amplitude analysis resulted in 4 data points above the normal area, the so-called "cloud."
In this patient with muscular dystrophy, the turns In this patient with muscular dystrophy, the turns/amplitude analysis resulted in 14 of 20 data points below the cloud of normal values, thus supporting the impression of a myopathic condition.

For each muscle, a normal area called "cloud" exists that was defined by plotting recordings from a large number of healthy subjects at the 95% confidence level. Normal values fall into this cloud, and for a healthy individual no more than 1 data point of 20 recordings should be outside the cloud. A recording indicates a neurogenic condition if 2 or more data points are above the cloud of normal values and a myopathic condition if 2 or more data points are below the cloud.


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