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Electromyography: Processing. D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada. Types of Signal Processing . Raw (with or without band-pass filtering) Full-wave rectified (absolute value)

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electromyography processing
Electromyography: Processing

D. Gordon E. Robertson, PhD, FCSB

Biomechanics Laboratory,

School of Human Kinetics,

University of Ottawa, Ottawa, Canada

types of signal processing
Types of Signal Processing
  • Raw (with or without band-pass filtering)
  • Full-wave rectified (absolute value)
  • Averaged or root-mean-square (RMS)
  • Linear envelope
  • Ensemble-averaged
  • Integrated EMG (iEMG)
  • Frequency or power spectrum (Fourier)
  • Fatigue analysis (sequential Fourier)
  • Amplitude probability distribution function (APDF) and CAPDF
  • Conduction velocity
  • Wavelet transform

Biomechanics Laboratory, University of Ottawa

raw emg
Raw EMG
  • wide frequency spectrum (20-500 Hz)
  • most complete information
  • needs 1000 Hz or greater sampling rates
  • requires large memory storage
  • difficult to determine “levels” of contraction
  • bursts of activity and “onset times” may be determined from this signal
  • best for examining problems with recording
  • following slides show some errors that can be detected from the raw signal

Biomechanics Laboratory, University of Ottawa

errors when recording emgs

heart rate detected

Errors when Recording EMGs
  • “clean” signal
  • ECG crosstalk

Biomechanics Laboratory, University of Ottawa

ecg crosstalk
ECG Crosstalk
  • ECG crosstalk occurs when recording near the heart (ECG has higher voltages then EMG)
  • EEG crosstalk when near scalp (rare)
  • difficult to resolve
    • use right side of body (away from heart)
    • move electrodes as far away from heart as possible
    • “signal averaging” (average many trials)
    • indwelling electrodes

Biomechanics Laboratory, University of Ottawa

muscle crosstalk
Muscle Crosstalk
  • one muscle’s EMG is picked up by another muscle’s electrodes
  • can be reduced by careful electrode positioning or double differential amplifier
  • can be determined by cross-correlation
  • difficult to distinguish crosstalk from synergistic contractions, however, biarticular muscles have “extra” bursts of activity compared to monoarticular muscles (thus crosstalk is not a problem)

Biomechanics Laboratory, University of Ottawa

errors when recording emgs7

60 Hz noise

baseline not at zero volts

Errors when Recording EMGs
  • line (AC) interference
  • DC-offset or DC-bias

Biomechanics Laboratory, University of Ottawa

solutions
Solutions

To interference (line AC and radio frequency RF etc.)

  • Keep away from fluorescent lighting
  • Keep away from large electrical devices and power cords (especially leads and cabling)
  • Use room lined with grounded conductive material
  • Keep leads short and braided (vs. radio)
  • Use preamplified electrodes (signal is stronger)
  • Use extremely narrow “notch filter” in post processing (e.g., 59.5-60.5 Hz)

For DC-offsets (telemetry systems often have DC-offsets)

  • Use a good ground electrode over electrically neutral area
  • Use high-pass filter (5–10 Hz) to remove in post-processing

Biomechanics Laboratory, University of Ottawa

errors when recording emgs9

electrodes were struck

clipped at +/–0.5 V

Errors when Recording EMGs
  • movement artifact
  • amplifier saturation (+/–0.5 V)

Biomechanics Laboratory, University of Ottawa

solutions10
Solutions

To movement artifacts

  • Affix leads to subject (tape, wrap, webbing)
  • Prevent electrodes from being struck (use lateral muscles)
  • Avoid rapid motions
  • Use strong high-pass filter in post-processing

Amplifier saturation

  • Test with maximal contractions before recording
  • Reduce gain if peaks and valleys “top out” or “bottom out”
  • Use larger range A/D converter (+/–10 V vs. +/–5 V)

