This document provides an overview of electromyography (EMG) techniques and normal EMG findings. It describes how EMG is used to study electrical activity in muscles to aid in neurological examination. It explains the motor unit, action potential generation, different electrode types, equipment, procedures, and normal EMG findings like insertional activity, end plate noise and spikes, fibrillation and fasciculation potentials, and motor unit action potentials. Precautions for the procedure and factors that can influence EMG readings are also summarized.

1. Technique of Electromyography
and
Normal EMG

2. Electromyography
•Needle electromyography refers to methods of
studying the electrical activity of muscle
•It serves as additional aid to neurological examination
in clinical practice for identifying and characterising
disorders of motor unit, including anterior horn cells,
peripheral nerves, NM junctions and muscles
•It requires Knowledge and skill

3. Motor unit
•A single motor neuron with all
the muscle fibres innervated by it
constitutes a motor unit
•It is the functional unit of Motor
component of peripheral nervous
system
•There are 2 different types of
motor units –slow firing fatigue
resistant and fast firing fatigable
units

4. Action potential generation in Skeletal muscle fibre

5. •Once the Action potential is generated at the End Plate, a
depolarising wave travels all along the length of muscle fibre
•Small segment of the muscle fibre act as dipole with
positive charge at the advancing end negative charge at the
other end resulting an electrical field through the
surrounding connective tissue
•The strength of the current decreases in proportion to the
square of the distance from the dipole in the skeletal muscle
fibre.

6. •When two electrodes are placed apart in interstitium, due to
difference in the distance from the dipole the field strength
varies at the two electrodes there by generating a potential
difference b/w them
Equipment
•Electrodes
•Amplifier
•Filter
•Display method

7. Electrodes
For clinical Electromyography following Needle electrodes are used
Needle electrode
Concentric Bipolar
Monopolar
Coaxial Bipolar
Single Fibre Macro electrodeBipolar

8. Types of Electrodes

9. Concentric Bipolar Needle
•This electrode consists of a cannula with 2 wires within it
•It records the potential difference b/w the two wires, as
one acts as active and other as reference
•It records from a very localised area, activity from only few
muscle fibres is picked up
•Amplitudes of MUPs are reduced due to reduced area

10. Monopolar electrode
•Electrode inserted into muscle acts as active
electrode, reference electrode is placed over the
surface
•Due to wide separation of the electrodes the
resolution of the low amplitude signals is better,
how ever the noise also gets amplified

11. Single fibre EMG needle
•It has smaller leading edge to record from single muscle
fibre rather than motor unit
•Like concentric bipolar needle it has a cannula with a wire
inside it but the wire is bent towards the side of the
cannula few mm behind the tip
Macro electrode
•It is suited for recording both from the single fibre and
motor unit

12. Amplifiers
•Bioelectrical potentials recorded will be in the range of 1μV to
1mV these signals need to be amplified by 1million to
thousand times for deflection of 1cm in 1v/cm recording
•Differential amplifiers increases the amplitude of the desired
response while rejecting unwanted noise
•Amplifiers ability to reject common signals is known
as its common mode rejection ratio (CMRR). The higher
the CMRR, the better the rejection

13. Gain
•Amplifier gain describes the extent to which the input signal
is increased in voltage.
Display sensitivity
•Describes the visible waveform and is expressed as volts per
division or volts per centimeter
•Usually kept at 50-200μV/cm
Filters
•They are used to selectively attenuate the noise preserving
the signal
•Band pass filters extending from 10HZ to 10KHZ is
commonly used

14. Display
•Once the wave form is recorded and processed it is
displayed for visual analysis
•As the EMG potentials have distinct auditory
characteristics presenting them as audible sounds also
helps in differentiating various responses

15. Preparing the patient
•Prior to the test Patient should be briefly explained about
the procedure and insertion of needle would cause some
discomfort
•Wipe the skin over the each puncture site with spirit before
needle is inserted
•Though most patients tolerate the pain some may require
oral analgesic
Selecting the muscle
•It is done on the basis of clinical indication
•Ideally muscle selected should be superficial, easily
palpated, Located away from major blood vessels and nerve
trunks

16. Abductor pollices brevis
Needle insertion:at mid point of
1st metacarpel
Abductor digiti minimi
Needle insertion at mid
point of 5th metacarpel

