2. EMG is the recording of the electrical activity of
muscle based on motor unit activity.
Motor units are composed of one anterior horn
cell, one axon, its neuromuscular junctions, and all
the muscle fibers innervated by that axon.
3. The single axon conducts an impulse to all its
muscle fibers, causing them to depolarize at
relatively the same time. This depolarization
produces electrical activity that is manifested as a
motor unit action potential (MUAP) and
recorded and displayed graphically as the EMG
The characteristics of the MUAP will change when
there is damage to either the nerve or muscle.
4. Motor unit action potential
The sum of action potential produced in muscle.
Characterized by its duration, number of phase,
amplitude & rate of rise of its first component.
5. Feature of MUP
Duration • measured from the initial take-off to the
point of return to the baseline.
• 5-15 ms
Phases • portion of the MUP between the departure & the
return to the baseline
• triphasic (positive, negative, positive)
• Polyphasic-MUP with more than four phase (5-
Amplitude • measured from maximum peak of negative phase
to maximum peak of the positive phase
• 0.5mV to 2mV
Rise time • duration from the initial positive to subsequent
• normal ↓ 500 µs
6. EMG Examination
I. Input Phase:
The apparatus used in routine electrodiagnosis
includes electrodes, amplifiers, display, loudspeaker
and data storage devices.
- Needle electrode
- Surface electrode
Most common types of electrodes are,
8. Monolpolar electrode : composed of a single fine
needle, insulated except at the tip. A second
surface electrode placed on the skin near the site
of insertion serves as the reference electrode.
These electrodes are less painful than concentric
electrodes because they are smaller in diameter.
Bipolar electrode: is a hypodermic needle, through
which a single wire of platinum or silver is
9. The cannula shaft and wire are insulated from each
other, and only their tips are exposed. The wire and the
needle cannula act as recording and reference
II. Processing phase:
Electrodes convert the bioelectric signal from muscle or
nerve depolarization into an electrical potential which is
processed by an amplifier.
10. a) Amplifiers: 10-5000Hz
The electrodes will pick-up both desired bioelectric
activity and interference signals.
It restricts the frequency domain of a signal.
To eliminate noise and bring out characteristics of
11. Common Mode Rejection Ratio: (CMR):
The amplifier amplifies differences that exist in the
signals from its two inputs, but rejects signals that
are common to both inputs, such as those caused
It is a measure of how much the desired signal is
amplified in relative to the unwanted signal.
12. Signal-to-noise ratio:
Noise also can be generated by the electric
components of an amplifier, including resistors,
transistors and ICs, which is manifested as a
hissing sound on an oscilloscope.
The ability of the amplifier to limit this noise
relative to the amplified signal is the signal-to noise
13. Gain and frequency response:
Ratio of the output voltage to the input voltage is
gain ( gain over the range of frequencies of the
This is achieved by using a filter which selectively
restricts the particular frequency domain of a
The frequency response is the range of
frequencies between the low and high cut-off
16. Factors that effect MUAP
- Type of needle electrode
- Characteristics of recording surface
- Electrical characteristics of cable
- Preamplifire & Amplifire
- Method of recording
- Age of patient
- Muscle examined
17. Phases of EMG Process
a) Insertional activity
b) Spontaneous activity:
1. Fibrillation potential
2. Positive sharp wave
4. Complex Repetitive Discharge
5. Myotonic Discharge
a) Minimal voluntary activity/Motor unit action
b) Maximal voluntary activity/ Recruitment and
18. a) Insertional Activity:
- Needle electrode is placed in the muscle and electrical activity
associated with its insertion is evaluated
This normally lasts less than 300 milliseconds (msec).
Insertional activity can be described as normal, reduced, absent,
increased, or prolonged.
Burst of high frequency positive and negative spikes occurring
during the movement of needle electrode.
It occurs due to stimulation of muscle fibre due to mechanical
irritation/ injury by the penetrating needle
19. The level of response depends on magnitude and speed
of needle movement.
- Though its normal response exaggeration/attenuation of
this response may suggest pathology. Like: Muscle
necrosis , neuropathy and myopathies
21. A normal relaxed muscle will exhibit electrical silence,
which is the absence of electrical potentials.
Observation of silence in the relaxed state is an
important part of the EMG examination. Potentials
arising spontaneously during this period are significant
After observing the muscle at rest, the patient is asked
to contract the muscle minimally.
22. This weak voluntary effort should cause individual
motor units to fire.
