Electromyography:Recording

D. Gordon E. Robertson, Ph.D.

Biomechanics Laboratory,

School of Human Kinetics,

University of Ottawa, Ottawa, CANADA

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EMG Recording

• Surface or indwelling• Electrode placement• Type of amplifier• Common Mode Rejection Ratio

(CMRR)• Dynamic range and Gain• Input impedance and skin resistance• Frequency response• Telemetry versus directly wired

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Surface Electrodes

• lower frequency spectrum (20 to 500 Hz)

• relatively noninvasive, cabling does encumber subject, telemetry helps

• skin preparation usually necessary

• surface muscles only

• global pickup (whole muscle)

• inexpensive and easy to apply

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Indwelling Electrodes

• fine wire or needle

• produce higher frequency spectrum (10 to 2000 Hz)

• invasive, possible nerve injury

• can record from deep muscles

• localized pickup

• expensive and difficult to insert

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Types of Electrodes

Bipolar surface

Needle

Fine-wire

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Electrode Placement

• electrode pairs in parallel with fibres

• midway between motor point and myotendonous junction (belly of muscle)

• approximately 2 cm apart, better if electrodes are fixed together to reduce relative movements

• leads should be immobilized to skin

• ground electrode placed over electrically neutral area usual bone

• N.B. there should be only one ground electrode per person

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Surface Electrode System

Differential amplifier

Leads

Electrodes

Ground electrode

Cable

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Surface Electrode Geometry

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Surface Electrode Placement

motor point frequency spectra

strongest EMG

best

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Type of Amplifier

• because EMG signal is small (< 10 mV) and external signals (radio, electrical cables, fluorescent lighting, television etc.) are relatively large, EMG signals cannot be distinguished from background noise

• background noise is a “common mode signal” (arrives at all electrodes simultaneously)

• common mode signals can be removed by differential amplifiers

• single-ended (SE) amplifiers may be used after differential preamplified electrodes

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Common Mode Rejection Ratio (CMRR)

• ability of a differential amplifier to perform accurate subtractions (attenuate common mode noise)

• usually measured in decibels (y=20 log10x)

• EMG amplifiers should be >80 dB (i.e., S/N of 10000:1, the difference between two identical 1 V sine waves would be 0.1 mV)

• most modern EMG amplifiers are >100 dB

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Dynamic Range and Gain

• dynamic range is the linear amplification range of an electrical device

• typical A/D computers use either +/–10 V or +/–5 V

• amplifiers usually have +/–10 V or more, oscilloscopes and multimeters (+/–200 V or more)

• tape or minidisk recorders have +/–1.25 V

• EMG signals must be amplified usually 1000x or more but not too high to cause amplifier “saturation” (signal overload)

• if too low, numerical resolution will comprised (too few significant digits, from 12 bit to 8 bit or less)

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Input Impedance

• impedance is the combination of electrical resistance and capacitance

• all devices must have a high input impedance to prevent “loading” of the input signal

• if loading occurs the signal strength is reduced

• typically amplifiers have a 1 M input resistance, EMG amplifiers need 10 M or greater

• 10 G amplifiers need no skin preparation

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Skin Impedance

• dry skin provides insulation from static electricity, 9-V battery discharge etc.

• unprepared skin resistance can be 2 M or greater except when wet or “sweaty”

• if using electrodes with < 1 G input resistances, skin resistance should be reduced to < 100 k

• Vinput = [ Rinput/(Rinput + Rskin) ] VEMG

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Skin Impedance Example

• Vinput = [ Rinput/(Rinput + Rskin) ] VEMG

If skin resistance is 2 M and input resistance is 10 M then voltage at amplifier will be [10/(10 + 2) = 0.833] 83.3% of its true value.

By reducing skin resistance to 100 k this can be improved to 99%.

By also using a 100 M resistance amplifier the signal will be 99.9%.

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Frequency Response

• frequency responses of amplifier and recording systems must match frequency spectrum of the EMG signal

• since “raw” surface EMGs have a frequency spectrum from 20 to 500 Hz, amplifiers and recording system must have same frequency response or wider

• since relative movements of electrodes cause low frequency “artifacts,” high-pass filtering is necessary (10 to 20 Hz cutoff)

• Since surface EMG signals only have frequencies as high as 500 Hz, low-pass filtering is desirable (500 to 1000 Hz cutoff)

• therefore use a “band-pass filter” (20 to 500 Hz)

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EMG Sampling Rate

• since highest frequency in surface EMG signal is 500 Hz, A/D (computer) sampling rate should be 1000 Hz or greater (2 times maximum frequency)

• raw EMGs cannot be correctly recorded by pen recorders since pen recorders are essentially 50 Hz low-pass filters

• mean or median frequencies of unfatigued muscles is around 70 to 80 Hz

• “notch” filters should not be used to remove 50/60 cycle (line frequency) interference because much of the EMG signal strength is in this range

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Telemetry versus Direct• telemetry has less encumbrance and permits greater

movement space• radio telemetry can be affected by interference and

external radio sources• radio telemetry may have limited range due to

legislation (e.g., IC, FCC)• cable telemetry (e.g., Bortec) can reduce interference

from electrical sources• telemetry more expensive than directly wired systems• telemetry has limited bandwidth (more channels

reduces frequency bandwidth)