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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)
- 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

- 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

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

- 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

baseline not at zero volts

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

Biomechanics Laboratory, University of Ottawa

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

clipped at +/–0.5 V

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

Biomechanics Laboratory, University of Ottawa

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

- 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

- raw EMG (band-passed filtered, 20-500 Hz)
- full-wave rectified

Biomechanics Laboratory, University of Ottawa

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

- raw EMG (1010 Hz sampling rate)
- averaged EMG (moving average, 51 points)

Biomechanics Laboratory, University of Ottawa

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

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

Biomechanics Laboratory, University of Ottawa

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

- 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

abscissa must be normalized to % cycle

ordinate may also be normalized

Ensemble-Averages from Squat LiftBiomechanics Laboratory, University of Ottawa

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

- 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

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

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

- 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

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

- 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

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)

- 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

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

- 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

- 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

- 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

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