EMG also known as electromyography; very important assessment tool for the assessment purpose of neuromuscuar junction.

1. Electromyography
By: Radhika Chintamani
 Luig; Galvini in 1971 presented the first report on electrical properties of muscle and
nerves. He demonstrated muscle activity followed by stimulation of nerves and recorded
potentials from muscle fibers in frog.
 It was accepted in early past century, when instrumentation was developed to make
recording of such activity.
 EMG form the basis to evaluate the scope of neuromuscular disease or trauma and as
kinesiologic tool to study muscle function. EMG involves the detection and recording of
electrical potentials from skeletal muscle fibers.
 EMG is the study of motor unit activity. Together with other clinical assessment EMG
can provide information about the extent of nerve injury or muscle disease and the
prognosis of surgical intervention and rehabilitation. It compares the electrical activity of
skeletal muscle fibers at rest and during voluntary activities of muscle.
 Motor units are composed of:
i) One Anterior horn cell (AHC)
ii) One axon
iii) All muscle fiber innervated by that axon
 Axon conducts an impulse to the fibers causing them to depolarize at the same time
producing an electrical activity known as Motor Unit Action Potential (MUAP) and
recorded graphically as EMG.
 Recording EMG requires 3 phases:
i) An input phase: electrodes to pick electrical activities from muscle.
ii) A processor phase: where small signal is amplified
iii) An output phase: where electrical signals are converted into audio or visual
signals and are displayed and analyzed.
 Instrumentation:
i) Electrodes
ii) Amplifier/pre-amplifier
iii) Fillers
iv) Display unit. (Oscilloscope, speaker, tape recorder, computer)
Electrodes
 An electrode is a transducer, a devise for converting one form of energy into another.
 Types of electrodes are:
i) Surface electrodes: used to test NCV, and EMG. Picks up the signals from
superficial muscle and group of muscles.
ii) Fine wire dwelling electrodes: for study of small and deep muscle.

2. iii) Needle electrodes: are necessary to record single motor unit potential.
iv) Ground electrode: It is a surface electrode, important electrode which must be
applied to provide mechanism of cancelling out the interference effect of external
electrical noise.
 Surface electrodes: These electrodes applied over skin, consists of small metal disc
commonly made of silver/silver chloride (3 to 5mm in diameter) skin resistance to be
reduced before applying surface electrodes by the emollient. These electrodes are
secured to the skin by tapes or straps.
 Fine wire indwelling electrodes: made with two strands of small diameter wire (100μm).
Inserted into the muscle belly with help of needle and then after insertion the needle is
removed out. Capable of picking up single motor unit potentials.
 Needle electrodes:
i) Concentric electrode: stainless steel, single wire platinum or silver threaded.
ii) Bipolar concentric: with two wires threaded through cannula.
iii) Monoploar needle electrode: composed of single fine needle, second electrode placed
on the skin as reference electrode.
Amplifier and Pre-amplifier.
 Amplifier: converts the electrical potentials picked up by electrode to a voltage large
enough to be display.
 Differential amplifier: are preferred as they control unwanted part of signals to be
amplified.
 Amplified signals have Common Mode Rejection Ratio (CMMR), which is a measure of
how much the desired signal voltage is amplified relative to unwanted signal. CMMR is
expressed in decibels. A good amplifier should have a CMMR exceeding 100,000:1.
Even signal to noise ratio limit the noise.
Filters.
 Low filters-limit high frequency artifacts.
 High filters-limit low frequency artifacts.
Displaying the EMG signal.
 Cathode ray oscilloscope.
 Computer monitor for analysis
 Graph records.
 Magnetic tape recorder.
 Audio signals.
These are the various instrument used for displaying EMG signals.

3. EMG examination:
Normal Motor Unit Potential.
 Represents the sum of action potential supplied by an anterior horn cell.
 Motor unit potential is also characterized by its duration, number of phases, amplitude,
turn phase and rate of rise of fast component.
 Duration:
- The duration of MUAP is measured from initial take off to the point of return to the
baseline.
- It normally varies from 5-15ms.
- Short in children, longer in adults, and still longer in elderly person.
 Rise time:
- Duration form initial positive to subsequent negative peak.
- Indication of distance of needle from the muscle fiber.
- Rise less than 500µs acceptable, a greater rise time is attributed to resistance and
capacitance of the investing tissue.
 Amplitude:
- Measured peak to peak.
- Depends upon size and density of muscle fibers, synchrony of firing, proximity of
needle to the muscle fiber, age of the subject, muscle examined and muscle
temperature.
- Normally it lies between 200Mv-3Mv.
 Phase of motor unit potential:
- The portion of MUP from departure and return to the baseline.
- Biphasic or inphasic or triphasic.
Frequency
 5-15per second (<60/sec)
Evaluation of EMG
Done at four stages:
i) Insertional activity.
ii) Rest
iii) Minimum muscle contraction.
iv) Maximum muscle contraction with resistance.
Insertional activity:
 Patient is asked to relax the muscle.
 Needle inserted into the muscle.

