The electromyography (or electromyography ) (EMG) is a electrophysiological method in the neurological diagnosis measured, in which the electric muscular activity based on action currents of the muscles, and is represented (graphically).
With the help of concentric needle electrodes, the potential fluctuations of individual motor units can be derived. With special needles also let individual muscle fibers capture ( Einzelfasermyografie ). It is also possible to measure the changes in potential on the skin with surface electrodes, but it is much less precise, as this technique measures the total action potential of a whole muscle or even several muscles.
When performing an EMG, the electrical activity is measured in the resting muscle ( spontaneous activity ) and when the muscle is arbitrarily contracted to different degrees ( muscle action potentials ).
In medical electrodiagnostics, the EMG can be used to make statements about diseases of the nerve and muscle cells. In biomechanics , the relationships between the frequencies or the amplitudes of the registered electrical signals and the strength of a muscle are examined in order to optimize the movements of athletes, for example.
The recording of nerve action potentials is known as electroneurography (ENG). It is mostly subsumed under the generic term electromyography.
Two types of EMG are widely used: surface EMG and intramuscular (needle and fine wire) EMG. In intramuscular EMG, a needle electrode or a needle with two fine wire electrodes is inserted through the skin into the muscle tissue. Depending on the mechanical dimensions of the electrodes used, a high spatial resolution and statements about individual muscle fibers or a low spatial resolution and summary statements about entire muscle groups can be achieved.
Due to their mechanical dimensions, surface electrodes detect electrical activity up to a few centimeters and beyond, but cannot depict the activity of individual motor units. Such electrodes are used, for example, in sports physiology when it comes to determining the point in time at which a muscle contraction begins . In neurology, surface derivation is primarily used for tremor analysis .
To derive the electrical signals of the muscle, concentric needle electrodes are used for routine diagnostic EMG in neurology . These needles consist of a central conductor (wire) around which an electrical insulation layer is applied, around which the outer metal casing (tube) is then mounted. A typical EMG needle is 50 mm long, has an outer diameter of 0.45 mm and an oblique bevel at the tip, which shows the central conductor isolated from the oval bevel of the metal sleeve. The potential is measured between the center conductor and the metal shell. With this structure, above all, near-peak potential changes are recorded essentially up to a distance of 1–2 millimeters. This enables very targeted diagnostics.
In single- fiber electromyography with special needles, the recorded half-radius is only 0.2-0.3 mm. Monopolar electrodes are also used for special purposes. These electrodes consist of a mostly Teflon- insulated needle, the metal of which is only exposed at the tip. The voltage is measured against a separate reference electrode (usually a surface electrode on the skin).
The potential source of the EMG is the membrane potential of the muscle cell, inside −70 mV versus outside. When a muscle cell is excited, e.g. via the motor endplate , ion channels open and lead to a brief (approx. 1 ms) and local reversal of the membrane potential. This change in potential can be measured. The potential difference recorded by the input amplifier of the electromyograph depends on many physical and physiological and ultimately also pathological factors: on the distance between the electrode and the signal source (attenuation with the third power of the distance), on the number and type of muscle fibers that are active at the same time , the spatial arrangement of conductive and insulating tissue components (blood conductor = good conductor, fat = bad conductor) and many other factors. The result is a very complex electrical field. The measurement result is determined by both capacitive and ohmic resistances. As a result, a signal is measured in a typical medical neurological measuring arrangement for an EMG, which records potentials in the range from 50 μV to a few millivolts. The discharges of individual motor units then appear as potential fluctuations of around 10 ms duration and contain frequency components up to a few kilohertz . Electrically low-noise, potential-free instrument amplifiers with very high input resistance are used (typically: 200 MOhm input impedance , noise level (noise) 0.7 μV RMS , common-mode rejection > 100 dB ).
