Emg

Armin Allahverdy1935

ElectromyographyElectromyography• Defined as the preparation, Defined as the preparation,

study of, and interpretation of study of, and interpretation of electromyogramselectromyograms

Electromyogram:Electromyogram:• Defined as the electrical Defined as the electrical

activity associated with the activity associated with the contraction of a musclecontraction of a muscle

Greek derivative:Greek derivative:• elektronelektron, amber, + , amber, + mysmys, ,

muscle, + muscle, + grammagramma, something , something writtenwritten

Dendrite

Soma (body)

Axon

A motor A motor unit is unit is composecomposed of a d of a motor motor neuron neuron and all of and all of the the muscle muscle fibers it fibers it innervateinnervatess

each muscle has many motor units (m.u.)

# of fibers in a m.u. is dependent on the precision of movement required of that muscle (average: 100-200 fibers per m.u.)more precision is obtained with more

neurons100 to 2000 motor neurons per muscle

# of m.u.’s in a muscle decreases in the elderly

Motor neurons release acetylcholine.

This has an excitatory effect on muscle fibres.

A single action potential in the motor nerve is enough to trigger an action potential in the muscle fibre.

Depolarisation triggers the release of calcium ions, which trigger contraction.

The potential (voltage) generated by one fibre is small (<100mV).

Clinical applicationsInvasive recording used in assessment.

BiofeedbackCan aid rehabilitation after injury or

strokeMay have other applications e.g.

forehead EMG biofeedback may reduce tension headache.

EMG and covert behaviour e.g.subvocalisation while readingFacial EMG and emotion

Uses:Uses:• Provides an indication of how Provides an indication of how

much a muscle is being used much a muscle is being used during particular types of during particular types of activityactivity

• Is a muscle on or off?Is a muscle on or off?• Is a muscle fatigued?Is a muscle fatigued?

By placing electrodes over the muscle we can record the signal generated by muscle contraction.

Voltage is displayed continuously and recorded for analysis.

The signal includes positive and negative waves and varies rapidly: we also analyse integrated EMG (averages over 20 samples, ignoring the sign)

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

Distance between electrodesDistance between electrodesIncreased amplitude vs. misaligning Increased amplitude vs. misaligning

electrodes, Multiple motor unit action electrodes, Multiple motor unit action potentials (MUAP) potentials (MUAP)

Muscle fibers of motor units are Muscle fibers of motor units are distributed evenly, thus large muscle distributed evenly, thus large muscle coverage is not necessary coverage is not necessary (De Luca).(De Luca).

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

Away from motor pointMUAP traveling in opposite directions

Simultaneous (+) & (-) AP’s Resultant increased frequency components

More jagged signalMiddle of muscle belly is generally accepted

Away from tendonFewer, thinner muscle fibersCloser to other muscle origins, insertionsMore susceptible to cross-talk

Away from outer edge of muscleCloser to other musculature

Orientation parallel to muscle fibersMore accurate conduction velocity

Increased probability of detecting same signal

As far away as possible from recording electrodes

Electrically neutral tissueBony prominence

Good electrical contactLarger sizeGood adhesive properties

Muscle contraction is graded by varying

The number of active motor units within the muscle.

The frequency of firing of each motor unit.

Skeletal muscles are normally slightly contracted, known as tonus. At any given time a small proportion of the motor units will be active.

Exertion can increase the size of individual fibres and increase energy stores: Hypertrophy.

Muscle contractionMuscle contractionExcitation-contraction couplingExcitation-contraction coupling• Process of nerve-muscle Process of nerve-muscle

excitation leading to Caexcitation leading to Ca++++ release and force generationrelease and force generation

Muscle contractionMuscle contraction1) Nerve stimulation1) Nerve stimulation• motor nervemotor nerve2) Electrochemical transmission2) Electrochemical transmission• saltatory conductionsaltatory conduction3) Terminal nerve (axon) 3) Terminal nerve (axon)

activityactivity• CaCa++++ influx influx• Vesicle dockingVesicle docking• Ach release into synaptic cleftAch release into synaptic cleft

Muscle contractionMuscle contraction4) End-plate potential4) End-plate potential• Activation of motor end plateActivation of motor end plate• Ach-receptor binding leading Ach-receptor binding leading

to Nato Na++ influx (depolarization) influx (depolarization)5) Membrane propagation5) Membrane propagation• muscle fiber membrane muscle fiber membrane

excitation spreads over the excitation spreads over the fiberfiber

• THIS IS THE EMG SIGNAL!THIS IS THE EMG SIGNAL!

