Electromyography Definition .
Electromyography is basically the study of motor unit activity. In electromyography, the study of the electrical activity of contracting muscle provides information concerning the structure and function of the motor units. Motor units are composed of one anterior horn cell, one axon, its neuromuscular junctions and all the muscle fibers innervated by the axon . The nerve cell and the muscle fiber it supplies are defined as a motor unit. Whenever a muscle fiber contracts, the surface membrane undergoes depolarization so that an action potential is recorded from the fiber.
When the fibers of a motor unit are activated, they contract nearly but not quite synchronously and their action potential is added up and hence relatively large complex potential known as motor unit action potential is recorded. Electromyography makes it possible to localize the site of pathology affecting either muscle or its innervation and also provides evidence regarding the nature of pathological process. Electromyography is a technique by which the action potentials of contracting muscle fibers and motor units are recorded and displayed.
Phase System of EMG .
Recording the EMG requires a three phase system .
1. an input phase .
2. a processor phase .
3. an output phase.
An input phase includes electrodes to pick up electrical potential from contracting muscle, a processor phase amplifies the very small electrical potentials and an output phase includes the display and analysis of electrical potential by visual and auditory means.
Types of Electromyography .
1. Diagnostic or clinical electromyography .
2. Kinesiological electromyography
1. Diagnostic or clinical electromyography .
It is used for the study of diseases of muscles, neuromuscular junctions and nerves. It is used for the purpose of electrodiagnosis. The electric potentials from the skeletal muscle fibers are recorded and analyzed for the study of some disease processes. Diseases in which the structure and function of the motor unit is affected, the motor unit action potential may have an abnormal configuration and the pattern of motor unit activity during voluntary contraction may be altered.
Healthy muscle fibers contract only when they are activated by neurons and hence under normal conditions, only the motor unit action potentials are seen. In neuromuscular disease, single muscle fiber may contract apparently spontaneously and this may be recognized by the action potential derived from small group of fibers.
2. Kinesiological electromyography .
It is used in the study of muscle activity and to establish the role of various muscles in specific activities. Kinesiological EMG is beneficial for producing the objective means for documenting the effects of treatment on muscle impairments. It is used to examine the muscle function during the specific, purposeful tasks or therapeutic regimen.
The Components of Electromyography .
The components of electromyography apparatus are:
1. Electrodes .
2. Amplifier system .
3. Display system.
1. The Electrodes .
They are used in the input phase for picking up of electrical potentials from the contracting muscle fibres. The electrodes are of following types:
a. Surface electrodes .
b. Needle electrodes .
c. Fine wire indwelling electrodes .
d. Single fibre needle electrodes .
e. Macroelectrode .
f. Intra cellular electrode .
g. Multi lead electrode .
a. Surface electrodes .
Surface electrodes are basically used for kinesiological investigations. These are made up of small disc of electrodes most commonly of silver-silver chloride. The diameter of electrode is generally 3 to 5 mm . Skin preparations are important in order to reduce skin resistance. Skin preparation includes washing of skin, rubbing to remove dry and dead cells and cleaning with alcohol to remove dust.
They are generally considered adequate for monitoring large superficial muscles or muscle groups. They are not considered selective enough to record activity accurately from an individual motor unit or from specific small or deep muscles unless special recording procedures with adequate amplifiers and filtering procedures are used.
b. Needle electrodes .
Needle electrodes are used for clinical electromyography for recording single motor unit potential from different parts of a muscle. The different types of needle electrodes used are:
1. Concentric (coaxial) needle electrode .
This type of electrode consists of a stainless steel cannula through which a single wire of platinum or silver comes out. The cannula shaft and wire are insulated from each other and only their tips are exposed. They act as electrodes and potential difference between them is thus recorded .
2. Monopolar needle electrode .
These are composed of single fine needle which is insulated except at its tip. A second surface electrode is placed on the skin near the site of insertion which serves as a reference electrode. These electrodes are less painful than concentric electrodes because they are much smaller in diameter .
3. Bipolar needle electrode .
These consist of a cannula containing two insulated wires with their bare tips. The bared tips of both wires act as the two electrodes and the needle serves as the ground .
c. Fine wire indwelling electrodes .
