Transcript Electromyography (EMG) Instrumentation

Electromyography
(EMG)
Instrumentation
David Groh
University of Nevada – Las Vegas
Research Applications of
Surface EMG
Indicator for muscle activation/deactivation
Relationship of force/EMG signal
Use of EMG signal as a fatigue index
Types of EMG
Electrode Categories

Inserted
Fine-wire (Intra-muscular)
Needle

Surface
Fine-wire Electrodes
Advantages




Extremely sensitive
Record single muscle activity
Access to deep musculature
Little cross-talk concern
Disadvantages




Extremely sensitive
Requires medical personnel, certification
Repositioning nearly impossible
Detection area may not be representative of entire
muscle
Surface Electrodes
Advantages



Quick, easy to apply
No medical supervision, required certification
Minimal discomfort
Disadvantages





Generally used only for superficial muscles
Cross-talk concerns
No standard electrode placement
May affect movement patterns of subject
Limitations with recording dynamic muscle activity
Electrode Comparison Studies
Giroux & Lamontagne - Electromyogr. Clin.
Neurophysiol., 1990


Purpose: to compare EMG surface electrodes
and intramuscular wire electrodes for
isometric and dynamic contractions
Results
No significant difference in either isometric or
dynamic conditions
However: dynamic activity was not very
“dynamic”
EMG Manufacturers
Noraxon
Motion Lab Systems
Delsys
General Concerns
Signal-to-noise ratio

Ratio of energy of EMG signal divided by
energy of noise signal
Distortion of the signal

EMG signal should be altered as minimally as
possible for accurate representation
Characteristics of EMG Signal
Amplitude range: 0–
10 mV (+5 to -5) prior
to amplification
Useable energy:
Range of 0 - 500 Hz
Dominant energy: 50
– 150 Hz
Characteristics of Electrical
Noise
Inherent noise in electronics equipment
Ambient noise
Motion artifact
Inherent instability of signal
Inherent Noise in Electronics
Equipment
Generated by all electronics equipment
Frequency range: 0 – several thousand Hz
Cannot be eliminated
Reduced by using high quality
components
Ambient Noise
Electromagnetic radiation sources



Radio transmission
Electrical wires
Fluorescent lights
Essentially impossible to avoid
Dominant frequency: 60 Hz
Amplitude: 1 – 3x EMG signal
Motion Artifact
Two main sources


Electrode/skin interface
Electrode cable
Reducible by proper circuitry and set-up
Frequency range: 0 – 20 Hz
Inherent Instability of Signal
Amplitude is somewhat random in nature
Frequency range of 0 – 20 Hz is especially
unstable
Therefore, removal of this range is
recommended
Factors Affecting the EMG
Signal
Carlo De Luca

Causative Factors – direct affect on signal
Extrinsic – electrode structure and placement
Intrinsic – physiological, anatomical, biochemical


Intermediate Factors – physical &
physiological phenomena influenced by one
or more causative factors
Deterministic Factors – influenced by
intermediate factors
Factors Affecting the EMG
Signal
Maximizing Quality of EMG
Signal
Signal-to-noise ratio


Highest amount of information from EMG signal as
possible
Minimum amount of noise contamination
As minimal distortion of EMG signal as possible



No unnecessary filtering
No distortion of signal peaks
No notch filters recommended
Ex: 60 Hz
Solutions for Signal Interruption
Related to Electrode and Amplifier
Design
Differential amplification


Reduces electromagnetic radiation noise
Dual electrodes
Electrode stability


Time for chemical reaction to stabilize
Important factors: electrode movement, perspiration,
humidity changes
Improved quality of electrodes

Less need for skin abrasion, hair removal
Differential Amplification
Ambient
(electromagnetic)
noise is constant
System subtracts two
signals
Resultant difference
is amplified
Double differential
technique
Electrode Configuration
Length of electrodes



# of included fibers vs. increased noise***
Delsys – 1 cm
Noraxon - ?
Distance between electrodes




Increased amplitude vs. misaligning electrodes,
Multiple motor unit action potentials (MUAP)
Muscle fibers of motor units are distributed evenly,
thus large muscle coverage is not necessary
(De Luca).
Delsys – 1 cm
Noraxon – 2 cm?
Electrode Placement
Away from motor point


MUAP traveling in opposite directions
Simultaneous (+) & (-) AP’s
Resultant increased frequency components
More jagged signal

Middle of muscle belly is generally accepted
Electrode Placement
Away from tendon


Fewer, thinner muscle fibers
Closer to other muscle origins, insertions
More susceptible to cross-talk
Away from outer edge of muscle

Closer to other musculature
Orientation parallel to muscle fibers


More accurate conduction velocity
Increased probability of detecting same signal
EMG Electrode Placement
Surface Electrode Placement
Reference Electrode Placement
(Ground)
As far away as possible from recording
electrodes
Electrically neutral tissue

Bony prominence
Good electrical contact


Larger size
Good adhesive properties
Off to the Lab!
References
Basmajian JV, De Luca CJ. Muscles Alive: their functions
revealed by electromyography (fifth ed.). Williams &
Wilkins, Baltimore, Maryland, 1985
Cram JR, Kasman GS. Introduction to surface
electromyography. Aspen Publishers, Inc. Gaithersburg,
Maryland, 1998
De Luca CJ: Surface electromyography: detection and
recording. DelSys, Inc., 2002
De Luca CJ: The use of surface electromyography in
biomechanics. J App Biomech 13: 135-163, 1997
MyoResearch: software for the EMG professional.
Scottsdale, Arizona, Noraxon USA, 1996-1999