No announcement yet.

Summary: techniques to reduce artefact content of EMG data(posted 12 sep 2002)

This topic is closed.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Summary: techniques to reduce artefact content of EMG data(posted 12 sep 2002)

    Thank you to all those who responded. A special thanks to those who offered
    to view sample data.

    Original message posted Thu, 12 Sep 2002 16:05:06 +1000 with subject
    "techniques to reduce artefact content of EMG data" has been successfully
    distributed to the BIOMCH-L list:

    "I am attempting to collect telemetered EMG data during running, but the
    artefact content of the data is overwhelming.

    I have searched the Biomech-L archives for previous postings relating to
    EMG noise content, but all suggestions mentioned in these postings have
    failed to resolve my current problems.

    The data is being captured with a Noraxon TeleMyo 900 8 channel
    telemetered EMG system. This system has a fixed output voltage range (+/-
    2.5 volts when in bipolar mode) and a fixed gain of 2000.

    To date, all measures taken to reduce the noise content of the data have
    failed. Any running, even at the slowest of speeds, causes clipping and
    thus a high frequency artefact which cannot be filtered - rending the data
    useless. Even a comparatively gentle foot 'stomp' causes an artefact of
    amplitude beyond the range of the voltage band and thus clipping.

    When the signal does not contain clipped data (for example: from isolated
    limb movements), filtering is effective, and the quality of the data,
    while not extraordinary, is reasonable. With a fixed output range and
    fixed gain, I can not see any adjustments that can be made to expand the
    voltage band and thus prevent the clipping without actually reducing the
    noise. However, if such adjustments are possible, I would welcome advice
    as to how these adjustments can be made.

    Interestingly, the artefact content of the data is enhanced when fine wire
    electrodes are used, but regardless, the artefact is at levels which
    render the data useless even when surface electrodes are used as per the
    intended use the Noraxon system (thus the fine wire electrodes themselves
    do not appear to be the main/only source of the problem).

    The fine wire electrodes used consist of 75 micrometer teflon coated
    stainless steel with ~1mm bared tips for recording. Custom made connectors
    were used to connect the wire to the Noraxon EMG cables, via an alligator
    clip to allow connection to the electrode wires, and a 'male' press stud
    to fit the 'female' press stud connection of the Noraxon cable.

    All cables were secured to the subject as best as possible to minimize
    movement, and all cables were wound around one another. Channels not in
    use were shorted out with back to back surface electrodes. The cable was
    secured to prevent tension on the input point to the transmitter (which
    was strapped as tightly as possible around the subject's waist) and the
    input point to the electrodes.

    Varying the ground electrode position does not alter the artefact content
    of the recorded signals.

    I would appreciate any suggestions as to how I can reduce the noise
    content of my EMG recordings? I can provide sample data if this will
    assist others in determining the cause of this noise.

    Andrew Chapman

    PhD Candidate
    Department of Physiotherapy
    The University of Queensland
    St. Lucia, 4072, QLD, Australia.


    Department of Physical Therapies
    Australian Institute of Sport
    P.O.Box 176
    Belconnen, 2616, ACT, Australia.

    phone: + 61 2 6214 7943
    fax: + 61 2 6214 7953
    mobile: + 61 438 115 605"

    Summary of responses:

    We currently use the 8 channel MIE telemetry system and don't have artefact
    In the past with a wire based system or for smaller experiments we use Pals
    electrodes which are a steel mesh woven into a cloth pad with a gel surface
    that sticks to the skin very well. The electrodes are available from . I would also try with a non-telemetry system to
    locate further the source of the artefact. In fact trying each part
    individually should help here.

    Hope this is useful,
    Jonathan Norton.
    It sounds like you are getting motion artifact from movement between the
    electrodes and the skin and/or movement of the electrode wires between the
    electrodes and the preamp (you are using "passive" electrodes). With fine
    wire electrodes the problem would be even greater since there is no
    preamplification of the emg signal and movement of the wires would cause
    artifact which would be amplified by the preamps. I havent worked with
    telemetry systems so I dont know if the telemetry may add additional noise.

    I don't know of any sure fire solution. If it is possible you may try to
    increase the cutoff frequency of the highpass filter to take out more
    motion artifact. Active electrodes where the preamp is built into the back
    of the electrode may also help. I think the best you can do with what
    you've got is to try to constrain the movement of any wires and electrodes
    as much as possible, but I guess you've already done that.