These are used for kinesiological study of small and deep muscle. It is made by using two fine wires of small diameter with polyurethane coating or nylon insulation. Insulation is removed from the tip of the wires and hooks are created to keep the wires imbedded while the needle is removed from the muscle .
d. Single fibre needle electrodes .
These are concentric wires of 25 µm diameter and contain stainless steel cannula of 0.5 mm diameter. This gives information about propagation velocity along the muscle fibres. Single fibre needle records from a small area and hence it cannot be used for motor estimation of motor unit size. Single fibre EMG is employed to study neuromuscular transmission abnormality and fibre density.
e. Macroelectrode .
Macroelectrode is a concentric needle electrode of 15 mm shaft. It records from a large number of motor units along the shaft of the needle. The recording from one motor unit is separated by using a single fibre needle attached to macroelectrode in the midshaft. This method gives information concerning the whole motor unit but has not at present widely applied to the study of pathological motor units.
f. Intra cellular electrode .
This is an extremely fine electrode of diameter 0.5 µm and is used to record the potential changes inside the membrane across a cell. It is made so fine so as to penetrate deep inside a cell or intracellular matrix.
g. Multi lead electrode .
This electrode consists of a common steel cannula which comprises of at least three insulated electrodes at regular intervals inside it.
In addition to recording electrodes (surface or needle), a ground electrode must be applied in order to cancel the interference effect of the external electrical noise and vibrations such as caused by mobile phones, fluorescent lights, broadcasting facilities, elevators and other electrical appliances. The ground electrode is a surface electrode which is attached to the skin near the recording electrode but usually not over the muscle.
The myoelectric signal .
The EMG electrodes convert bioelectric signal resulting from muscle or nerve depolarization into an electrical potential capable of being processed by an amplifier. The difference of electric potential between the two recording electrodes is processed. The potential difference is measured in volts.
The amplitude or height of potential is measured in microvolts. The potential difference and the amplitude are directly proportional to each other, the greater the potential difference between the electrodes the greater the amplitude. The amplitude of motor unit potential is measured from the highest to the lowest point (i.e. from peak to peak).
2. The Amplifier system .
Before the motor unit potential can be visualized, it is necessary to amplify the small myoelectric signals. An amplifier converts the electric signal large enough to be displayed.
Differential amplifier .
The electric potential is composed of the EMG signal from the muscle contraction and unwanted noise from the static electricity in the air and power lines. To control for the unwanted part of the signal, the differential amplifier is used, as noise is transmitted to the amplifier as a common mode signal when the difference of potential is reduced at both the ends, the noise being cancelled out both the ends of amplifier.
Common mode rejection ratio .
Actually, noise is not eliminated completely in the differential amplifier. Some of the recorded voltage includes noise. The common mode rejection ratio (CMRR) is a measure of how much the desired signal voltage is amplified relative to the unwanted signal. A CMRR of 1000:1 indicates that the wanted signal is amplified 1000 times more than the noise. It can also be expressed in decibels (dB). A good differential amplifier should have a CMRR exceeding 100000 : 1.
Signal to noise ratio .
Noise can be generated internally by the components of the amplifier system such as resistors, transistors, or the circuit. This noise can be observed by the hissing sound on an oscilloscope. The factor that reflects the ability of the amplifier to limit this noise relative to the amplified signal is the signal to noise ratio. This ratio can also be described as the wanted signal to the unwanted signal.
Gain .
The gain refers to the ratio of the output level of signal to the input level of signal. This characteristic refers to the amplifier’s ability to amplify the signals. A higher gain will make a smaller signal to appear larger on the display system.
Input impedance .
Impedance is a resistive property present in the alternating current circuits. Impedance is present at the input of the amplifier and as well as at the output of the electrodes and they are directly related to the voltage. As per law, if the impedance at the amplifier is more than the impedance at the electrodes, the voltage will drop more and more accurately it represents the signal.
On contrary, if the impedance at the electrodes is more than the impedance at the amplifier, the voltage drop will be less. Also, the impedance depends on many factors such as skin resistance, material of the electrodes, size of the electrodes, length of the leads and electrolyte, etc. Blood, skin and adipose tissue also offer resistance to the electrical field.
