Transcript Measurement of internal work during running

Electromyography:
Recording
D. Gordon E. Robertson, PhD, FCSB
Biomechanics Laboratory,
School of Human Kinetics,
University of Ottawa, Ottawa, Canada
EMG Recording: Topics
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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
Bipolar surface
Needle
Fine-wire
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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
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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
frequency
spectra
motor point
best
strongest
EMG
myotendinous
junctions
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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)
Ground or reference electrode
Differential amplifier
Cable
Leads
Electrodes
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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
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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 >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
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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 < 1 GW input
resistances, skin resistance should be
reduced to < 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
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
20–15 000 Hz
20–20 000 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 (>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
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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
• Delsys’s Trigno EMG and accelerometry telemetry
system
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