Electromyography: Recording

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

2. 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 Biomechanics Laboratory, University of Ottawa

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

4. 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 Biomechanics Laboratory, University of Ottawa

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 Ottawa

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 Biomechanics Laboratory, University of Ottawa

7. 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 Biomechanics Laboratory, University of Ottawa

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

9. 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 Biomechanics Laboratory, University of Ottawa

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

11. 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 Biomechanics Laboratory, University of Ottawa

12. 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 >80 dB (i.e., S/N of 10 000:1, the difference between two identical 1 mV sine waves would be 0.1 mV) • most modern EMG amplifiers are >100 dB Biomechanics Laboratory, University of Ottawa

13. 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) Biomechanics Laboratory, University of Ottawa

14. 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 Biomechanics Laboratory, University of Ottawa

15. 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 Biomechanics Laboratory, University of Ottawa

16. 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%. Biomechanics Laboratory, University of Ottawa

17. 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) Biomechanics Laboratory, University of Ottawa

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

19. Typical Band Widths Biomechanics Laboratory, University of Ottawa

20. 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” filtersshould not be used to remove 50/60 cycle (line frequency) interference because much of the EMG signal strength is in this range Biomechanics Laboratory, University of Ottawa

21. 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) Biomechanics Laboratory, University of Ottawa

22. Telemetered EMG • Delsys’s Trigno EMG and accelerometry telemetry system Biomechanics Laboratory, University of Ottawa