If, for example, the internal structure of the muscular excitation is not to be recorded in sports physiology, but rather the rough course of a muscle twitch, the EMG signal is often electrically rectified and filtered with a low pass . This results in an “integrated” and “smoothed” signal with a correspondingly low spatial and temporal resolution. The rectification, however, is based on a number of often inapplicable basic assumptions (it is simply not a simple, sinusoidal alternating voltage signal such as the mains voltage). The result should therefore only be interpreted with great caution.
The EMG signal was initially displayed on soot-blackened, rotating drums on which a mechanical pointer drew a track (similar to seismology ). Oscillographs and magnetic tape recorders were later used for storage. Since digital storage technology became more widely established, commercially available PCs or laptops with color screens and corresponding printers have been used.
When a skeletal muscle is completely relaxed, no nerve action potentials reach the muscle via the supplying nerves. The muscle is slack. The muscle fibers of a healthy skeletal muscle that does not receive any stimulating nerve impulses then show a constant membrane potential, which is shown on the screen as a straight, undeflected horizontal line. Under certain, mostly pathological conditions, however, spontaneous activity occurs, i.e. the muscle fibers' own activity without being stimulated by nerve impulses. This spontaneous activity manifests itself in various forms, which can be differentiated in terms of frequency and amplitude values. The puncture activity must be separated from the spontaneous activity. It occurs when needle electrodes are used and is explained by a temporary mechanical irritation of the muscle cell.
Miniature endplate potentials
Miniature endplate potentials, also known as endplate noise, have negative amplitudes of less than 50 µV and last for 0.5 to 2 ms. They arise at the contact points between axon and muscle cell through spontaneous transmitter releases, which however do not trigger any forwarded action potential on the muscle cell membrane. They therefore do not indicate any muscle damage. End plate potentials last approx. 20 ms and are always supra-threshold.
Fibrillation potentials and positive waves
Fibrillation potentials and positive waves arise in individual muscle cells and are signs of a lack of innervation ( denervation ). Fibrillation potentials last 1 to 5 ms, have amplitudes up to several 100 µV, are mostly biphasic or triphasic and occur strictly rhythmically (in contrast to the modulated discharge frequency of healthy motor units).
Positive sharp waves last about 4 ms, have amplitudes up to several 100 µV, are biphasic with a characteristic shape and occur rhythmically with a frequency between 3 and 50 Hz.
Fasciculation potentials are generated by a motor unit . The origin of the excitation lies in the supplying neuron . Damage to the innervating neuron can lead to depolarization of the nerve cell membrane, which reaches the motor unit as a forwarded action potential. The place of origin can be far away in the cell soma in the spinal cord ( motor neuron ) or also distally in the end branches of the axon to the individual muscle fibers. Fasciculation potentials occur irregularly (e.g. every 1–30 s). Fasciculations also suggest neuropathy .
Myotonic discharges are high-frequency sequences of action potentials (60–150 per second) with a duration of one to two (or three) seconds and an amplitude of 10 μV to around 1 mV. They indicate a malfunction of the muscle membrane , the ion channels of which are damaged.
Action potentials of motor units (MUAP)
After entering the muscle, each nerve fiber branches into several end branches, each of which innervates a muscle fiber via the motor end plates. A single, transmitted action potential of a single motor neuron therefore triggers a depolarization ( action potential ) and, subsequently, a contraction in several muscle fibers almost simultaneously . The sum of the depolarizations of a motor unit can be observed in the EMG as a characteristic deflection on the monitor. The height of the deflection gives a rough measure of the number of innervated muscle fibers (large versus small motor unit). The duration of the potential can indicate whether all muscle fibers are discharging synchronously or whether there are delays in individual end branches. During the evaluation, parameters such as age, type of muscle and muscle group must be taken into account. The results are compared with normal values in order to make statements about possible diseases. The normal value for the duration in middle age is 8 to 10 ms and for the amplitude 1 to 3 mV.
A special case of the MUAP examination is the interference pattern analysis, in which one measures the electrical activity of a muscle that has intentionally contracted maximally.
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