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Electromyography: Recording D. Gordon E. Robertson, Ph.D. Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, CANADA 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 2 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 3 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 4 Types of Electrodes Bipolar surface Needle Fine-wire 5 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 6 Surface Electrode System Ground electrode Differential amplifier Cable Leads Electrodes 7 Surface Electrode Geometry 8 Surface Electrode Placement motor point best frequency spectra strongest EMG 9 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 10 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 11 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) 12 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 MW input resistance, EMG amplifiers need 10 MW or greater • 10 GW amplifiers need no skin preparation 13 Skin Impedance • dry skin provides insulation from static electricity, 9-V battery discharge etc. • unprepared skin resistance can be 2 MW or greater except when wet or “sweaty” • if using electrodes with < 1 GW input resistances, skin resistance should be reduced to < 100 kW • Vinput = [ Rinput/(Rinput + Rskin) ] VEMG 14 Skin Impedance Example • Vinput = [ Rinput/(Rinput + Rskin) ] VEMG If skin resistance is 2 Mw and input resistance is 10 Mw then voltage at amplifier will be [10/(10 + 2) = 0.833] 83.3% of its true value. By reducing skin resistance to 100 kw this can be improved to 99%. By also using a 100 Mw resistance amplifier the signal will be 99.9%. 15 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) 16 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 17 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) 18