17. First dorsal interossei
Needle insertion: mid point
b/w 1st and 2nd MCP joints
Biceps Brachii
Needle insertion: Just
proximal to mid point b/w
biceps tendon and anterior
part of shoulder

18. Vastus lateralis
Needle insertion: lateral thigh 4 finger
breadths proximal to lateral part of knee
Tibialis anterior
Needle insertion: just lateral to
tibial shin at junction of proximal
1/3rd and distal 2/3rd of tibia

19. Needle insertion
•Prior to needle insertion the muscle should be palpated during
intermittent contraction to localise its borders
•Skin over the puncture site is made taut and needle is inserted
smoothly into superficial layers of the muscle
•When testing the small muscles needle should be inserted
obliquely to increase the needles path
Needle movement
•Needle is moved along a straight line in to the muscle in short steps
of 0.5-1mm as large movements are more painful
•Needle is advanced in 5-30 such steps with brief pause b/w each
step

20. •Once the diameter of the muscle is traversed needle is
withdrawn till subcutaneous plain and reinserted from a
different angle at same location
•All the 4 quadrants should be sampled for achieving good
recording
Precautions
•For patient with bleeding disorders or those on anticoagulants
INR should be <2
Platelet count >20,000
•Caution should be taken in patients with skin infection, cellulitis
•Patient s with prosthetic heart valves may have risk of infective
endocarditis

21. Findings in Normal EMG
Insertional activity
•Burst of high frequency positive or negative spikes
occurring during the movement of the needle electrode
•It occurs due to stimulation of muscle fibres due to
mechanical irritation/injury by the penetrating needle
•The level of response depends on magnitude and speed
of needle movement
•It lasts for about few hundred milliseconds
•Though it is a normal response exaggeration/attenuation
of this response may suggest pathology

22. End plate noise
•It is frequent irregular low amplitude (10-50μv )negative
waveform with duration of 1-2ms
•It correspond to miniature end plate potential
•It occurs with the release of acetylcholin due to irritation of
intramuscular nerve terminals by the needle tip at the end
plate region
•Sounds like seashell held to the ear
•Following botulinum inj analysis of end plate noise helps to
evaluate the neuromuscular transmission

23. End plate spike
•It is irregular high amplitude(100-200μv)negative
waveform with duration of 3-4ms
•It occurs due to stimulation of the single muscle fibre by
the tip of the needle at the end plate
Small irregular positive discharges may also occur at the
end plate particularly with concentric needles, these are
considered to be normal

24. Fibrillation potentials
•These are spontaneous action potentials that arise from single
muscle fibre.
•Oscillations in resting membrane potential triggering action
potential result in fibrillation
• Occasionally they may occur in normal healthy muscle
•They have regular firing pattern with freq 1-30Hz
•Amp litude:20-500μv, Duration of 1-5ms
•Pathologically significant when detected in at least three
separate sites within the muscle being examined.

25. FASCICULATION POTENTIALS
•They are similar to motor unit action Potentials occurs due to
spontaneous activation of the muscle fibres of individual motor
units.
•Stimulus can originate at any level from anterior horn cell to
axon terminal
•About 77% of normal individuals can have fasciculations
•Association with fibrillations, positive sharp waves suggest
pathological fasciculations
•Generally Benign fasciculations fire at higher frequency(1-2Hz)
than pathological fasciculations(<1Hz), however it is difficult to
differentiate benign from pathological

26. Morphology Frequency Amplitude Duration rhythm Auditory
component
End plate
noise
Monophasic
Negative
wave form
20-40Hz 10-50μv 1-2ms Irregular Sea shell held
to ear
End plate
spike
Biphasic with
initial Sharpe
negative
wave form
5-50Hz 100-200μv 3-4ms Irregular Cracking
sound
Positive
Sharpe wave
Biphasic with
initial
positive
followed by
slow
negative
0.5-30Hz 20-200μv 10-15ms Regular Dull pop
Fibrillation Bi/Triphasic
with initial
positive
1-30Hz
Avg 13 Hz
20-500μv 1-5ms Regular High pitched
,rain drop on
roof
Fasciculation Tri/poly
phasic
1-2Hz >300μv 15-20ms Irregular Low pitched
popping

