Transcript Measurement of internal work during running

Electromyography:
Recording
D. Gordon E. Robertson, Ph.D.
Biomechanics Laboratory,
School of Human Kinetics,
University of Ottawa, Ottawa, CANADA
EMG Recording
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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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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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Indwelling Electrodes
• fine wire or needle
• produce higher frequency spectrum
(10 to 2000 Hz)
• invasive, possible nerve injury
• can record from deep muscles
• localized pickup
• expensive and difficult to insert
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Types of Electrodes
Bipolar surface
Needle
Fine-wire
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Electrode Placement
• electrode pairs in parallel with fibres
• midway between motor point and myotendonous
junction (belly of muscle)
• approximately 2 cm apart, better if electrodes are
fixed together to reduce relative movements
• leads should be immobilized to skin
• ground electrode placed over electrically neutral
area usual bone
• N.B. there should be only one ground electrode per
person
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Surface Electrode System
Ground electrode
Differential amplifier
Cable
Leads
Electrodes
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Surface Electrode Geometry
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Surface Electrode Placement
motor point
best
frequency
spectra
strongest
EMG
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Type of Amplifier
• because EMG signal is small (< 10 mV) and
external signals (radio, electrical cables,
fluorescent lighting, television etc.) are relatively
large, EMG signals cannot be distinguished from
background noise
• background noise is a “common mode signal”
(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 log10x)
• EMG amplifiers should be >80 dB (i.e., S/N
of 10000:1, the difference between two
identical 1 V 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 linear amplification range of an
electrical device
• typical A/D computers use either +/–10 V or +/–5 V
• amplifiers usually have +/–10 V or more,
oscilloscopes and multimeters (+/–200 V or more)
• tape or minidisk recorders have +/–1.25 V
• EMG signals must be amplified 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, from 12 bit to 8 bit or less)
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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 input
resistance, EMG amplifiers need 10 MW or
greater
• 10 GW amplifiers 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 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 system 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” (20 to 500 Hz)
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EMG Sampling Rate
• since highest frequency in surface EMG signal is
500 Hz, A/D (computer) sampling rate 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
is 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
• telemetry has less encumbrance and permits
greater movement space
• radio telemetry can be affected by interference and
external radio sources
• radio telemetry may have limited range due to
legislation (e.g., IC, FCC)
• cable telemetry (e.g., Bortec) can reduce
interference from electrical sources
• telemetry more expensive than directly wired
systems
• telemetry has limited bandwidth (more channels
reduces frequency bandwidth)
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