Frequency band width .
The EMG waveforms as processed by an amplifier are actually the summation of signals of varying frequencies. The frequency is measured in Hertz. The frequency of an EMG signal is inversely proportional to the interelectrode separation. Consequently the frequency spectrum extends from 10 to 500 Hertz for most surface electrodes and from 10 to 1000 Hertz for fine wire electrodes.
3. The display system .
The amplified or processed signal is displayed in a useful manner. The form of output used depends upon the desired information and the instrumentation available. The electrical signal can be displayed visually on a cathode ray oscilloscope or computer monitor for analysis. A cathode ray oscilloscope consists of the electron gun, screen, horizontal and vertical plates.
The working of the cathode ray oscilloscope is, the electron gun which projects the electron beam toward the screen interiorly is phosphorescent in nature. There are two set of plates that is vertical and horizontal arranged, as the electron beam passes there is deflection in the vertical plate and sweep at the horizontal plate this is shown at vertical plate signal voltage in microvolts and sweep at the horizontal plate shows the duration of signal in millisecond but by conversion there is positive as well as negative deflection and below base line.
These signals are displayed by the loudspeaker which records both the cathode ray oscilloscope image sound and ink pen writers are also sometimes used, but they are limited to frequencies. Alternatively camera can be connected to the cathode ray oscilloscope and then photographs can be made for permanent record. Computers can also be used so that it performs the complex analysis of motor unit potentials and send results to printer. The data received can also be stored and monitored on a computer based system . It can be stored in an analog or digital form.
The conversion process is referred to as analog to digital conversion and the device that is used to perform this task is called A to D converters. The motor unit potential can also be converted into the sound in the same way as the radio signal is processed. For the same reason that every motor unit potential will look different it will also sound different. Normal and abnormal potentials have distinctive sounds that are helpful in distinguishing them.
Motor Unit Action Potential .
The motor unit action potential (MUAP) means when the depolarization of muscle fibres, which results in the electrical activity and graphically recorded by electromyogram, it represents potential derived from group of muscle fibres that are contracting nearly synchronously and are situated fairly close together and frequently activated by a single neuron. The motor unit action potential therefore represents a sample of activity of the fibres of motor unit and its characteristics are influenced by position of electrodes in relation to fibres of unit.
The muscle action potential can be recorded as a monophasic wave in a non conducting medium. Recording in a conducting medium, the current flow generated by the potential is same as a relative positive wave when recorded from a distance. Electromyography refers to recording of action potentials of muscle fibres firing singly or in groups near the needle electrode in a muscle. The distance of recording electrodes from the muscle fibre determines the rise time and fall time of the muscle fibres.
Indications of Electromyography .
- Myopathy .
- Myasthenia gravis .
- Nerve lesion .
- Sensory Nerve Conduction Velocity .
- Motor Nerve Conduction Velocity .
- Peripheral Nerve Injuries / Neuropathy .
- Severity of Nerve Injuries .
- Numbness .
- Muscle Pain or Cramping .
Contraindications of Electromyography .
Although electromyography with surface electrodes have very minimal contraindications, electromyography with needle electrodes is contraindicated in
- Dermatitis .
- Uncooperative patient .
- Pacemaker .
- Blood transmittable diseases .
- Extreme swelling .
- Abnormal blood clotting factor .
- Patient on an anticoagulant therapy.
- Gloves and eye protection should be used in case the EMG is to be done in individuals with blood transmittable diseases.
The Electromyographic Examination .
An electromyography is used to assess the integrity of neuromuscular system including the upper and lower motor neuron, the neuromuscular junction and muscle fibres. The test is done for detecting the muscle action potential in a group or individual in the different stage of contraction. Peripheral nerve lesions are also detected by electromyography.
The Technique of EMG Recording .
The patient is asked to relax and the needle is inserted inside the muscle, simultaneously spontaneous burst of potential is observed. The insertion activity is observed when the needle breaks the fibre membrane. The equipment of EMG recording is set up at sweep speed 5–10 ms/div; amplification 50 µV/division for studying spontaneous activity and 200 µV/divisions for motor unit potentials and filter setting 20–10000 Hz, the duration of MUP’s should be measured at a gain of 100µV/div and sweep speed of 5 ms/div and low filter at 2–3 Hz.
