Electromyography (EMG)

Electromyography (EMG)Mark David S. Basco, PTRPFacultyDepartment of Physical TherapyCollege of Allied Medical ProfessionsUniversity of the Philippines ManilaObjectivesAt the end of the session, you should be able to:Discuss the principles of EMGDescribe the characteristics of normal EMGDescribe the characteristics of abnormal EMGDefinitionStudy of muscle function through the examination of electrical signalsElectro electricMyo muscleGraphy to graph / to measureWhy use EMG?In vivo examination of muscle activityQuantifies muscle activitiesClinical vs. Kinesiological History1791Luiggi Galvani, depolarized frog legs with metal rods1838Carlo Matteucci, proved electrical currents originated in mm1849Du Bois-Reymond, designed a galvanometer, reduced impedance by popping blistersHistory

Principles

Involves detecting, amplifying, and displaying electrical changes that occur during mm contraction

Electrical events are amplified electronically, visualized on a tv-like monitor or even transformed into soundTV like monitor: Oscilloscope6Principles

TV like monitor: Oscilloscope7How can we detect electrical signals?ElectrodesTransducer; device converting one form of energy to anotherDifferent typesSurface electrodesFine wire indwelling electrodesNeedle electrodesElectrodesSurface ElectrodesApplied on the surface of the skinMeasures signals from large muscles that lie close to the skinOften silver/silver chloride, coated with a suitable conducting gel, and can be taped on the skin

Refer to Sullivan for pictures10ElectrodesFine-wire Indwelling ElectrodesTwo strands of 100micro-meter wires inserted into the muscle bellyUsed for monitoring activity from deep mm, small mm, or narrow mmMay not be useful for large mm ElectrodesNeedle ElectrodesUsed to record motor unit potentials from different parts of the muscleFine needles with electrodes inserted into the muscleThe bare tip of the needle serves as the recording electrode ElectrodesRecording / Detection ElectrodeEither surface, needle, or fine-wireGround ElectrodeProvides a mechanism for cancelling out the interference effect from external noiseSurface electrodeAttached near the recording electrode

Electrodes

Electrodes

15Electrodes

16Myoelectric SignalVariables that may affect MUAPProximity of electrodes to the fibers that are firingNumber and size of fibers in the motor unitDistance between the fibersSize of the electrodesDistance between the electrodes

MUAP Motor Unit Action Potential17Myoelectric SignalCross talkElectrical overflowElectrical activity from nearby contracting muscles reaches the electrodesControlled by:Careful electrode placement, spacing, choice of size and type of electrode

MUAP Motor Unit Action Potential18Myoelectric SignalArtifactsUnwanted electrical activityArises outside of the tissues beinng examinedMay distort output signals markedlySourcesMovementPower lineECG

MUAP Motor Unit Action Potential19Myoelectric SignalMovement ArtifactSkinElectrodeContracting muscleElectrode cablesProduces high voltage, high frequency artifactsControlled by:Firm fixation of electrodes, firmly taping cables, skin preparation

Movement of skin, movement of electrode or pressure on electrodes, and movement of electrodes within contracting muscles usually produces artifacts below 20Hz causing minimal interference can easily be controlled.20Myoelectric SignalPower line InterferenceHuman body attracts electromagnetic energy from power linesLoose electrode attachments, broken or frayed electrodes, broken electrode wiresDiathermy, ES, cellphones, radios, vibrators

21Myoelectric SignalECGcan occur if electrodes are placed on the trunk, upper arm, or upper thighHow to reduce this artifact?Correct application of ground electrodeUse of amplifier with appropriate characteristics22AmplifierSignals (microvolts) recorded have to be amplified about a thousand times for them to be displayed on an oscilloscope and be heard through speakers or be recorded on a chart

AmplifierThe electrical potential recorded by the electrodes is composed of EMG signal from mm contraction and unwanted noise EMGs use differential amplifiersGround electrode and active electrode supply input to the amplifierDifference between the signals will be amplified and recordedIf two electrodes receive equal signals, no activity is recorded 24Amplifier

25Amplifier

26AmplifierCommon Mode Rejection RatioMeasure of how much the desired signal is amplified relative to the unwanted signalThe higher the value, the betterA good differential amplifier should have a CMRR exceeding 100,000:1AmplifierSignal-to-noise RatioReflects the ability of the amplifier to limit noise relative to amplified signalRatio of wanted to unwanted signalNoise generated fromInternal electrical componentsAmplifierGainRefers to amplifiers ability to amplify signalsRatio of output signal to input signal level

AmplifierInput ImpedanceResistive property observed in AC circuitsReduced by:Larger electrodesSkin preparation

NORMAL EMG Insertional activityCaused by the needle breaking through muscle fiber membranesAlso seen during needle repositioningNormally stops when the needle stops moving.May be described as NormalReducedProlongedReducedabsence; electrode not on excitable tissue / fibrotic mm tissue