27. Motor Unit Action Potential(MUAP)
The motor unit action potential is a compound potential
representing the sum of the individual action potentials
generated in the few muscle fibres of the unit that are within
the pick-up range of the recording electrode
Components:
•Amplitude
•Duration
•Rise time
•Phases
•Area

28. Amplitude:
•It is measured between the greatest positive and the
greatest negative deflections of the potentials.
•When recorded by a concentric needle electrode, it is
usually between 200 μV and 3 mV
Factors influencing the Amplitude of MUP
•Predominantly determined by the action potentials of
fibres that lie close to the recording electrode
•Slight movement of the electrode has significant effect on
amplitude
•Temporal dispersion of the individual action potentials
also affects to some extent

29. Rise time of MUP
•It is the time lag from the initial positive peak to the
subsequent negative peak of the MUP.
• It reflects the distance between the recording electrode and
the muscle fibres of the motor unit
•Rise time less than 500μs indicate appropriate position of
the electrode within the motor unit territory
Duration of MUP
Measured from the initial deflection from the base line to the
final return to the base line
•It indicate the synchrony among various fibres of a motor
unit
•It is influenced by fibres in the recording region that may
extend to about 2-2.5mm radius from the needle
•Normally varies from 5-15ms

30. Area of the MUP
•It depends on the number of muscle fibres with in
2mm radius of the recording electrode
•Movement of the electrode has significant effect on
area
•Ratio of amplitude to area is stable and less affected
by electrode movement
•Helps to differentiate neuropathy from myopathy

31. Phases of MUP
It is determined by counting the number of base line crossings
of MUP plus one
•It indicates the synchrony among the individual muscle fibres
of a motor unit
•Usually MUP has 2-4 phases, when >4 it is called polyphasic
•In normal limb muscles about 12 percent may have five or
more phases (polyphasic)

32. Satellite potential
•It is a late spike distinct from main potential, that is
time locked to the main potential
•Generated by muscle fibre in a motor unit that has
long nerve terminal, distant end plate region
•Latency can rage from 8-32ms
•It can occur in both pathologic and normal muscle

33. Pick up area within a motor unit territory

34. Motor unit potentials

35. Physiologic Factors Influencing MUPs
•Patient age,
Increasing age from infancy to adulthood there is an increase in
the mean duration of motor unit action potentials in limb
muscles
•Intramuscular temperature,
As temperature declines Mean duration of motor unit
potentials and the number of polyphasic potentials also
increase
• The site of the recording electrode within the muscle, and
Particular muscle under examination

36. MOTOR UNIT DISCHARGE PATTERN
Recruitment
•As the force of contraction increases, first the firing rate of the
particular active unit increases until it reaches a certain frequency
when additional units are recruited
•Normal ratio of firing frequency to the number of motor units is 5:1
•Maximum firing rate of a motor unit is about 30-50HZ
•The order of recruitment depends on
Diameter of neuron, synaptic density, Threshold of recruitment, type
of motor units
•It depends on patients effort, input from cortex, number of active
motor units available

37. Interference
•During the maximum contraction of muscle several motor
units get activated simultaneously resulting in the over lap
of MUPs creating an interference pattern
•It gives a simple measure of number of firing units at
maximum effort
•It is influenced by factors that alter recruitment

38. Mechanical artefacts during EMG
Electrode noise:
•It occurs due to generation of potentials by the
active metals of electrodes at junction of needle tip
and interstitial fluid
•This can distort the signal that is being recorded
•Using inert metals –stainless steel, platinum this can
be minimized

39. Electrostatic and Electromagnetic interference
This occurs from electric appliances- Fans, lamps due to
use of AC current
Interference occurs at 50-60Hz
It can be minimised by bundling wires of the recording
and ground electrode, relocating the wires of
appliances, shielding the recording area
Mobile phone use at the recording area also produces
artefacts that resemble complex repetitive discharges

40. Conclusion
•EMG is an important additional aid in evaluating
patients with peripheral neuromuscular disorder
•It has great sensitivity and specificity
•Good recording of various waveforms require high
technical skills
•Knowledge about various normal and abnormal findings
is required for proper interpretation and application of
test results

41. Thank You