These motor unit potentials are examined with respect
to amplitude, duration, shape, sound, and frequency.
Finally, the patient is asked to increase levels of
contraction progressively to a strong effort, allowing
determination of recruitment patterns.
23. Gradually increasing the force of contraction will allow
the electromyographer to observe the pattern of
recruitment in the muscle.
With greater effort, increasing numbers of motor units
fire at higher frequencies, until the individual potentials
are summated and can no longer be recognized, and an
interference pattern is seen. This is the normal finding
with a strong contraction.
24. A. During single motor unit potential during
B. During maximal contraction
C. Interference pattern
25. Spontaneous Activity
Normal spontaneous activities originated from the NMJ or
(1) End plate Noise
(2) End plate potential
- Once insertional activity decays after a second or so, in normal
individual, there is no spontaneous electrical activity.
- In the end plate zone, miniature endplate potentials are
26. END PLATE POTENTIAL:
- On needle recording, end plate potential appears as
monophasic negative waves < 100microvolt & 1-3 ms
- Usually seen with an irregular baseline and called END
PLATE NOISE (Sea shell sound)
- In end plate region, Action potential which are brief,
spiky and irregular with an initial negative deflection
are known as End plate Spike.
27. End plate spike are due to mechanical activation of
nerve terminal by needle.
To avoid normally occuring spontaneous end plate
activities, the needle should be introduced slightly away
from the muscle end plate which is usually situated near
the center of muscle belly.
29. Fibrillation potentials are believed to arise from
spontaneous depolarization of a single muscle fiber.
- Their sound is a high-pitched click, which has been
likened to rain falling on a roof or wrinkling tissue
- Fibrillations fires regularly at a rate of 0.5 to 15 hz,
with an amplitude of 20-200 microvolt and duration of
1-5 ms when recorded by concentric needle.
- Rarely be regular.
- These are biphasic or triphasic waves with initial
positive, which is imp differential features from end
31. Positive sharp waves are longer duration biphasic
potentials with initial sharp positive waves followed by
a long duration negative phase resulting in a saw tooth
Appears regularly with an amplitude of 20-200
microvolt and duration of 10-30 ms.
Sound like dull pop.
Positive sharp waves are the action potential resulting
from injured muscle fiber.
32. Both fibrillation potential and positive sharp waves are
recognized by their characteristic sound on the
loudspeaker.(clicking sound of clock)
Fibrillations and sharp waves aare found in various
lower motor neuron disease such as AHC disorders,
radiculopathy, plexopathy, axonal type of neuropathies,
muscle disorders such as myositis, muscular dystrophy,
muscle trauma and relative NMJ, Myasthenia gravis.
33. Fibrillation potential and sharp waves are graged as
+1- Persistent single trains of potential in at least two
+2- Moderate number of potential in three or more area.
+3- Many fibrillations and sharp waves in all area
+4- Full interfernce pattern of fibrillations and sharp
34. The waves are typically biphasic, with a sharp initial
positive deflection (below baseline) followed by a slow
35. The negative phase is of much lower amplitude than the
positive phase, and of much longer duration, sometimes
up to 100 msec.
The sound has been described as a dull thud.
Both Fibrillations and positive sharp waves are
recognized by their characteristic sound on the
loudspeaker - clicking sound of clock
36. Myotonic Discharge: The action potentials of
muscle fibers firing for a prolonged period after external
excitation are the characteristics of myotonic discharge.
Increase and decrease in amplitude & frequency in a
waxing and waning fashion are found because of muscle
Myotonic discharge are rhythmic with frequency ranging
between 20 Hz to 150 Hz.
Variation in frequency and amplitude gives sound like a
37. Depending on the location of needle electrode to the
muscle fiber, there are two types of these potentials: (1)
Positive waves (2) Brief spikes.
Positive waves are similar to the runs of positive sharp
waves and are attribute to the muscle fibers.
Brief spikes are biphasic or triphasic 20-300 microvolt
and resemble the fibrillation potential.
40. Complex Repetitive Discharge (CRD) : It
refers to repetitive and synchronous firing of a group of
muscle fibers spontaneously or following needle
Amplitude of potential range from 50 microvolt to 1
macrovolt. Duration 50 ms to 100 ms.
The entire sequence repeats at slow or fast rate ranging
between 5 Hz and 100 Hz.
The unique repetitive pattern bears a superficial
resemblance to myotonic discharge and these are also
called pseudomyotonic discharge.
