ElectromyographyElectromyography (EMG) (EMG)

InstrumentationInstrumentationDavid GrohDavid Groh

University of Nevada – Las VegasUniversity of Nevada – Las Vegas

Research Applications of Research Applications of Surface EMGSurface EMG

Indicator for muscle activation/deactivationIndicator for muscle activation/deactivation

Relationship of force/EMG signalRelationship of force/EMG signal

Use of EMG signal as a fatigue indexUse of EMG signal as a fatigue index

Types of EMGTypes of EMG

Electrode CategoriesElectrode Categories InsertedInserted

Fine-wire (Intra-muscular)Fine-wire (Intra-muscular)

NeedleNeedle SurfaceSurface

Fine-wire ElectrodesFine-wire Electrodes

AdvantagesAdvantages Extremely sensitiveExtremely sensitive Record single muscle activityRecord single muscle activity Access to deep musculatureAccess to deep musculature Little cross-talk concernLittle cross-talk concern

DisadvantagesDisadvantages Extremely sensitiveExtremely sensitive Requires medical personnel, certificationRequires medical personnel, certification Repositioning nearly impossibleRepositioning nearly impossible Detection area may not be representative of entire Detection area may not be representative of entire

musclemuscle

Surface ElectrodesSurface Electrodes

AdvantagesAdvantages Quick, easy to applyQuick, easy to apply No medical supervision, required certification No medical supervision, required certification Minimal discomfortMinimal discomfort

DisadvantagesDisadvantages Generally used only for superficial musclesGenerally used only for superficial muscles Cross-talk concernsCross-talk concerns No standard electrode placementNo standard electrode placement May affect movement patterns of subjectMay affect movement patterns of subject Limitations with recording dynamic muscle activityLimitations with recording dynamic muscle activity

Electrode Comparison StudiesElectrode Comparison Studies

Giroux & Lamontagne - Giroux & Lamontagne - Electromyogr. Clin. Electromyogr. Clin. Neurophysiol., 1990Neurophysiol., 1990 Purpose: to compare EMG surface electrodes Purpose: to compare EMG surface electrodes

and intramuscular wire electrodes for and intramuscular wire electrodes for isometric and dynamic contractionsisometric and dynamic contractions

Results Results No significant difference in either isometric or No significant difference in either isometric or dynamic conditionsdynamic conditions

However: However: dynamic activity was not very dynamic activity was not very “dynamic”“dynamic”

EMG ManufacturersEMG Manufacturers

NoraxonNoraxon

Motion Lab SystemsMotion Lab Systems

DelsysDelsys

General ConcernsGeneral Concerns

Signal-to-noise ratioSignal-to-noise ratio Ratio of energy of EMG signal divided by Ratio of energy of EMG signal divided by

energy of noise signalenergy of noise signal

Distortion of the signalDistortion of the signal EMG signal should be altered as minimally as EMG signal should be altered as minimally as

possible for accurate representationpossible for accurate representation

Characteristics of EMG SignalCharacteristics of EMG Signal

Amplitude range: 0–Amplitude range: 0–10 mV (+5 to -5) prior 10 mV (+5 to -5) prior to amplificationto amplification

Useable energy: Useable energy: Range of 0 - 500 Hz Range of 0 - 500 Hz

Dominant energy: 50 Dominant energy: 50 – 150 Hz– 150 Hz

Characteristics of Electrical Characteristics of Electrical NoiseNoise

Inherent noise in electronics equipmentInherent noise in electronics equipment

Ambient noiseAmbient noise

Motion artifactMotion artifact

Inherent instability of signalInherent instability of signal

Inherent Noise in Electronics Inherent Noise in Electronics EquipmentEquipment

Generated by all electronics equipmentGenerated by all electronics equipment

Frequency range: 0 – several thousand HzFrequency range: 0 – several thousand Hz

Cannot be eliminatedCannot be eliminated

Reduced by using high quality Reduced by using high quality componentscomponents

Ambient NoiseAmbient Noise

Electromagnetic radiation sourcesElectromagnetic radiation sources Radio transmissionRadio transmission Electrical wiresElectrical wires Fluorescent lightsFluorescent lights

Essentially impossible to avoidEssentially impossible to avoid

Dominant frequency: 60 HzDominant frequency: 60 Hz

Amplitude: 1 – 3x EMG signalAmplitude: 1 – 3x EMG signal

Motion ArtifactMotion Artifact

Two main sourcesTwo main sources Electrode/skin interfaceElectrode/skin interface Electrode cableElectrode cable

Reducible by proper circuitry and set-upReducible by proper circuitry and set-up

Frequency range: 0 – 20 HzFrequency range: 0 – 20 Hz

Inherent Instability of SignalInherent Instability of Signal

Amplitude is somewhat random in natureAmplitude is somewhat random in nature

Frequency range of 0 – 20 Hz is especially Frequency range of 0 – 20 Hz is especially unstableunstable

Therefore, removal of this range is Therefore, removal of this range is recommendedrecommended

Factors Affecting the EMG Factors Affecting the EMG SignalSignal

Carlo De LucaCarlo De Luca Causative Factors – direct affect on signalCausative Factors – direct affect on signal

Extrinsic – electrode structure and placementExtrinsic – electrode structure and placement