In needle electromyography, following types of activities are recorded .
1. Insertional activity .
2. Spontaneous activity .
3. Motor unit potential .
4. Recruitment pattern.
1. Insertional activity .
Introduction of the needle into the muscle normally produces a brief burst of electrical activity due to mechanical damage by needle movement and it lasts slightly exceeding the needle movement (0.5–0.10 sec). It appears as positive or negative high frequency spike in a cluster. Insertional activity may be increased in denervated muscles and myotonia whereas it is reduced in periodic paralysis during the attack and myopathies when muscle is replaced by connective tissue or fat.
Prolonged insertional activity is sometimes found in normal individuals which is diagnosed by its widespread distribution. Trains of regularly firing positive waves sometimes are familial and may be due to a subclinical myotonia. On the other hand in muscular individuals, the insertional activity is reduced especially in the calf muscles.
2. Spontaneous activity .
When the cessation or decay of insertional activity occurs after a second or so, there is no spontaneous activity in a normal muscle, which is called Electrical silence. Observation of silence in the relaxed state is an important part of the EMG examination. In the end plate zone however miniature end plate potentials are spontaneously recorded instead of silence. On needle recording, end plate potentials appear as monophasic negative waves of less than 100 µV and duration of 1–3 ms.
The end plate potentials are usually seen with an irregular baseline and are called as end plate noise. In the end plate region, action potentials which are brief, spiky, rapid and irregular with an initial negative deflection are known as end plate spikes. These are compared with the sound of sputtering fat in a frying pan. End plate spikes are due to mechanical activation of nerve terminals by the needle. To avoid the normally occurring spontaneous end plate activities, the needle should be introduced slightly away from the motor point.
3. Normal motor unit action potential .
The normal motor unit action potential is the sum of electrical potential of the muscle fibres present in the single motor unit, having the capability of being recorded by the electrodes. The normal motor unit action potential depends on the given five factors that is amplitude, duration, shape, sound and frequency. In normal muscle, the amplitude of a single motor unit action potential may range from 300 mV to 5 mV from peak to peak. The total duration measured from initial baseline will normally range from 3 to 16 m-sec.
The shape of a motor unit action potential is diphasic or triphasic with a phase representing a section of potential. There are sometimes polyphasic potentials in two or more phase. The sound is a clear distinct thump and there is capability of the motor unit that it will fire up to 15 times per second with strong contraction, usually when muscle is at rest it represents electrical silence but if there is an activity it is considered as abnormal and denoted by spontaneous activity which is not represented by normal voluntary muscle contraction.
a. Duration of motor unit action potential .
The duration of motor unit action potential is measured from the initial take off to the point of return to the baseline. The duration of motor unit action potential normally varies from 5 to 15 ms depending upon the age of the patients, muscle examined and temperature. The facial muscles have a very short duration 4.3 to 7.5 cm compared to limb muscles. Duration of biceps brachii is 7.3 to 12.8 ms and that of interossei 7.9 – 14.2 ms.
The duration of the motor unit action potential is greatly influenced by age of the subject; motor unit action potential is short in children, longer in adults and still longer in elderly persons. Temperature also influences the duration significantly; 7ºC cooling increases the duration of motor unit action potential by 10 – 30%. The duration of motor unit action potential is a measure of conduction velocity, length of muscle fibre, membrane excitability and synchrony of different muscle fibres of a motor unit.
The initial and the terminal low amplitude portions of motor unit potential are also contributed by the fibres more than 1 mm away from the recording electrode. The duration of motor unit action potential, therefore, is much less influenced by the distance of recording electrode compared to the amplitude.
b. Rise time of motor unit action potential .
The rise time of motor unit action potential is the duration from initial positive to subsequent negative peak. It is an indicator of the distance of needle electrode from the muscle fibre. A greater rise time is attributed to resistance and capacitance of the intervening tissue.
c. Amplitude of motor unit potential .
