EMG Analysis and EMG Analysis and InterpretationInterpretation

Extracting information from Extracting information from the signalsthe signals

Interference EMGInterference EMG

Superimposed muscle action potentialsSuperimposed muscle action potentials– Positive deflectionsPositive deflections– Negative deflectionsNegative deflections– Isoelectric lineIsoelectric line

Algebraic sum of muscle action potentialsAlgebraic sum of muscle action potentials

Amplitude cancellationAmplitude cancellation– I.e., interferenceI.e., interference

Two Signal DomainsTwo Signal Domains

Time domainTime domain– AmplitudeAmplitude

Frequency domainFrequency domain– FrequencyFrequency

Examples…Examples…

Time DomainTime Domain

Changes in EMG Changes in EMG amplitude over timeamplitude over time– Root-mean-square Root-mean-square

(rms) value(rms) valueSame as standard Same as standard deviationdeviation

– Time-averaged Time-averaged integrated EMG valuesintegrated EMG values

Integrated = AUCIntegrated = AUC

AUC AUC ÷ time÷ time

N

iirms x

NEMG

1

21

N

iixN 1

1EMG Rectified Average

NormalizationNormalization

Drawbacks of surface (interference) EMG is Drawbacks of surface (interference) EMG is comparing raw valuescomparing raw values– Between subjectsBetween subjects– Between musclesBetween muscles– Repeated over timeRepeated over time

Solution = normalize EMG amplitude as a Solution = normalize EMG amplitude as a percentage of the maximal voluntary contraction percentage of the maximal voluntary contraction (MVC).(MVC).Result = comparing the patterns of response Result = comparing the patterns of response under different conditions or tasks, not the under different conditions or tasks, not the absolute value.absolute value.Example…HRmaxExample…HRmax

Frequency DomainFrequency Domain

Determining the frequency content of an EMG signal epochDetermining the frequency content of an EMG signal epoch– Epoch = any given signal portion/durationEpoch = any given signal portion/duration

Measured in Hertz (Hz)Measured in Hertz (Hz)– Cycles (turns) per secondCycles (turns) per second

Every signal can be represented by a set of sine and cosine terms = Every signal can be represented by a set of sine and cosine terms = Fourier series.Fourier series.– The fundamental, its harmonics, and the amplitude of each harmonicThe fundamental, its harmonics, and the amplitude of each harmonic

Fourier TransformationFourier Transformation– Determine the Power Density Spectrum (PDS)Determine the Power Density Spectrum (PDS)– Mean Power Frequency (MPF)Mean Power Frequency (MPF)

Mean (Hz) of the PDSMean (Hz) of the PDS– Median Power Frequency (MDF)Median Power Frequency (MDF)

Frequency (Hz) that bisects the PDS in halfFrequency (Hz) that bisects the PDS in half

EMG Signal ConditioningEMG Signal Conditioning

UnitsUnits– Volts (V), millivolts (mV), and microvolts (Volts (V), millivolts (mV), and microvolts (μμV)V)

Zero MeanZero Mean– Subtracting the mean valueSubtracting the mean value

Digital filteringDigital filtering– Reducing unwanted signal componentsReducing unwanted signal components

Low-frequency noise (movement artifact, etc)Low-frequency noise (movement artifact, etc)High-frequency noise (random white noise, etc)High-frequency noise (random white noise, etc)

– More about this after Fourier analysis… More about this after Fourier analysis…

Quantified ValuesQuantified Values

EMG AmplitudeEMG Amplitude– RMSRMS– Represents:Represents:

Algebraic sum of muscle action potentials traveling Algebraic sum of muscle action potentials traveling within the recording areawithin the recording area

– Number of motor units recruitedNumber of motor units recruited– Firing rate of activated motor unitsFiring rate of activated motor units

Linear or curvalinear relation with force productionLinear or curvalinear relation with force production

Quantified ValuesQuantified Values

EMG Median Power Frequency (MDF)EMG Median Power Frequency (MDF)– Derived from Fourier TransformationDerived from Fourier Transformation– Indication of what frequencies dominate the EMG Indication of what frequencies dominate the EMG

signal (power of the signal)signal (power of the signal)– Related to:Related to:

The shape of the action potentialsThe shape of the action potentials– Most significant contributor…Most significant contributor…

The relative timing of the action potentials discharged by The relative timing of the action potentials discharged by different motor unitsdifferent motor units

The discharge rate of motor units (firing rate)The discharge rate of motor units (firing rate)– Least significant contributor…Least significant contributor…

– Mostly used to track fatigueMostly used to track fatigue

Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003.

Displacement Sensor

Laser Beam

Accelerometer

Bipolar EMG Electrodes

Force Transducer

Isometric muscle

action at 30% MVC