4.  At this time the electromyographer observes a spontaneous onset of potential which is
possibly caused by needle breaking through muscle fiber membrane.
 This is called insertional activity.
 Lasts less than 300ms.
 Stops when needle stops moving.
 It is rapid, spiky and biphasic activity.
 It is described as normal, reduced or increased.
 It is a measure of muscle excitability and markedly decreased in fibrotic muscle or
increased when denervation or inflammation is present.
Clinical relevance:
 When EMG is done on the gluteus minimus muscle in a standing position of the patient,
the insertional activity is never lasts upto 300ms, as any individual can not stand without
swaying in antero-posterior and medio-lateral direction, leading to shifting of line of
gravity accordingly, which describes high level of action of muscel and sometimes low
level of action of muscle. Hence when EMG study should be done the individual must be
completely resting the part undergoing study, or necessary procedures must be done to
avoid such errors in the study.
The muscle at rest:
 After cessation of insertional activity a normal relaxed muscle exhibits electrical silence,
which is absence of electrical potential.
 Potential arising spontaneously during this period are significant abnormal findings.
Clinical relevance:
 When EMG is done on the gluteus minimus muscle in a standing position of the patient,
the insertional activity is never lasts upto 300ms, as any individual can not stand without
swaying in antero-posterior and medio-lateral direction, leading to shifting of line of
gravity accordingly, which describes high level of action of muscel and sometimes low
level of action of muscle. Hence when EMG study should be done the individual must be
completely resting the part undergoing study, or necessary procedures must be done to
avoid such errors in the study.
Minimum muscle contraction:
 Here the patient is asked to contract the muscle minimally.
 This causes individual motor unit to fire.
 These motor unit potentials are assessed with respect to amplitude, duration, shape, sound
and frequency.
 These parameters are essential to distinguish normal and abnormal potentials.

5. In normal muscle:
Amplitude= may range from 300µv-5µv.
Duration = may range from 3-15ms.
Phase = may be biphasic or triphasic
Sometime polyphasic phase is observed which may be normal, but when these polyphasic
phase represent more than 10% of muscle output, they are considered as abnormal.
Maximum muscle contraction:
 Ask patient to contract the muscle maximally.
 With greater effort, increasing number of motor units fire at higher frequencies until the
individual potentials are summated and can no longer be recognized.
 An influence pattern is seen.
 This is normal finding with strong contraction.
Abnormal potentials:
I. Spontaneous activity: Muscle at rest exhibits electrical silence. Any activity seen during
relaxed state is called as spontaneous activity, because it is not produced by voluntary
contraction. There are four types of spontaneous activity; as follows;
a) Fibrillation potentials:
- Due to spontaneous depolarization of muscle fiber.
- Small amplitude and duration of potentials.
- Indicative of lower motor neuron disorders such as peripheral nerve, anterior horn
cell disease, radiculopathies and polyneuropathies.
- High pitched clicks.
- Triphasic (3 phases).
- Spikes may vary in amplitude from 10-300µV.
- Average duration of 2ms.
- Recorded at frequency up to 30per second.
b) Positive sharp waves:
- Observed in denervated muscle at rest.
- Usually accompanied by fibrillation.
- Dipahsic with sharp initial positive deflection (below baseline) followed by slow
negative phase.
- Low amplitude than positive phase.
- Much longer duration.
- Amplitude 1mV.
- Frequency ranges from 2-100/sec.

6. - Dull thread sound.
*In Upper motor neuron lesion fibrillation and positive sharp waves may be seen together.
c) Fasciculation:
- Seen with irritation or degeneration of anterior horn cell, nerve root compression,
muscle spasm and cramps.
- Represents involuntary asynchronous contraction of a bundle of muscle fibers
- Amplitude and duration similar to MUP.
- Diphaisc, Triphasic, Polyphasic.
- Frequency rate= 50/sec.
- Low pitched thump.
d) Repetitive discharge:
- Called bizarre high frequency discharge.
- Seen with anterior horn cell and peripheral nerve with myopathies.
- Extended terrain and potential of various forms.
- Frequency = 5-100 impulses/second.
- Amplitude = 50µV – 1 µV.
- Duration=100ms.
II. Abnormal Voluntary Potentials:
 Polyphasic potentials occurring greater than 10%.
 Elicited by voluntary contractions.
 Seen in myopathies and peripheral nerve involvement.
 Potentials of smaller amplitude than motor units.
 Shorter durations.
III. Giant motor units:
 Increased amplitude.
 Increased durations.
 Seen in post polio residual syndrome.
EMG used in evaluating entire motor system at various level:
Sr. No Area EMG
1. Cerebral cortex CNA,
Neoplasm
Trauma
2. Corticospinal tract
3 Anterior horn cells MND,
Polio,

7. SMA
4 Peripheral nerves Neuropathies
5 Neuromuscular
junction
Myasthenia gravis
6 Muscle membrane Myotonia,
Inflammation
7 Muscle
(contractile unit)
Dystrophies
Precautions: Aseptic techniques.
General Principles of EMG testing:
 Examination of number of muscles, both above and below the suspected site of
pathology.
 Examination of muscles innervated by other nerves in the same limb.
 Sampling EMG activity of the full cross section of each muscle tested.
 Examination of muscle in contralateral limbs or both upper and lower limbs may be
appropriate.
 Examination should be performed at the appropriate time in the context of the suspected
disorders.
Contraindications/ Precautions in electrophysiological Testing.
 Abnormal blood clotting factors/ anticoagulant therapy.
 Extreme swelling.
 Dermatitis.
 Uncooeprative patient.
 Recent myocardial infarction.
 Blood transmittable disease.
 Immune suppressed conditions.
 Central going lines.
 Pacemakers.
 Hypersensitivity to stimulation.