Transcript EMG Signal Processing

Differential Amplifier
EMG Signal Processing
EMG Amplifier Specifications
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Amplifiers should be described by the following:
 If single, differential, double differential
 Input impedance
 Common Mode Rejection Ratio (CMRR)
 Signal to noise ratio
 Actual gain used
 Frequency range of amplifier
Noraxon Telemyo EMG Amplifier
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Input Noise < 1 μV RMS
Differential Input Impedance 16 Meg Ohms
Common Mode Rejection Ratio CMRR 85 dB
Frequency Response 16 – 500 Hz
Throughput Circuit Gain 1000
Electrodes
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Reports on surface EMG should
include:
Electrode material (Ag/AgCl)
Electrode geometry (disc, bars,
rectangular)
Size (diameter, radius, width x
length)
Use of gel or paste, alcohol
applied to cleanse skin, skin
abrasion, shaving of hair
Interelectrode distance
Electrode location, orientation
over muscle with respect to
tendons, motor points and fiber
direction
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Intramuscular wire electrodes
should be described by:
Wire material (stainless steel)
Single or multi strand
Insulation material
Method of insertion (hypodermic
needle)
Depth of insertion
Single or bipolar wire
Location of insertion in the muscle
Interelectrode distance
Type of ground electrode used,
location
EMG Amplitude
Root Mean Square
1 N
2
RMS 
EMG
(
i
)

N i 1
RMS 
1 N
2
EMG
(
i
)
N i 1
Average Rectified Value
1
ARV 
T

T
i 1
EMG (t ) dt
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Full wave rectify the EMG
EMG Linear Envelope
Low pass filter with a time constant?
EMG Filtering
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Since the power density
spectra of the EMG
contains most of its power
in the frequency range of 5500 Hz at the extremes,
surface EMG should not be
filtered above 10 Hz as a
low cutoff and below 350
Hz as a high cutoff.
Intramuscular signals
should be filtered in the 10450 Hz range.
EMG Frequency Spectrum
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Explain of Power Spectrum
should include:
Time epoch used for each
calculation
Type of window used prior
to FFT (Hamming,
Hanning)
Number of zero padding
points
Equation used to calculate
Median or Mean Frequency
Muscle length or fixed
joint angle
The Median Frequency is defined as the
frequency that divides the power
spectrum in two regions having the
same power or area under the
Amplitude – Frequency Curve
f med

0

S ( f )df 
 S ( f )df
f med
EMG - Force
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Recruitment of a motor unit. A
quanta of force in contributed to
the muscle contraction;
however, the contribution to the
EMG signal amplitude depends
on the proximity of the
detection surfaces of the
electrode to the nearest fibers of
the recruited motor unit.
 A newly recruited MU will
increase its firing rate as the
force demand increases.
 The force increase rapidly as a
function of the increasing firing
rate, whereas the contribution to
the amplitude of the EMG
signal increases less rapidly
EMG Normalization
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Subjects should be adequately trained to elicit an MVC.
Provide subjects with immediate feedback of obtained force/torque.
Give the following information relative to normalization:
How the subjects were trained to obtain MVC
Joint angle or muscle length
Rate of force development
Velocity of shortening or lengthening
Change in muscle length
Range of joint angle in non – isometric contractions
Load applied in non – isometric contractions
When normalizing the amplitude of the EMG signal, do so at values
less than 80% MVC. Above this level the EMG signal and the force
(torque) are exceptional unstable and do not provide a suitable
reference point (De Luca, JAB 13, p. 154)
EMG Crosstalk
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Make every effort to determine that EMG crosstalk from muscles near
the muscle of interest did not contaminate the recorded signal
Select an appropriate electrode size, interelectrode distance and
location when working on an area where many narrow muscles are
tightly gathered
Care also should be employed when recording surface EMG from
areas with subcutaneous adipose tissue as it is known that adipose
tissue enhances crosstalk.