Biomechanics Laboratory, University of Ottawa

full wave rectified emg
Full-wave Rectified EMG
  • same as taking the absolute value of the raw signal
  • mainly used as an intermediate step before another process (e.g., averaging, linear envelope, and integration)
  • can be done electronically in real-time

Biomechanics Laboratory, University of Ottawa

sample emgs
Sample EMGs
  • raw EMG (band-passed filtered, 20-500 Hz)
  • full-wave rectified

Biomechanics Laboratory, University of Ottawa

averaged emg
Averaged EMG
  • simple to compute
  • can be done in real-time
  • averaged EMG is a “moving average” of a full-wave rectified EMG
  • must select an appropriate “window width”that changes with sampling rate
  • easy for determining levels of contraction

Biomechanics Laboratory, University of Ottawa

sample averaged emg
Sample Averaged EMG
  • raw EMG (1010 Hz sampling rate)
  • averaged EMG (moving average, 51 points)

Biomechanics Laboratory, University of Ottawa

linear envelope emg
Linear Envelope EMG
  • requires two-step process: full-wave rectification followed by low-pass filter (4-10 Hz cutoff)
  • can be done electronically (but adds a delay)
  • reduces frequency content of EMG and thus lowers sampling rates (e.g., 100 Hz) and memory storage
  • easy to interpret and to detect onset of activity
  • can be ensemble-averaged to obtain patterns
  • difficult to detect artifacts
  • useful as a control (myoelectric) signal

Biomechanics Laboratory, University of Ottawa

sample le emg

can have a time lag

Sample LE-EMG
  • raw (band-passed filtered) EMG
  • linear envelope EMG (cutoff 4 Hz)

Biomechanics Laboratory, University of Ottawa

ensemble averaged emg
Ensemble-Averaged EMG
  • usually applied to cyclic activities and linear envelope EMGs
  • requires method for identifying start of a cycle or start and end of an activity
    • foot switches or force platforms can be used for gait studies
    • microswitches, optoelectric, or electromagnetic sensors for other activities
    • can also use a threshold detector of a kinematic or EMG channel
  • each “cycle” of activity must be time normalized

Biomechanics Laboratory, University of Ottawa

ensemble averaged emg cont d
Ensemble-Averaged EMG cont’d
  • amplitude normalization is often done
    • to maximal voluntary contraction (MVC)
    • to submaximal isometric contraction
    • to EMG of a functional activity (e.g., holding a load)
  • mean and standard deviations for each proportion of cycle are computed
  • mean and s.d. or 95% confidence interval may be presented to show representative contraction during activity cycle
  • easier to make comparisons among subjects
  • “grand” ensemble-averages (average of averages) for comparisons among several experimental conditions

Biomechanics Laboratory, University of Ottawa

ensemble averages from squat lift

mean +/– S.D.

abscissa must be normalized to % cycle

ordinate may also be normalized

Ensemble-Averages from Squat Lift

Biomechanics Laboratory, University of Ottawa

integrated emg iemg
Integrated EMG (iEMG)
  • important for quantitative EMG relationships (EMG vs. force, EMG vs. work)
  • best measure of the total muscular effort
  • useful for quantifying activity for ergonomic research
  • various methods:
    • mathematical integration (area under absolute values of EMG time series)
    • root-mean-square (RMS) times duration is similar but does not require taking absolute values
    • electronically (see next page)

Biomechanics Laboratory, University of Ottawa

electronically integrated emg
Electronically Integrated EMG
  • always requires full-wave rectification
  • various methods:
    • simple time integration (eventually saturates amplifier)
    • integration and reset after a fixed time interval
    • integration and reset after a particular value is reached
  • cannot recognize artifacts, noise become included
  • especially important to first remove DC-offsets
  • must compute amount of iEMG from amplitude or differences between 2 amplitudes

Biomechanics Laboratory, University of Ottawa

sample integrated emg

notice units are mV.s

read total iEMG from curve (i.e., 320 mV.s)