Electromyogram Electromyogram (EMG)(EMG)

• Algebraic summation of all Algebraic summation of all motor unit action potentials motor unit action potentials propagated along a muscle at propagated along a muscle at a point in timea point in time

• MUAPsMUAPs - Motor unit action - Motor unit action potentialspotentials

• Recording all the MFAPs of all Recording all the MFAPs of all the motor units activated by the motor units activated by the CNS in a given period of the CNS in a given period of timetime

Electromyography Electromyography (EMG)(EMG)

Generation of the EMG signalGeneration of the EMG signal• ““Wave” of depolarization and Wave” of depolarization and

the subsequent “wave” of the subsequent “wave” of repolarization are detected by repolarization are detected by recording electrodesrecording electrodes


Generation of the EMG signalGeneration of the EMG signal

Recording electrodeRecording electrode

Muscle surfaceMuscle surface

DepolarizationDepolarizationRepolarizationRepolarization

ElectromyographyElectromyography (EMG)(EMG)2 types of recording electrodes:2 types of recording electrodes:1) Surface1) Surface• most commonly usedmost commonly used• small recording surfacesmall recording surface• placed over the muscle bellyplaced over the muscle belly• requires adequate skin requires adequate skin

preparation: removal of hair, preparation: removal of hair, oil, dirt, skin, etc.oil, dirt, skin, etc.


2) Indwelling2) Indwelling• inserted directly into the inserted directly into the

muscle via a hypodermic muscle via a hypodermic needleneedle

• used for deep musclesused for deep muscles• used to identify different used to identify different

motor units recruited or how motor units recruited or how many motor units recruited many motor units recruited during an activityduring an activity


Characteristics:Characteristics:1) Muscle primarily involved 1) Muscle primarily involved

during a particular movementduring a particular movement• no direct indication of muscle no direct indication of muscle

forceforce• concentric or eccentric concentric or eccentric

contractionscontractions• related to muscle forcerelated to muscle force


Characteristics:Characteristics:2) Measures antagonist muscle 2) Measures antagonist muscle

activityactivityExample:Example:• hamstring activity at terminal hamstring activity at terminal

phase of knee extensionphase of knee extension


Characteristics:Characteristics:3) Assessment of muscle 3) Assessment of muscle

fatigue and recruitment of fatigue and recruitment of different motor units and fiber different motor units and fiber typestypes

• Analyzing the frequency of the Analyzing the frequency of the EMG signalsEMG signals

• related to type of motor unit related to type of motor unit activated and conduction activated and conduction velocityvelocity


Factors affecting the EMGFactors affecting the EMG1) Depth of the muscle fiber1) Depth of the muscle fiber• affecting distance to recording affecting distance to recording

electrodeelectrode• if distance increases, signal if distance increases, signal

amplitude decreasesamplitude decreases• muscle contraction: low to muscle contraction: low to

high intensities move from high intensities move from deep to surface of muscledeep to surface of muscle


Factors affecting the EMGFactors affecting the EMG2) Muscle fiber type2) Muscle fiber typeFast twitch fibers:Fast twitch fibers:• high conduction velocityhigh conduction velocity• high frequency and higher high frequency and higher

amplitudeamplitude


Factors affecting the EMGFactors affecting the EMG3) Motor unit size3) Motor unit size• ““all or none” responseall or none” response• a larger motor unit will result a larger motor unit will result

in a larger EMG recordingin a larger EMG recording


Factors affecting the EMGFactors affecting the EMG4) Muscle temperature4) Muscle temperature• increasing temperature increasing temperature

increases conduction velocityincreases conduction velocity• negligible increases in body negligible increases in body

temperature during exercisetemperature during exercise


Factors affecting the EMGFactors affecting the EMG5) Amount of tissue between 5) Amount of tissue between

muscles and electrodesmuscles and electrodes• skin and subcutaneous fatskin and subcutaneous fat• acts as a “low pass” filteracts as a “low pass” filter• decreases detection of EMG decreases detection of EMG

signals with higher signals with higher frequenciesfrequencies

Raw EMG signalRaw EMG signal• Can be assessed qualitativelyCan be assessed qualitatively• visually observing the size and visually observing the size and

density of the EMG signaldensity of the EMG signal

Assessing Assessing amplitudeamplitude

• Method of evaluating the Method of evaluating the “amount” of EMG activity “amount” of EMG activity collectedcollected