    Kelvin J. Chen []
    I encountered similar difficulties when collecting data for my
    dissertation with the same system. Interestingly, I had clean data for
    surface EMG channels, but I was encountering the same impact spikes for
    my single indwelling channel. I had a home-built interface unit for the
    fine-wires, which I ended up replacing with a unit professionally built
    from Run Technologies and that took care of my problem. I came to the
    conclusion that the problem was at the interface between the wires and
    the fine-wire adapter (where they snap together). I don't know how old
    your cables are, but perhaps new cabling will help for all of the
    channels. In general, the Noraxon unit provided clean data during
    running after I fixed that one problem. I don't know if this is
    helpful, but feel free to contact me if you have any questions.

    Kristian M. O'Connor, Ph.D.
    Assistant Professor of Biomechanics
    Department of Human Movement Science
    University of Wisconsin - Milwaukee
    P.O. Box 413
    Milwuakee, WI 53201
    (414) 229-2680
    The main issue in your case is that the gain is too high. 2.5V /
    2000 = 1.25 mV
    For surface EMG (not fine-wire) this might do, but in movement
    there are artifacts of much higher amplitude.
    Can Noraxon provide you with 200x amplifiers?
    The artifacts, which are of high amplitude but low frequency, can
    then be filtered out with a high-pass filter.
    If the output with 200x preamp. is too low for your A/D, apply first
    an analog high-pass filter and the amplify some 10x after the filter.
    An other (but $$) option is to abandon the Noraxon system and
    buy the PORTI system as we use. This system has some very
    clever interference suppression techniques and 22-bits A/D.
    Good luck,
    ************************************************** *****
    At Hof
    Institute of Human Movement Sciences &
    Laboratory of Human Movement Analysis AZG
    University of Groningen
    A. Deusinglaan 1, room 321
    postal address:
    PO Box 196
    Tel: (31) 50 363 2645
    Fax: (31) 50 363 3150
    ************************************************** *******
    Hi Andrew,
    I am sorry you are having such difficulties. Has the unit made the
    trip back to Noraxon? I suspect something major with the electronics, but
    who knows. Can you collect walking data with acceptable results? Is it the
    impacts that cause the clipping? Try leaving the unused channels/wires
    unplugged--perhaps movement of these electrodes is the cause. Use coban or
    some other stretchy tape to bind the wires to the runners legs. Do not wind
    the wires to avoid an induction coil.
    I have seen clipping, but only on a Noraxon system with two analog
    channels designed for A to D (accelerometers, etc.). Never have I seen this
    with EMG. If you don't get it figured out I'd ask Noraxon for a loaner and
    send back the suspect unit.

    Seattle VA Gait Lab

    Michael Orendurff []
    Hi Andrew

    Are your subjects running on a treadmill? Static electricity from the belt
    cause huge artefacts.

    Regards, Brian
    Brian L. Davis, Ph.D.
    Department of Biomedical Engineering (ND20)
    Cleveland Clinic Foundation

    Dear Andrew

    Can you send me any noisy signal to analyze.
    I hope that I can help you!



    Rodrigo LĂ­cio Ortolan []
    Dear Andrew,

    Have you tried using an antialiasing lowpass filter before recording?, you
    stablish the cut off frequency of the filter to the maximum emg frequency
    you want as long as the sampling frequency you are using is at least 5 times
    the desired frequency. This will avoid any higher frequencies to affect the
    data. I am not familiar with that EMG system and dont' know if this filter
    be added. My guess is to place it, if possible between the receiver and
    computer (I am assuming you are using a computer to store your data)

    Try avoiding the alligator clips, the movement between the alligator and the
    electrode is a source of noice

    I hope this helps

    Kind regards


    PS, Do you have more explicit specifications of this emg system? or a
    where they can be found?

    Jose Salazar []
    Hi Andrew.

    I'm interpreting your note to mean that the raw EMG voltages from running
    are so strong that they overload the signal conditioner and lead to

    If this is the case, why not reduce the magnitude of the signals before
    they get to the EMG processing unit?

    The signal path would look like:

    wire electrode--> amplifier with gain 0.5 --> noraxon signal processing unit

    The amplifier gain could be as small as necessary to avoid clipping.
    This could introduce additional noise, and would reduce the signal content
    but if it eliminated the clipping it might be an overall benefit. It might
    also require a custom designed amplifier to make it small and portable, but
    hopefully your university has some good electronics people. A first test
    might be to use a research quality but full size amplifier in the lab and
    see if it works for a foot stomp.

    Paul Ostic

    Paul Ostic []

    My first thought is that the fixed gain of 2000 may be excessive for
    recording EMG during high effort such as running. This would account for
    the clipping. If you plot peak (rectified and smoothed) EMG amplitude vs.
    gait speed, what does this curve look like? Can you verify or predict that
    EMG amplitude will saturate for a certain level of effort, and clip above
    that level? If so, you will need to reduce the gain for high-effort tasks.

