Electromyography: Recording

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ElectromyographyRecording

• D. Gordon E. Robertson, PhD, FCSB
• Biomechanics Laboratory,
• School of Human Kinetics,
• University of Ottawa, Ottawa, Canada

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EMG Recording Topics

• 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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Types of Electrodes
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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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Surface Electrodes

• pre-gelled disposable electrodes are most common
  and inexpensive
• MLS pre-amplified electrodes reduce movement
  artifact
• Delsys Trigno includes 3D accelerometers

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Indwelling Electrodes

• fine wire or needle
• localized pickup
• difficult to insert
• invasive, possible nerve injury
• produces higher frequency
• spectrum (10 to 2000 Hz)
• can record deep muscles

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Electrode Placement

• electrode pairs in parallel with 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

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Surface Electrode Placement
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Noise Reduction and Grounding

• leads should be immobilized to skin
• surgical webbing can help reduce movement
  artifacts
• ground electrode placed over
• electrically neutral area usually
• bone
• N.B. there should be only one ground electrode
  per person to prevent ground loops that could
  cause an electrical shock

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Surface Electrode System(preamplifier type)
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Type of Amplifier

• because EMG signals are small (lt 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 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 log10 x)
• EMG amplifiers should be gt80 dB (i.e., S/N of 10
  0001, the difference between two identical 1 mV
  sine waves would be 0.1 mV)
• most modern EMG amplifiers are gt100 dB

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Dynamic Range and Gain

• dynamic range is the range of linear
  amplification 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 V
• EMG signals must be amplified by 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)

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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 MW (megohm) input
  resistance, EMG amplifiers need 10 MW or greater
• 10 GW bioamplifiers 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 MW or greater
  except when wet or sweaty
• if using electrodes with lt 1 GW input
  resistances, skin resistance should be reduced to
  lt 100 kW
• Vinput Rinput / (Rinput Rskin) VEMG

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Skin Impedance Example

• Vinput Rinput / (Rinput Rskin) VEMG
• If skin resistance is 2 MW (megohm) 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.

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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 systems 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 (e.g., 20 to
  500 Hz)

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Frequency Response

• Typical frequency spectrum of surface EMG

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Typical Band Widths
EMG 20500 Hz 101000 Hz surface indwelling
ECG 0.0530 Hz 0.05100 Hz standard diagnostic
EEG 13 Hz 47 Hz 812 Hz 1230 Hz 30100 Hz delta waves theta waves alpha waves beta waves gamma waves
muscle forces or human movements DC10 Hz muscle moments joint trajectories
audio 208000 Hz 2015 000 Hz 2020 000 Hz voice tape CD
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EMG Sampling Rate

• since highest frequency in surface EMG signal is
  500 Hz, A/D (computer) sampling rates should be
  1000 Hz or greater (gt2 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
  are 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 Wire

• telemetry has less encumbrance and permits
  greater movement volumes
• radio telemetry can be affected by interference
  and external radio sources
• radio telemetry may have limited range due to
  legislation (e.g., IC, FCC, CRTC)
• 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 bandwidths)

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Telemetered EMG

• Delsyss Trigno EMG and accelerometry telemetry
  system