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The EMG Signal Artifact & Interference Sampling Rate Signal References Signal Processing.1.

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Presentation on theme: "The EMG Signal Artifact & Interference Sampling Rate Signal References Signal Processing.1."— Presentation transcript:

1 The EMG Signal Artifact & Interference Sampling Rate Signal References Signal Processing.1

2 EMG Noise u A form of artifact –Interference with signal recording u Obscures a clean signal –Electromagnetic sources from the environment may overlay or cancel the signal being recorded from a muscle »Especially problematic when the interfering frequency is the same as being recorded from muscle u Example: 60 Hz from power lines vs. 20 - 125 Hz slow twitch motor units

3 Sources of Noise (Interference) u Driver amplifier u Poor quality –High CMRR > 100,000 u Broken u Ground fault –Amps not tied together –Ground prong on cable »Broken »Absent u Loose cable connection u Loose controls

4 Sources of Noise (Interference) u Driver amplifier u Electrodes u Pre-amp faulty u Broken/cracked u Poor skin prep –Increases resistance –Attenuates signal u Poor electrode-to-skin contact –Electrode tipped –No/too little conducting gel/paste

5 Sources of Noise (Interference) u Driver amplifier u Electrodes u Small electrodes may cause poor contact u Different electrode disk impedance u Fixation failure over time –Tape loosens 2 0 to »Movement »Perspiration

6 Sources of Noise (Interference) u Driver amplifier u Electrodes u Cable fatigue –Along length –At connector –Stripped insulation u Poor reference (ground) contact

7 Sources of Noise (Interference) u Driver amplifier u Electrodes u Cable movement artifact u Swinging cables –Especially if un- or poorly-shielded uSwing frequency will probably be under 10 Hz –Slow twitch mus: (20) 70 - 120 Hz

8 Sources of Noise (Interference) u Driver amplifier u Electrodes u Cable movement artifact u Shorter cable minimizes swing u Use shielded cable 1 u Apply shield cables - tie to ground 1 Digi-Key Corp 701 Brooks Ave South Thief River Falls, MN 56701-0677 1-800-344-4539 www.digikey.com

9 Sources of Noise (Interference) u Driver amplifier u Electrodes u Cable movement artifact u Electro-static/- magnetic radiation u Light bulbs –Especially florescent u Motors –AC –Fans –Experiment component u Power lines - 60 Hz u Phone lines u Ethernet cables u Cable dishes

10 Sources of Noise (Interference) u Driver amplifier u Electrodes u Cable movement artifact u Electro-static/- magnetic radiation u Radio waves u AM u FM

11 Cross-Talk u Electrodes over an adjacent muscle pick-up a signal via skin conduction M1 M2

12 Cross-Talk u Visually inspect a tracing (monitor or printout) of a signal –If they have the same shape there is probably cross-talk Muscle 1 Muscle 2

13 Cross-Talk Fixes u Check skin prep u Check skin resistance u Reposition electrodes u Check reference (ground) electrode –Move between electrode sets u Use a narrower OC distance between electrodes, if available

14 Sampling Rate u Number of data points (cycles) collected per unit of time - usually seconds –Example: 1000 cps = 1000 Hertz (Hz) u An adequate sampling rate ensures that whats being recorded is truly representative of the signal

15 Sampling Rate Lost Data Points Sampling rate Baseline Signal Adequately Sampled Signal Under-sampled

16 Consequences - Sampling u Under-sampling u Lost data points u Signal not truly representative –Cant be trusted

17 Consequences - Sampling u Under-sampling u At or over-sampling rate u Signal adequately sampled u With over-sampling more data points are recorded than necessary –Could tax storage capacity

18 Selecting the Sampling Rate The Two Times Rule u Analyze the signal (or movement) and determine the highest possible operating frequency –Example: motor unit frequency range = (10) 70 - 250 Hz u Double the top rate –Sampling rate: 250 Hz x 2 = 500 Hz ~ 1000Hz

19 Sampling at 1000 Hz u For data plotted on a graph sampled at 1000 Hz, each tic on the X-axis is 1msec 1000 msec 1 second

20 Signal Reference (Events) u Event marker stamps the point-in-time (point-in-the range, etc.) from which to start counting –Voltage spike –Concurrent video »Ariel synch method - drop a ball –Electrogoniometer –Torque signal

21 Voltage Spike from Event Marker Event Raw Rectifie d Voltage Spike

22 Correlate EMG Signal with Torque Channel Torque Rectified EMG

23 Signal Processing.1 u Timing - Phase transition –Onset - Offset u Duration OffsetOnsetDuration

24 Phase Transition u Visual assessment of phasic activity 1st 2nd3rd

25 Question: At what (data) point do I start counting? ?

26 Baseline Noise vs. Signal Differentiation u Manual visual identification using a cursor

27 Baseline Noise vs. Signal Differentiation u 2 SD Method –Select a filtered segment of the pre-signal baseline to analyze »Example: 500 points »Zoom-in on baseline –Calculate descriptive statistics for the segment using full-wave rectification »Mean & SD –Double the SD and add to mean value = point where the true signal rises from the baseline

28 Baseline Noise vs. Signal Differentiation Baseline Raw Signal Baseline Rectified Signal 500 pts

29 Reference Sources Soderberg, G.L., Cook, T.M., Rider, S.C., & Stephenitch, B.L. (1991). Electromyographic activity of selected leg musculature in subjects with normal and chronically sprained ankles performing on a BAPS board. Physical Therapy, 71, 514-522. Winter, D.A. (1991). Electromyogram recording, processing and normalization: procedures and consideration. Journal of Human Muscle Performance, 1, 5-15. Soderberg, G.L., & Cook, T.M. (1984). Electromyography in biomechanics. Physical Therapy, 64, 1813-1820

30 Reference Sources DeLuca, C.J. (1997). The use of surface electromyography in biomechanics. Journal of Applied Biomechnics, 13, 135-163. Powers, C.M., Landel, R., & Perry, J. (1996). Timing and intensity of vastus medialis muscle activity during functional activites in subjects with and without patellofemoral pain. Physical Therapy 76, 946-967. Winter, D.A., Fugerlan, A.J. & Archer, S.E. (1994). Crosstalk in surface electromyography: theoretical and practical estimates. Journal of Electromyography and Kinesiology, 4, 15-26.

31 Reference Sources Koh, T.J., Grabiner, M.D. (1993). Evaluation and methods to minimize cross talk in surface electromyography. Journal of Biomechnics, 26(supplement 1), 151-157. Karst, G.M., & Willett, G.M. (1995). Onset timing of electromyographic activity in vastus medialis oblique and vastus lateralis muscles in subjects with and without patellofemoral pain syndrome. Physical Therapy, 75, 813-823 Hodges, P.W., & Bui, B.H. (1996). A comparison of computer- based methods for the determination of onset of muscle contractions using electromyography. Electroencephalography and Clinical Neurophysiology, 101,511-519.

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