Different methods:Different methods:• rectificationrectification• integrationintegration• linear envelopelinear envelope• root mean square (RMS)root mean square (RMS)

Assessing Assessing amplitudeamplitude

• Raw EMG signal is a bi-polar Raw EMG signal is a bi-polar signalsignal

• positive and negative phasespositive and negative phasesRectificationRectification• Method of translating the raw Method of translating the raw

EMG signal to a single polarityEMG signal to a single polarityHalf-wave rectificationHalf-wave rectification• eliminating one polarity of the eliminating one polarity of the

signal (Ex. Deleting the signal (Ex. Deleting the negative phase)negative phase)

RectificationRectificationFull-wave rectificationFull-wave rectification• Inverting the negative polarity Inverting the negative polarity

onto the positive polarityonto the positive polarity• Preferred and most often used Preferred and most often used

methodmethod• Preserves all the energy in the Preserves all the energy in the

raw EMG signalraw EMG signal

RectificRectificationationRaw signalRaw signal Full-wave rectified signalFull-wave rectified signal

RectificationRectification• Once rectified, now can Once rectified, now can

perform calculations on the perform calculations on the signalsignal

Why not perform calculations on Why not perform calculations on the raw signal itself?the raw signal itself?

• Bi-polar signal (positive and Bi-polar signal (positive and negative phases)negative phases)

• positive + negative = ZEROpositive + negative = ZEROWhat do you do now with the What do you do now with the

rectified signal?rectified signal?

IntegrationIntegration• Mathematical operation of Mathematical operation of

calculating the area calculating the area underneath the rectified curveunderneath the rectified curve

• This measure is a function of This measure is a function of timetime

• The longer the EMG is The longer the EMG is collected, the more integrated collected, the more integrated EMG will appearEMG will appear

IntegrIntegrationation• Area = length × widthArea = length × width

• Units for integrated EMG = Units for integrated EMG = mVmVsecsec

• depends upon the amplitude, depends upon the amplitude, duration and frequency of duration and frequency of action potentialsaction potentials

secondsseconds

mVmV

Linear envelopeLinear envelope• Method of “smoothing” the Method of “smoothing” the

rectified EMG signalrectified EMG signal• Used most often in gait Used most often in gait

studies (cyclical muscle studies (cyclical muscle contractions)contractions)

• Use of a moving average to Use of a moving average to smooth the signalsmooth the signal

Linear Linear envelopeenvelope• Calculating the average EMG Calculating the average EMG amplitude (mV) over a fixed amplitude (mV) over a fixed window lengthwindow length

• moving the window across the moving the window across the whole EMG signal at fixed whole EMG signal at fixed intervalsintervals

secondsseconds

mVmV

Rectified signalRectified signal

Linear envelopeLinear envelope

Root mean squareRoot mean square• Different method of processing Different method of processing

the raw EMG (AC) signalthe raw EMG (AC) signal• also gives a moving average also gives a moving average

over time (smoothing effect)over time (smoothing effect)• is a measure of signal poweris a measure of signal power• converts the raw signal to a converts the raw signal to a

single polaritysingle polarity

Root mean squareRoot mean square• The value of the raw EMG The value of the raw EMG

signal (amplitude) is squared signal (amplitude) is squared (V(V22))

• the raw signal is the square the raw signal is the square root of the resultant, or root of the resultant, or processed, signalprocessed, signal

• units = mV.units = mV.

EMG frequency EMG frequency analysisanalysis

• Method of assessing the Method of assessing the frequency domain of the EMG frequency domain of the EMG signalsignal

• Amplitude measures Amplitude measures (integration, RMS, etc.) are in (integration, RMS, etc.) are in the time domainthe time domain

• Frequency analysis: assessing Frequency analysis: assessing the “quality” of the EMG the “quality” of the EMG signalsignal

• Reflection of the conduction Reflection of the conduction velocity of the recorded MFAPs velocity of the recorded MFAPs (muscle fiber type specific)(muscle fiber type specific)

EMG frequency EMG frequency analysisanalysis

• The EMG signal is a composite The EMG signal is a composite measure of many simple measure of many simple sinusoid wavessinusoid waves

• Alternating wavesAlternating waves• the EMG signal is non-periodic the EMG signal is non-periodic

(i.e. - they do not repeat any (i.e. - they do not repeat any any regular interval)any regular interval)

• Transformation of the EMG Transformation of the EMG signal from the time domain to signal from the time domain to the frequency domainthe frequency domain

Frequency Frequency spectrumspectrum

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