42. CRD do not have waxing and waning pattern, instead
of these appear and disappear abruptly and this feature
helps in differentiate it from myotonic discharge.
CRD are identical in morphology from one discharge to
another giving a machine –like sound on EMG.
- Myogenic: Polymyositis, Muscular dystrophies,
Schwartz jample syndrome
- Neurogenic: Polymyositis, ALS, Spinal muscular
atrophy, Chronic radiculopathy and neuropathy
43. Abnormal spontaneous activity originating from
Motor neuron or Axon
Fasciculation: Is a spontaneous contraction of a
number of muscle fibers belonging to whole or a part of
It occurs randomly and irregularly at variable rates
ranging between 0.1 Hz and 10 Hz.
The size and shape of fasciculations depends upon the
motor unit from which they arise and to the distance of
recording electrode to the motor unit.
44. Fasciculations may have the appearance of normal and
Fasciculation can occur in normal individual also as
well as in a wide variety of disease such as ALS, Root
compression, Axonal neuropathy and metabolic
The differentiation between bening and pathological
fasciculation is difficult.
45. Bening: are not companied by muscle weakness,
muscle wasting and reflex changes. These are of normal
MUAP , fire at faster rate and same site repetitively.
During voluntary contraction, muscle twitching may b
seen in patient with chronic neurogenic lesion
47. Doublets, Triplets & Multiplets: MUAP that fire in
groupof two, three or multiple potentials are reffered as
doublets, triplets and multiplets.
It is because spontaneous depolarization of motor unit or
its axons similar to fasciculations.
These are seen in hyperventilation, tetany, MNDand other
Ischemia can also induce these abnormality.
49. Myokymic Discharge: Are spontaneous bursting,
repetitive discharge of the same MUAP, which is
associated with fine, worm like quivering of muscles.
On EMG,these discharges reveal normal MUAPs,
which fire in fixed pattern and rhythm, occuring in
bursts of 2 to 10 potential firing at 40-60 Hz.
The burts reoccur regular intervals of 0.1-10 s creating
a marching round.
Mainly seen in demylinating neuropathy.
51. C) Minimal voluntary Activity:
Motor unit potential:
The motor unit consists of a group of muscle fibers
innervated by a single anterior horn cell
Represents the sum of muscle action potentials
supplied by an AHC
Higher amplitude and longer duration than action
potential produced by a single muscle fiber
They are characterized by duration, phases,
amplitude, turns and rate of rise.
53. A. Normal Motor unit action potential
B. Long duration polyphasic potential (appears twice)
C. Shirt duration (Low amplitude, polyphasic potential)
54. Abnormal MUAP:
1) Short Duration/Low amplitude/Recruitment
increased at minimal effort
- Myopathies, inflammatory disorders of
muscles and NMJ disorders, Early stage of
reinnervation after severe PNI
2) Long Duration/ High amplitude/ Recruitment
- MND, Axonal Neuropathies, radiculopathies,
chronic myositis (Polymyositis)
55. c) Polyphasic: >= 4 phase:
- Myopathies, regeneration of axons
D) Mixed pattern / mixed variety of MUAPs:
- Myopathies and neuropathies
E) Double/triplets/ amplitude vary / fired two or more
times at one interval of 10 to 30 sec:
- Tetany, MND and Metabolic disease
56. d) Maximal Voluntary Contraction:
The change in electrical activity is assessed as the
level of muscle contraction gradually increases and
reaches a maximum
Recruitment and interference pattern are evaluated
- The firing rate of MUAPS for a muscle is constant
- During a voluntary contraction, the motor units are
recruited in an orderly fashion; the smallest appearing
first, larger later and the largest later
57. A reduced or increased pattern indicates a fewer or
greater no. of discharging units than expected
In the early stage of neuropraxia, reduced
recruitment pattern is evident
In myopathies, more motor units are recruited at
relatively low force which is known as rapid
recruitment, where the firing rate remains same
58. Interference pattern:
With stronger contraction, many motor units begin
to fire very rapidly.
The discharge of many action potentials results in
pattern which unable to differentiate of the
individual muscle fibre potentials, hence the name
The spike density and the average amplitude of
the summated response are determined.
60. Abnormal Interference pattern:
- In disorders of the motor neuron, root or peripheral
nerve with reduced no. of excitable motor units,
recruitment and interference pattern are limited.
- LMN disorders present with a rapid rate of
- LMN and UMN both shows reduced interference