Sample Integrated EMG
  • raw (band-passed filtered) EMG
  • integrated EMG (over contraction)

Biomechanics Laboratory, University of Ottawa

other iemgs

add each peak to get total IEMG

notice units are mV.s

multiply number of peaks by 20 mV.s

Other iEMGs
  • integrate after preset time (0.1 s)
  • integrate after preset voltage (20 mV.s)

Biomechanics Laboratory, University of Ottawa

frequency spectrum
Frequency Spectrum
  • useful for determining onset of muscle fatigue
  • mean or median frequency of spectrum in unfatigued muscle is usually between 50–80 Hz
  • as fatigue progresses fast-twitch fibres drop out, shifting frequency spectrum to left (lowering mean and median frequencies)
  • mean frequency is less variable and therefore is better than median
  • useful for detecting neural abnormalities

Biomechanics Laboratory, University of Ottawa

sample power spectrum

gradual increase to >95% after 200 Hz

median frequency approx. 70 Hz

Sample Power Spectrum
  • flexor digitorum longus (MVC)

Biomechanics Laboratory, University of Ottawa

fatigue analysis
Fatigue Analysis
  • essentially a series of frequency analyses
  • select duration of window (1 to 5 s)
  • can overlap intervals to increase resolution
  • usually normalized to percentage of initial mean or median frequency
  • mean frequencies are less variable than median
  • need to decide a threshold for when fatigue occurs (i.e., fatigue has occurred when mean or median frequency is below a threshold)

Biomechanics Laboratory, University of Ottawa

sample fatigue analysis

gradual decline of mean and median frequencies

medians are more variable

Sample Fatigue Analysis
  • erector spinae over 60 seconds (50% overlap)

Biomechanics Laboratory, University of Ottawa

amplitude probability distribution function apdf capdf
Amplitude Probability Distribution Function (APDF & CAPDF)
  • developed by Hagberg & Jonsson for ergonomics research (Ergonomics, 18:311-319)
  • EMG is amplitude normalized to %MVC then sampled to compute frequencies of various amplitudes, usually for long durations (hours)
  • Cumulative APDF is calculated to compute three thresholds:
    • 10%tile < 2–5% MVC for level of rest
    • 50%tile < 10–14% MVC for work load
    • 90%tile < 50–70% MVC for heavy contractions

Biomechanics Laboratory, University of Ottawa

sample apdf capdf

90%tile =52%MVC

50%tile =8%MVC

10%tile =2%MVC

Sample APDF & CAPDF
  • neck flexor (only 5 minutes)

Biomechanics Laboratory, University of Ottawa

muscle fibre conduction velocity
Muscle Fibre Conduction Velocity
  • requires two amplifiers and three electrodes
  • electrodes are arranged in a line over a known distance (15 mm)
  • middle electrode connected as ground to both amplifiers
  • divide distance between electrodes by time difference between similar peaks (Andreassen & Arendt-Neilsen, J Physiology, 319:561-71, 1987)

Biomechanics Laboratory, University of Ottawa

wavelet analysis
Wavelet Analysis
  • decomposition of EMG time series into a time-frequency space, to determine the dominant modes of variability and their temporal changes
  • figure shows EMG signal and

its wavelet transform (SIMI)

  • used to “de-noise” EMG signals,

to detect fatigue and for feature

extraction

Biomechanics Laboratory, University of Ottawa

other techniques
Other Techniques
  • auto-correlation (correlate signal with itself shifted in time, gives signal characteristics)
  • cross-correlation (correlate signal with another EMG signal, tests for crosstalk)
  • zero-crossings (the more crossings the greater the level of recruitment)
  • spike (peak) counting (number of spikes above a threshold)
  • single motor unit detection
  • double differential amplifier (velocity of propagation)

Biomechanics Laboratory, University of Ottawa