Prolonged- Hyperexcitable mm / during denervation / mm inflammation32Electrical Silence(-) electrical activityIf needle is at the NMJ/motor end plate, very small spontaneous potentials may be observedAny other activity observed indicates pathology

Interference PatternNormally seen with strong muscular contractionIndividual potentials are summated with increasing number of motor units firing at higher frequencies

Abnormal EMGFibrillation PotentialsRepresents spontaneous, repetitive depolarization of a single mm fiberNOT visibleMay result fromDenervationMetabolic dysfunctionInflammatory diseases (polio)TraumaIndicative of LMN lesions

Fibrillation Potentials

Positive sharp wavesRepresents asynchronous discharge of a number of denervated mm fibersReflects an altered membrane excitabilityUsually accompanied by fibrillation potentials

Fasciculation PotentialRepresent spontaneous discharges from a group of mmOften visible; small mm twitchesSeen with irritation or degeneration of ant horn cell, chronic pni, nerve root compression, cramps or spasmsFasciculation PotentialMay be seen in NORMAL individualsPathological significance if seen with fibrillation and positive sharp waves

Myotonic / Complex Repetitive DischargeSeen with lesions of the anterior horn cell and peripheral nerves, myopathiesDive-bomber sound that occurs on needle insertion, voluntary contractionProbably triggered by movement of needle electrode within unstable mm fibers Myotonic / Complex Repetitive Discharge

Nerve Conduction Velocity (NCV)Mark David S. Basco, PTRPFacultyDepartment of Physical TherapyCollege of Allied Medical ProfessionsUniversity of the Philippines ManilaObjectivesAt the end of the session, you should be able to:Discuss the principles of NCVDescribe motor NCV testingDescribe sensory NCV testing

PrinciplesInvolves direct stimulation to initiate an impulse in motor or sensory nerves (evoked potential)The conduction time is measured to determine the presence or absence of a lesionIt provides data on the ability of the nerve to transmit impulses

Stimulating electrodeBipolar electrode with anode and cathodePulse duration: 0.1 msecFrequency: 1HzContraindicated in patients withIndwelling cardiac catheterCentral venous pressure line

Contains a trigger mechanism that corresponds to a sweep on the screenCathode stimulating electrode (black)Anode inactive electrode (red)46Stimulating electrode

47Stimulating electrode

48MOTOR NCV testingStimulation and RecordingEvoked potential recorded from a distal muscle innervated by the nerve under studySmall surface electrodes are used to record the evoked potential from musclesStimulation and Recording Recording electrodePlaced over the belly of test muscleReference electrodeOver the tendon of the mm; distal to the active electrodeGround electrodePlaced over a neutral area between the electrodes and the stimulation siteStimulation and Recording

Stimulation and Recording

Stimulation and Recording Stimulus starts low and is slowly increasedIntensity is increased until the evoked response no longer increases in sizeSupramaximal stimulusM-waveM-waveMAP (motor action potential) CMAP (compund motor action potential) Represents the summated activity of all motor units in the muscle that responded to the stimulation54Stimulation and Recording Conduction velocityDetermined by dividing the distance between the two points by the difference between the two latenciesMeters/secondProximal latencyDistal LatencyLatency time elapsed from the initial propagation of nerve impulse to the depolarization of mm fibers 55Stimulation and Recording For example:Proximal latency: 7msecDistal latency: 2 msecConduction distance: 300 mm or 30 cmCV = 30 cm / (7msec 2 msec) = 30 cm / 5 msec = 60 m/s Latency time elapsed from the initial propagation of nerve impulse to the depolarization of mm fibers 56Stimulation and Recording How to interpret?Upper Extremity: 50 70 m/s [60 m/s]Lower Extremity: 60 m/s57Sensory ncv testingStimulation and Recording Ring electrodes

Stimulation and RecordingTechniquesOrthodromic conductionAntidromic conductionStimulation and RecordingOrthodromic conductionNormal way physiologic responses travel

Stimulation and RecordingAntidromic conductionStimulus applied to proximal sites and is recorded at the index or long finger

Stimulation and RecordingHow to interpret?Normal sensory NCV40 75 m/sSensory NCVs have been found to be faster compared with the motor NCVs Factors that may affect motor and sensory conductionIncrease in body temperature5% increase in conduction velocity per degreeReduced latenciesUE faster than LEProximal segments faster than distal segmentsOTHERSH ReflexHoffman ReflexMeasures the integrity of both motor and sensory fibersFor exampleSubmaximal stimulus to the tibial n. in the popliteal fossaMotor response recorded from the medial soleus (rich in mm spindles) Norm: 29.8 msecF WaveElicited bySupramaximal stimulation of a peripheral nerve in a distal site resulting in both orthodromic and antidromic conduction.Antidromic conduction extends to the spinal cordMost helpful in conditions where the most proximal portion of the nerve is affectedUE: 30 secondsLE: less than 60 seconds