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

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Presentation on theme: "Electromyography: Recording D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada D."— Presentation transcript:

1 Electromyography: Recording D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada

2 Biomechanics Laboratory, University of Ottawa2 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

3 Biomechanics Laboratory, University of Ottawa3 Types of Electrodes Bipolar surface Needle Fine-wire

4 Biomechanics Laboratory, University of Ottawa4 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

5 Surface Electrodes pre-gelled disposable electrodes are most common and inexpensive MLS pre-amplified electrodes reduce movement artifact Delsys Trigno includes 3D accelerometers Biomechanics Laboratory, University of Ottawa5

6 6 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

7 Biomechanics Laboratory, University of Ottawa7 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

8 Biomechanics Laboratory, University of Ottawa8 Surface Electrode Placement motor point frequency spectra strongest EMG best myotendinous junctions

9 Biomechanics Laboratory, University of Ottawa9 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

10 Biomechanics Laboratory, University of Ottawa10 Surface Electrode System (preamplifier type) Differential amplifier Leads Electrodes Ground or reference electrode Cable

11 Biomechanics Laboratory, University of Ottawa11 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 amplifiers single-ended (SE) amplifiers may be used after differential preamplified electrodes

12 Biomechanics Laboratory, University of Ottawa12 Common Mode Rejection Ratio (CMRR) ability of a differential amplifier to perform accurate subtractions (attenuate common mode noise) usually measured in decibels (y = 20 log 10 x) EMG amplifiers should be >80 dB (i.e., S/N of :1, the difference between two identical 1 mV sine waves would be 0.1 V) most modern EMG amplifiers are >100 dB

13 Biomechanics Laboratory, University of Ottawa13 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 1000 x or more but not too high to cause amplifiersaturation (signal overload) if too low, numerical resolution will comprised (too few significant digits)

14 Biomechanics Laboratory, University of Ottawa14 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 (megohm) input resistance, EMG amplifiers need 10 M or greater 10 G bioamplifiers need no skin preparation

15 Biomechanics Laboratory, University of Ottawa15 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 V input = [ R input / (R input + R skin ) ] V EMG

16 Biomechanics Laboratory, University of Ottawa16 Skin Impedance: Example V input = [ R input / (R input + R skin ) ] V EMG If skin resistance is 2 M (megohm) 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%.

17 Biomechanics Laboratory, University of Ottawa17 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)

18 Biomechanics Laboratory, University of Ottawa18 Frequency Response Typical frequency spectrum of surface EMG

19 Biomechanics Laboratory, University of Ottawa19 Typical Band Widths EMG20–500 Hz 10–1000 Hz surface indwelling ECG0.05–30 Hz 0.05–100 Hz standard diagnostic EEG1–3 Hz 4–7 Hz 8–12 Hz 12–30 Hz 30–100 Hz delta waves theta waves alpha waves beta waves gamma waves muscle forces or human movements DC–10 Hzmuscle moments joint trajectories audio20–8000 Hz 20– Hz 20– Hz voice tape CD

20 Biomechanics Laboratory, University of Ottawa20 EMG Sampling Rate since highest frequency in surface EMG signal is 500 Hz, A/D (computer) sampling rates 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 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

21 Biomechanics Laboratory, University of Ottawa21 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)

22 Biomechanics Laboratory, University of Ottawa22 Telemetered EMG Delsyss Trigno EMG and accelerometry telemetry system

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