Intrinsic – physiological, anatomical, biochemicalIntrinsic – physiological, anatomical, biochemical Intermediate Factors – physical & Intermediate Factors – physical &

physiological phenomena influenced by one physiological phenomena influenced by one or more causative factorsor more causative factors

Deterministic Factors – influenced by Deterministic Factors – influenced by intermediate factorsintermediate factors

Factors Affecting the EMG Factors Affecting the EMG SignalSignal

Maximizing Quality of EMG Maximizing Quality of EMG SignalSignal

Signal-to-noise ratioSignal-to-noise ratio Highest amount of information from EMG signal as Highest amount of information from EMG signal as

possiblepossible Minimum amount of noise contaminationMinimum amount of noise contamination

As minimal distortion of EMG signal as possibleAs minimal distortion of EMG signal as possible No unnecessary filteringNo unnecessary filtering No distortion of signal peaksNo distortion of signal peaks No notch filters recommendedNo notch filters recommended

Ex: 60 HzEx: 60 Hz

Solutions for Signal Interruption Solutions for Signal Interruption Related to Electrode and Amplifier Related to Electrode and Amplifier

DesignDesign

Differential amplificationDifferential amplification Reduces electromagnetic radiation noiseReduces electromagnetic radiation noise Dual electrodesDual electrodes

Electrode stabilityElectrode stability Time for chemical reaction to stabilizeTime for chemical reaction to stabilize Important factors: electrode movement, perspiration, Important factors: electrode movement, perspiration,

humidity changeshumidity changes

Improved quality of electrodesImproved quality of electrodes Less need for skin abrasion, hair removalLess need for skin abrasion, hair removal

Differential AmplificationDifferential Amplification

Ambient Ambient (electromagnetic) (electromagnetic) noise is constantnoise is constant

System subtracts two System subtracts two signalssignals

Resultant difference Resultant difference is amplifiedis amplified

Double differential Double differential techniquetechnique

Electrode ConfigurationElectrode Configuration

Length of electrodesLength of electrodes # of included fibers vs. increased noise***# of included fibers vs. increased noise*** Delsys – 1 cmDelsys – 1 cm Noraxon - ?Noraxon - ?

Distance between electrodesDistance between electrodes Increased amplitude vs. misaligning electrodes, Increased amplitude vs. misaligning electrodes,

Multiple motor unit action potentials (MUAP) Multiple motor unit action potentials (MUAP) Muscle fibers of motor units are distributed evenly, Muscle fibers of motor units are distributed evenly,

thus large muscle coverage is not necessary thus large muscle coverage is not necessary (De Luca).(De Luca).

Delsys – 1 cmDelsys – 1 cm Noraxon – 2 cm?Noraxon – 2 cm?

Electrode PlacementElectrode Placement

Away from motor pointAway from motor point MUAP traveling in opposite directionsMUAP traveling in opposite directions Simultaneous (+) & (-) AP’s Simultaneous (+) & (-) AP’s

Resultant increased frequency componentsResultant increased frequency components

More jagged signalMore jagged signal Middle of muscle belly is generally acceptedMiddle of muscle belly is generally accepted

Electrode PlacementElectrode Placement

Away from tendonAway from tendon Fewer, thinner muscle fibersFewer, thinner muscle fibers Closer to other muscle origins, insertionsCloser to other muscle origins, insertions

More susceptible to cross-talkMore susceptible to cross-talk

Away from outer edge of muscleAway from outer edge of muscle Closer to other musculatureCloser to other musculature

Orientation parallel to muscle fibersOrientation parallel to muscle fibers More accurate conduction velocityMore accurate conduction velocity Increased probability of detecting same signalIncreased probability of detecting same signal

EMG Electrode PlacementEMG Electrode Placement

Surface Electrode PlacementSurface Electrode Placement

Reference Electrode PlacementReference Electrode Placement(Ground)(Ground)

As far away as possible from recording As far away as possible from recording electrodeselectrodes

Electrically neutral tissueElectrically neutral tissue Bony prominenceBony prominence

Good electrical contactGood electrical contact Larger sizeLarger size Good adhesive propertiesGood adhesive properties

Off to the Lab!Off to the Lab!

ReferencesReferences

Basmajian JV, De Luca CJ. Muscles Alive: their Basmajian JV, De Luca CJ. Muscles Alive: their functions revealed by electromyography (functions revealed by electromyography (fifth edfifth ed.). .). Williams & Wilkins, Baltimore, Maryland, 1985Williams & Wilkins, Baltimore, Maryland, 1985Cram JR, Kasman GS. Introduction to surface Cram JR, Kasman GS. Introduction to surface electromyography. Aspen Publishers, Inc. Gaithersburg, electromyography. Aspen Publishers, Inc. Gaithersburg, Maryland, 1998Maryland, 1998De Luca CJ: Surface electromyography: detection and De Luca CJ: Surface electromyography: detection and recording. DelSys, Inc., 2002recording. DelSys, Inc., 2002De Luca CJ: The use of surface electromyography in De Luca CJ: The use of surface electromyography in biomechanics. J App Biomech 13: 135-163, 1997 biomechanics. J App Biomech 13: 135-163, 1997 MyoResearch: software for the EMG professional. MyoResearch: software for the EMG professional. Scottsdale, Arizona, Noraxon USA, 1996-1999 Scottsdale, Arizona, Noraxon USA, 1996-1999