18 Nov 2010 ... EMG Recording. • Surface or indwelling. • Electrode placement. • Type of
amplifier. • Common Mode Rejection Ratio. (CMRR). • Dynamic range ...
Electromyography: Recording D. Gordon E. Robertson, PhD, FCSB
Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada
EMG Recording • • • • • • • • 18 Nov. 2010
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 Biomechanics Laboratory, University of Ottawa
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Surface Electrodes • lower frequency spectrum (20 to 500 Hz) • relatively noninvasive, noninvasive cabling does encumber subject, telemetry helps • skin preparation usually necessary • surface muscles only • global pickup (whole muscle) • inexpensive and easy to apply 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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I d lli Electrodes Indwelling El t d • • • •
fine wire or needle localized pickup difficult to insert possible nerve invasive,, p injury • produces higher frequency spectrum (10 to 2000 Hz) • can record deep muscles 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Types of Electrodes Bipolar surface
Needle
Fine wire Fine-wire
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Biomechanics Laboratory, University of Ottawa
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Electrode Placement • electrode l t d pairs i in i parallel ll l with ith fib fibres • midway between motor point and myotendinous junction (or near belly of muscle) • approximately 2 cm apart, better if electrodes are fixed together to reduce relative movement • leads should be immobilized to skin • ground electrode placed over electrically neutral area usually bone • N.B. there should be only one ground electrode per person 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Surface Electrode System (preamplifier type) Ground electrode
Differential amplifier Cable
Leads Electrodes
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Biomechanics Laboratory, University of Ottawa
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Surface Electrode Geometry
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Biomechanics Laboratory, University of Ottawa
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Surface Electrode Placement frequency spectra
motor point best
strongest EMG
myotendinous junctions
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Biomechanics Laboratory, University of Ottawa
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Type of Amplifier • because EMG signals are small (< 5 mV) and external signals (radio, electrical cables, fluorescent lighting, television, etc.) are relatively large, EMG signals cannot be distinguished from background noise • background noise (hum) is a “common mode signal” (i.e., arrives at all electrodes simultaneously) • common mode signals can be removed by differential a p e s amplifiers • single-ended (SE) amplifiers may be used after differential preamplified electrodes 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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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 log10 x) • EMG amplifiers lifi should h ld be b >80 80 dB (i.e., (i S/N of 10 000:1, the difference between two identical 1 V sine waves would be 0.1 mV)) • most modern EMG amplifiers are >100 dB
18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Dynamic Range and Gain • dynamic range is the linear amplification range of an electrical device • typical A/D computers use +/–10 V or +/–5 V • amplifiers usually have +/–10 V or more, oscilloscopes and multimeters +/–200 V or more • audio tape or minidisk recorders have +/–1.25 +/ 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) 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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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 l di occurs the th signal i l strength t th is i reduced d d • typically amplifiers have a 1 MΩ (megohm) input resistance resistance, EMG amplifiers need 10 MΩ or greater • 10 GΩ bioamplifiers need no skin preparation 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Skin Impedance • dry skin provides insulation from static electricity, 9-V battery discharge etc. • unprepared skin resistance can be 2 MΩ or greater except when wet or “sweaty” • if using i electrodes l t d with ith < 1 GΩ input i t resistances, skin resistance should be reduced to < 100 kΩ • Vinput = [ Rinput / (Rinput + Rskin) ] VEMG 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Skin Impedance Example Vinput = [ Rinput / (Rinput + Rskin) ] VEMG If skin resistance is 2 MΩ (megohm) and input resistance is 10 MΩ then voltage at amplifier will be [10/(10 + 2) = 0.833] 83 3% of its true value. 83.3% value By reducing skin resistance to 100 kΩ this can be improved p to 99%. % By also using a 100 MΩ resistance amplifier the signal will be 99.9%. 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Frequency Response • frequency responses of amplifier and recording systems must match frequency spectrum of the EMG signal raw surface EMGs have a frequency spectrum • since “raw” 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) band pass filter” filter (20 to 500 Hz) • therefore use a “band-pass 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Frequency Response • Typical frequency spectrum of surface EMG
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Biomechanics Laboratory, University of Ottawa
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Typical Band Widths EMG
20-500 Hz 10-1000 Hz
surface indwelling
ECG
0.05-30 Hz 0.05-100 Hz
standard diagnostic
EEG
11-33 H Hz 4-7 Hz 8-12 Hz 12-30 12 30 Hz 30-100 Hz
delta d lt waves theta waves alpha waves beta waves gamma waves
muscle forces, human movements
DC-10 Hz
audio
20-8000 Hz 20-15 000 Hz 20-20 000 Hz
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voice tape CD
Biomechanics Laboratory, University of Ottawa
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EMG Sampling Rate • since i highest hi h t frequency f i surface in f EMG signal i l is i 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 p filters • mean or median frequencies of unfatigued muscles are around 70 to 80 Hz • “notch” “ h” fil filters should h ld not be b used d to remove 50/60 cycle (line frequency) interference because much of the EMG signal strength is in this range 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Telemetry versus Direct Wire • telemetry has less encumbrance and permits greater movement volumes • radio telemetryy can be affected byy interference and external radio sources • radio telemetry may have limited range due to g ((e.g., g , IC,, FCC,, CRTC)) legislation • cable telemetry (e.g., Bortec) can reduce interference from electrical sources • telemetry is usually more expensive than directly wired systems • telemetry has limited bandwidth (more channels reduce frequency bandwidth) 18 Nov. 2010
Biomechanics Laboratory, University of Ottawa
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Telemetered EMG • Trigno EMG and accelerometry telemetry system
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Biomechanics Laboratory, University of Ottawa
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