The amplitude of motor unit action potential is measured peak to peak. It depends upon size and density of muscle fibre, synchrony of firing, proximity of needle to the muscle fibre, age of the subject, muscle examined and muscle temperature. Decreasing muscle temperature results in higher amplitude and longer duration of MUPs.
d. Phase of motor unit action potential .
Motor unit potential recorded by a concentric or monopolar needle reveals as inverted triphasic potential (positive-negative-positive). The phase is defined as the portion of MUP between departure and return to the baseline. A motor unit action potential with more than four phases is called as polyphasic potential. Some potentials show directional changes without crossing the baseline and these are known as turns.
4. Recruitment pattern .
The firing rate of motor unit action potential for a muscle is constant. When voluntary contractions are initiated, the motor units are recruited in an orderly fashion, the smallest appearing first, larger later and largest still later. This pattern of recruitment is based on Hanneman’s size principle. If there is loss of motor unit action potential, the rate of firing of individual potentials during muscle contraction will be out of proportion to the number of firing and it is termed as reduced recruitment. During strong voluntary contraction, normally there is dense pattern of multiple superimposed potentials which are called as interference pattern. Less dense pattern may occur with a loss of motor units, poor effort or in upper motor neuron lesions.
Abnormal spontaneous potentials .
As a normal muscle at rest exhibits electrical silence, any activity seen during the relaxed state is considered as abnormal. These activities are termed as spontaneous because these are not produced by the voluntary contraction of the muscles. The common abnormal spontaneous activities are:
1. Fibrillation potential .
2. Positive sharp waves .
3. Fasciculation potential .
4. Repetitive discharges.
1. Fibrillation potential .
Fibrillations are spontaneously occurring action potentials from a single muscle fibre. Fibrillation potential is seen in the denervated muscle as they give spontaneous discharges due to circulating acetylcholine. Fibrillation potential are classically indicative of lower motor neuron disorders such as peripheral nerve lesions, anterior horn cell disease, radiculopathies, and polyneuropathies with axonal degeneration. Fibrillation potentials are found to a lesser extent in myopathic diseases such as muscular dystrophy, dermatomyositis, polymyositis and myasthenia gravis.
2. Positive sharp waves .
Positive sharp waves are found in denervated muscles at rest and are usually accompanied by fibrillation potentials. These are recorded as a biphasic with a sharp initial positive deflection followed by slow negative phase. Positive sharp waves are seen in primary muscle disease like muscular dystrophy, polymyositis but sometimes it is also seen in upper motor neuron lesions.
3. Fasciculation potential .
Fasciculation potentials are random twitching of muscle fibre or a group and may be visible through skin. These are spontaneous potentials seen with irritation or degeneration of anterior horn cell, nerve root compression and muscle spasm or cramps. They may be biphasic, triphasic or polyphasic.
4. Repetitive discharges .
These are also called as bizarre high-frequency discharges. These are characterized by an extended train of potentials of various forms. These are seen with lesions of the anterior horn cells, peripheral nerves and with the myopathies.
Normalization of EMG .
It is not reasonable or justified to compare the EMG activity of one muscle to another or from one person to another. This is because of the variability inherent in the EMG signal and interindividual differences in anatomy and movement. Therefore, some form of normalization is required to validate these studies, as for many studies the quantified EMG signal is used to compare activity between different muscles or subjects.
Kinesiological Electromyography .
Kinesiological electromyography is used to study the muscle activity and to establish the role of various muscles in specific activities. Surface electromyography can be used as a kinesiological tool to examine muscle function during specific and purpose tasks. Kinesiological electromyography presents an objective means for documenting the effect of treatment on muscle impairments. Surface as well as fine wire indwelling electrodes is used for kinesiological study.
Smaller muscles obviously require the use of smaller electrodes, with a small interelectrode distance. The ground electrode should be located reasonably close to the recording electrodes. The EMG signal can be stored, averaged and sampled in a variety of ways to permit detailed and complete analysis. For kinesiological electromyography the therapist should be interested at looking the overall muscle activity and quantification of the signal is often desired to describe and compare changes in the magnitude and pattern of the muscle response.
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