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

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

4. Biomechanics Laboratory, University of Ottawa
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. Biomechanics Laboratory, University of Ottawa
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. Biomechanics Laboratory, University of Ottawa
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. Surface Electrode Placement
myotendinous junctions
frequency spectra
motor point
best
strongest EMG
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. Surface Electrode System (preamplifier type)
Differential amplifier
Ground or reference electrode
Cable
Leads
Electrodes
Biomechanics Laboratory, University of Ottawa

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

19. Biomechanics Laboratory, University of Ottawa
Typical Band Widths
EMG
20–500 Hz
10–1000 Hz
surface
indwelling
ECG
0.05–30 Hz
0.05–100 Hz
standard
diagnostic
EEG
1–3 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 Hz
muscle moments
joint trajectories
audio
20–8000 Hz
20– Hz
20– Hz
voice
tape CD
Biomechanics Laboratory, University of Ottawa

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