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In the name of God
Electromyography
(EMG)
By:sahar shahbazi
What is EMG?
EMG stands for electromyography.
It is the study of muscle electrical
signals. EMG is sometimes referred
to as myoelectric activity. Muscle
tissue conducts electrical potentials
similar to the way nerves do and the
name given to these electrical
signals is the muscle action
potential.
What is motor unit?
o The smallest subunit that can be controlled
is called motor subunit because it is
separately innervated by a motor axon.
o The number of the muscle fibers that
are under control of 1 motor unit varies
from 3 to 20,000 depending on the
fineness of the control required.
o An EMG signal is the train of Motor Unit
Action Potential (MUAP) showing the muscle
response to neural stimulation.
Samples of EMG signal
o In response to a stimulus from the neuron, a
muscle fiber depolarizes as the signal
propagates along its surface and the fiber
twitches. This depolarization, accompanied
by a movement of ions, generates an
electric field near each muscle fiber.
Motor unit
EMG Applications
o Indicator for muscle
activation/deactivation
o Relationship of force/EMG signal
o Use of EMG signal as a fatigue index
o Chronic pain study
Signal processing and
amplification
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1. rectification
2. Filtering and linear envelope detection
3. Integration
4. Amplification :
Amplifier impedance
common mode rejection
Outcome measures:
 horizontal axis (time)
 timing (percent
on/off; on/off ratio)
duration (sec;
percent of cycle)
coactivation (percent
of cycle)
Area under the curve
(related to force)
 vertical axis (voltage)
 rate of rise of voltage
peak amplitude
 Ellaway's cumulative sum histogram
technique:
Where Si is the cumulative sum up to sample i, X is the
mean voltage over the trial, and xi is the voltage at
sample
The measurement of EMG
 When the muscle is activated, the length of
the muscle decreases and the muscle, skin
and electrodes will be moving with respect to
each other. The electrode signal will therefore
probably show some movement artifacts.
 This is not the only difficulty when measuring
an EMG. When the muscle is moving, the
body will show some movements as well, and
the cable may move in the space between the
electrode and the input of the amplifier. This
space is often filled with an electro-magnetic
field caused by the mains cables in the area.
So we can expect more disturbances. The
moving cable will show a movement artifact
as well and the influence of the mains will
cause a 50 or 60 Hz mains interference in the
signal.
50 Hz noise in the signal
electrode is moving with
respect to the skin.
Artifact when the cable is
moving.
 To get rid of mentioned noise, several
measures can be taken.
 1: Movement artifacts that are generated by
the movement of the electrode with respect
to the skin:
 High pass filtering
 TMS-I
 2: Mains interference:
 TMS-I
 In many EMG systems, the input
amplifier is put on the electrode. This is
often called an active electrode
Types of EMG
o Inserted
• Fine-wire (Intra-muscular)
• Needle
o Surface
• Active
• passive
Fine-wire Electrodes
o The most important
difference of
inserted and surface
electrodes is in their
band width.
o Inserted one has
higher frequency
o Bw:2_1000Hz(surfac
e electrode’s
BW:10_600Hz)
Fine-wire Electrodes
o Advantages
• Extremely sensitive
• Record single muscle activity
• Access to deep musculature
• Little cross-talk concern
o Disadvantages
• Extremely sensitive
• Requires medical personnel
• Repositioning nearly impossible
• Detection area may not be representative of
entire muscle
Surface Electrodes
o
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o
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Active:
with amplifier
Reduce skin impedance( no need to gel)
Reduce motion artifact
Increase s/n
Passive:
Without amplifier
Need to gel & skin preparing
Decrease s/n
Surface Electrodes
o Advantages
• Quick, easy to apply
• No medical supervision
• Minimal discomfort
o Disadvantages
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Generally used only for superficial muscles
Cross-talk concerns
No standard electrode placement
May affect movement patterns of subject
Factors affecting the surface
EMG
o It’s a random signal and can’t be
represented mathematically.
o Many factors affect the observed surface
EMG like:
• different days even for same person
• different people; depending upon individual
anatomy, thickness of fat tissue ,cross talk…
• Presence of gland & blood vessels.
• Characteristic of recording equipment
• Position of electrodes.
Observed difference from
different positions
Surface electrodes
Needle electrodes
Characteristics of EMG Signal
o Amplitude range: 0–
10 mV (+5 to -5)
prior to
amplification
o Useable energy:
Range of 0 - 500 Hz
o Dominant energy:
50 – 150 Hz
detecting
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o
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Raw EMG signal is important
Need filtering or not
It’s difficult to detect signal from noise
Sometimes we need LP,HP or BP filter
Motion artifact, low frequency
Sharp peaks , sudden movement
60 Hz, ambient noise
Data :
o Two main parameters that extracted:
• Beginning and finishing EMG signal (on & off)
(first data)
• Increase and decrease of muscle activity.
o
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What can not we conclude?
Strength of muscle
Compare of muscle strength
Voluntary or involuntary motion
Abnormal results
EMG is used to diagnose two general categories of
disease:neuropathies and myopathies
Neuropathic disease has the following defining EMG
characteristics
An increase in duration of the action potential
A decrease in the number of motor units in the muscle (as
found using motor units number estimation techniques)
Myopathic disease has these defining EMG characteristics:
A decrease in duration of the action potential
A reduction in the area to amplitude ratio of the action potential
Maximizing Quality of EMG
Signal
o Signal-to-noise ratio
• Highest amount of information from EMG
signal as possible
• Minimum amount of noise contamination
o As minimal distortion of EMG signal as
possible
• No unnecessary filtering
• No distortion of signal peaks
• No notch filters recommended
Electrode Configuration
o Length of electrodes
• About 1 cm
o Distance between electrodes
• Increased amplitude, will increase noise…
• 1 cm,2 cm
Electrode Placement
o
Away from motor point
Middle of muscle belly is generally
accepted
o Away from tendon
o Away from outer edge of muscle
EMG Electrode Placement
Surface Electrode Placement
Reference Electrode Placement
(Ground)
o As far away as possible from recording
electrodes
o Good electrical contact
• Larger size
• Good adhesive properties
Example of the hand and wrist control using the EMG signals: the
subject executed eight hand and wrist motions for about 30 s. The darkened areas
indicate no motions because the square sum of EMG (n), which was defined as
(1), was below the prespecified threshold. “SUS” in the discrimination results
indicates the suspended discrimination where E(n) exceeds the prespecified
threshold E .
The End