    Best wishes,

    Andy Hoffer
    This is a very real problem especially with surface electrodes. If you
    bump a set of electrodes will always get such an artifact. I suggest that
    you try to prevent electrodes from rubbing against clothing etc. and space
    pairs of electrodes far enough apart that they cannot hit each
    other. Another solution might be to glue pairs together so that they move
    as-one or obtain electrodes from Delsys which come paired.

    I assume you are not using a treadmill, which may collect a static
    charge. If so, try to ground the belt, like some trucks or high-speed
    printers, with charge dissipators.

    D. Gordon E. Robertson, Ph.D. []
    Hi Andrew,

    Are you sure the gain is fixed? I'm not sure what model our Noraxon is, but
    the gains are adjustable. There are dip switches inside the transmitter box
    on the waist band that allow us to alter gain.

    There shouldn't be any movement artifact with the Noraxon. I don't know
    what kind of filtering goes on, I imagine it is substantial, because we can
    have quite gross cable movements and still find no artifact present.

    Another thing to look for is a loose electrical connection somewhere. We
    have had occasional problems with the battery pack causing the signal to
    drop in and out. When it comes in, there is a big spike in the signal.

    If you want to send me a text file containing some data you've collected, I
    would be happy to have a look and try to interpret your problem. It's
    really hard to guess the cause without seeing what the trace looks like.

    Are you going to LaTrobe in November? I believe abstracts are due on Monday.


    Peter Sinclair
    Lecturer in Biomechanics
    School of Exercise and Sport Science
    The University of Sydney
    East St Lidcombe NSW 1825
    Phone (02) 9351 9137

    When we used alligator clips to connect to male press
    studs of EMG electrodes we found that considerable
    noise was introduced by the metal-on-metal
    movement between the alligator clip and press-stud.
    This was still present in walking with leads and pre-
    amps securely fastened. This noise was removed by
    simply securing the respective alligator clip to the
    EMG electrode to prevent any movement between the

    Hope this helps

    Allan Carman
    Research Fellow
    School of Physiotherapy
    University of Otago

    Perhaps what you are getting are the result of auto-gain control. And, this
    could be compounded by motion artefacts. My suggestion is that try to repeat
    the process with the muscle steady (isometric) and the load varies suddenly.
    The result of this experiment may shed more light. Let me know.

    I have never used this equipment though.


    Dinesh Kant Kumar []
    Hi Andrew

    We recently upgraded our old Noraxon system (which we could alter the output
    range) and discovered, to our horror, that the new "updated" Noraxon system
    has several features that now make it not nearly as user friendly in
    research (ie fixed output range; an inability to display the output on the
    receiver of more than one channel at a time etc). We have also experienced
    similar types of problems as you have with either extremely small signals
    and clipping. If you would
    like to give me a ring I can chat through some of the option we tried (with
    varying levels of success) to obtain cleaner signals. Let me know via email
    when a good time is to call.


    Julie Steele
    Dear Andrew,

    My name is Peter Konrad and besides my own scientific work here in
    Cologne/Dortmund - Germany, I am associated with Noraxon since 91, now being
    responsible for the software design.

    The Noraxon EMG telemetry system is a very well established standard in
    numerous labs all over the world, so I must conclude that there is a local
    problem in your lab. It may relate to hardware or handling problems.

    I propose to first try the following tests, based on surface EMG:

    1) Check system noise vs external noise

    What is the average baseline noise if you short connect the two electrodes
    of one lead? Too high noise could mean amplifier problems or heavy
    interfering external noise (typically 50 or 60Hz ground noise which can
    easily been check in the EMG power spectrum). Next step: is it constant over
    all the channels you have? If so, it indicates external noise problems.

    2) Noise Constancy

    Assuming that you stay in battery mode(recommended because more stable
    against ground noise interferences), move around (with the complete system),
    check other rooms to verify that you do not struggle against local
    electrical problems (=strong electrical motors, bad electricity conditions
    3) Skin impedance
    Unfortunately you didn't describe your technique of skin preparation
    (surface EMG), which should be checked prior to fine wire use. Did you clean
    and slightly rubber the skin? Did you check the impedance between two
    This could be an important point...
    We do not use fine wire electrodes, but plan to do, so any exchange is
    appreciated! But what we already learned is that it needs a high knowledge
    and training. Obvisiously it is much more difficult to sample clean wire
    EMGs than surface (where we never had a problem).

    4) Records

    You report about high amplitudes "clipped" away by the system signal range.
    Did you properly setup the software A/D settings/Hardware/Device? Did you
    run an FFT to check your power spectrum?

    Anyways, yes I would like check some of your sample data
    Best regards - Peter
    Dr. Peter Konrad
    KinLab - University of Dortmund FB 16 Sports Science Germany.
    End of message

    To unsubscribe send SIGNOFF BIOMCH-L to
    For information and archives: