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The EMG Signal
Filtering
Signal Processing.2
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Filters - Overview
Primary function is noise attenuation If the frequency of the noise source is
sufficiently different from the frequency components of the signal waveform of interest - the noise can be removed providing a “cleaner” EMG signal
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Filters - Overview
Frequency range of muscle - slow twitch motor units
(20) 70 - 125 Hz
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Filters - Overview
Frequency range of muscle - slow twitch motor units
Fast twitch motor units
125 - 250 Hz
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Filters - Overview
Frequency range of muscle - slow twitch motor units
Fast twitch motor units
Sources of noise that “compete” with these frequency ranges
Attenuate or make the true signal less visible and difficult to interpret– Example: 60 Hz from
120 V power lines
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Filter Types
Hardware filters– Analog electronic circuit
» Amplifiers, resistors, capacitors
Software filters– “Digital” filters
» Mathematical algorithms
Butterworth Filter.vi
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Frequency Components
Bandwidth– Range of frequencies from the low frequency
limit of the EMG signal to the high frequency limit = the band pass
Low frequency cut-off High frequency cut-off Roll off
– Rate at which frequency attenuation occurs
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Frequency Components
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Filter Types by Frequency Component
LP Filter
20 Hz 250 Hz
High frequency filter– Removes high
frequency components above a certain “cut-off”
Low pass filter (LP)– Pass = retain
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Filter Types by Frequency Component
BPFilter
20 Hz 250 Hz
Low - High frequency filter– Removes frequency
components below and above certain “cut-offs”
Band pass filter (BP)Filter
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Filter Types by Frequency Component
BS
20 Hz 250 Hz
Mid-range frequency filter– Removes a specific
frequency component within a range
Band stop filter (BS)– Example: 60 Hz filter
60 Hz
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Roll Off
Rate at which frequency attenuation occurs
Expressed by the order– The higher the
order the more rapid the roll off
– Index of sensitivity of attenuation
2
B u tte rw o rth F ilte r.v i
2
Butterworth Filter.vi
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Phase Shift
Filtering causes a change in phase = shift– A time delay frequency
component as it passes through the filter
– May cause waveform distortion especially if the the shift occurs near the cut-off frequency
If the phase shift is small it may be a tolerable error source
Shift
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Phase Shift Solution
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Filter Use Turn filter “On” Select type Insert cut-off(s) Run the VI
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Practical Effect - Filtering
Filtering will “sharpen” the image permitting better approximation of important events– Onsets
– Offsets
– Etc.
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Practical Effect - Filtering
Raw
Filtered
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Signal Processing.2
Descriptive statistics– Signal spike counts– Peak amplitude (voltage - mV) detection– Averaging– Variability analysis
Root Mean Square
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Descriptive Statistics
Often used as a basic means of analysis after visual inspection of the raw data– Probably more useful in quantifying “On-Off”
phenomena– May be used in conjunction with time-based
analyses: onset, duration & offset
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Signal Spike Counts
More useful when muscle force levels are relatively low– The interference pattern typical of high force
levels (e.g., MVC) makes spike counts difficult
4.0
-2.0
0.0
2.0
20000 500 1000 1500
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Signal Spike Counts
Spike Counting by Window Spike Counting of RawSignal - (could also be donewith rectified signal)
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Peak Amplitude Has traditionally been issued as an index of
maximal muscle activity– Probably valid when electrical activity is
relatively constant– A peak amplitude that exists more as an outlier
may not be truly representative of typical or average activity
Full-Wave Rectified Signal
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Averaging (Mean)
A data smoothing technique useful when high signal variability is of concern
Moving average - the mean amplitude of a full-wave rectified window (segment) of data points for:– Baseline noise (last session: “2 SD Method”)– The true EMG signal
Ensemble average - a mean of mean segments across subjects or trials
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Variability Analysis.1 Reproducibility of
recording electrodes (e.g., Δ’s in skin resistance; number of motor units sampled) with repeated measures designs is problematic– Within subjects (e.g.,
over several days)
– Between subjects
Report EMG amplitude (e.g., mean amplitude or integral - next session) as a percentage of a baseline MVIC
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Variability Analysis.2
Variability assessment of EMG will document reproducibility/consistency– SD: measure of dispersion about the mean
stated in units of interest (mV)– CV: describes dispersion of a group mean as a
percentage– SE: low SE argues sample mean is a good
estimate of the population mean
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Root Mean Square (RMS)
One of several methods of quantifying EMG output (in mV) using– Hardware
or
– Software The “effective” value (quantity) of the EMG
signal (i.e., not an average) Measures electrical power A form of linear envelope procedure
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RMS Value Reflects
Motor unit– Firing rates– Duration– Velocity of the electrical signal
Electrode configuration Instrument (amplifier characteristics)
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RMS Procedure
Individual amplitudes are squared A mean of the squared amplitudes is
calculated Square root is calculated
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RMS - Time Constant Selected
Hardware
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RMS - Time Constant Selected
Hardware Software
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RMS - Time Constant
Should be consistent with the nature of the activity being performed– Slow movement
» Use a longer time
– Fast movement» Use a shorter time
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Reference Sources
Gitter, A.J., & Stolov, W.C. (1995). AAEM minimongraph #16: instrumentation and measurement in electrodiagnostic medicine-part I. Muscle & Nerve, 18, 799-811.
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Reference Sources
Soderberg, G.L., & Knutson, L.M, (2000). A guide for use and interpretation of
kinesiologic electromyographic data. Physical Therapy, 80, 485-498.
Winter, D.A. (1990). Biomechanics and motor control of human movement (2nd Ed). New
York: John Wiley & Sons, Inc., 191-212.
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