Electromyography
Principle
Electromyography (EMG) is a mainly clinical electrophysiological technique for evaluating
and recording physiologic properties of muscles at rest and while contracting. EMG
is performed with an electromyograph recording the potentials generated by the muscle
cells.
The electrical source of the EMG is the muscle membrane
potential of about -70mV. The resulting measured potentials range between about
50 μV and 30 mV. Typical repetition rate of muscle unit firing is about
7–20 Hz. Damage to motor units can be expected at ranges between 450 and 780
mV.
Muscle tissue at rest is
normally electrically inactive. After the electrical activity caused by the irritation
of needle insertion subsides, the electromyograph should detect no abnormal
spontaneous activity (i.e. a muscle at rest should be electrically silent, with
the exception of the area of the neuromuscular junction, which is normally
electrically very spontaneously active). When the muscle is voluntarily
contracted, action potentials begin to appear. As the strength of the muscle
contraction is increased, more and more muscle fibers produce action
potentials. When the muscle is fully contracted, there should appear a
disorderly group of action potentials of varying rates and amplitudes (a
complete recruitment and interference pattern).
EMG can be recorded invasively with an intramuscular needle
electrode (see for electrodes Electrophysiology:
general). Abnormal spontaneous activity might indicate nerve and/or
muscle damage. Then the patient is asked to contract the muscle smoothly. The
shape, size and frequency of the resulting motor unit action potentials (MUAPs)
are judged. Because skeletal muscles differ in the inner structure, the
electrode has to be placed at various locations to obtain an accurate study.
So, the electrode is retracted a few mm, and again the activity is analyzed
until at least 10-20 units have been collected.
The non-invasive technique with a surface electrode may
be used to monitor the general picture of muscle activation. This technique is mostly
used. Optimal electrode position and applied voltage are adjusted by auditory
or visual inspection of the recorded signal (biofeedback).
The characteristics of the MUAP are determined by the
number of muscle fibers per motor unit, the metabolic type of muscle fibers (red
or pale) and other factors.
Nerve conduction testing (for conduction speed and
amplitude decay) is also often done at the same time as an EMG in order to
diagnose neurological diseases.
Of importance are amplitude (dependent on fibers/unit and MUAPs in the EMG,
end the duration of the EMG (dependent on duration and synchronization of MUAPs).
Application
EMG is used to diagnose two general categories of
disease: neuropathies (e.g. alcoholic neuropathy, peripheral neuropathy, sensorimotor
polyneuropathy, denervation
(reduced nervous stimulation), poliomyelitis, carpal tunnel syndrome) and
myopathies (e.g. myasthenia gravis, Duchenne muscular dystrophy, and myopathy).
EMG is also applied in physiotherapy by biofeedback (auditory
or visual inspection of the recorded signal) to learn to control muscles involved in backaches, headaches, neck pain, the breathing
muscles and heart muscle.
Fig. 1
Flexible multi-electrode grid for high-density surface EMG. The
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Fig. 2
Data from high-density multi-electrode grid recoding. Top: “butterfly”
presentation of a 6x10 grid. Bottom: 3 activity maps of the 6*10 signals,
constructed at the times indicated after stimulation.
For a thorough analysis, the measured EMG signals can be
decomposed into their constituent MUAPs. MUAPs from different motor units tend
to have different characteristic shapes, while MUAPs recorded by the same
electrode from the same motor unit are typically similar. Notably MUAP size and
shape depend on where the electrode is located with respect to the fibers and
so can appear to be different if the electrode moves position. EMG
decomposition is non-trivial, although many methods have been proposed.
A development of the last decade is high-density surface
EMG, which can recently be performed with flexible multi-electrode grids.