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  • SUMMARY: Active Electrode Advice Needed

    Thanks very much to all of those who took the time to address my question.
    I received a lot of good advice, which has solved my problem, without the
    need for active electrodes. Below I list (1) my original question, (2) the
    result I obtained when I tried the different suggested solutions & (3) the
    individual responses I received.

    To answer a question asked to me by a few of the people who responded: YES,
    I am using differential amplifiers. Also, I should have mentioned that the
    data was hardware high pass filtered at 10Hz (& low pass filtered at 1kHz)
    as it was collected.
    *********************************************
    (1) MY ORIGINAL QUESTION

    I have been collecting EMG from the Soleus muscle while subjects walk on a
    treadmill. On some subjects there is an oscillation in the EMG baseline
    immediately around the time of heel strike, which is large enough to make
    the EMG level useless. The electrodes are firmly attached and sway of the
    wires is minimized as much as I see is possible. I have not used active
    electrodes before, but I assume that one of their purposes is to boost the
    EMG signal well beyond this noise level.
    I use standard passive disc electrodes, Grass Amplifiers, and Cambridge
    Electronic Designs A/D system.

    So here are three questions:
    1) Will the use of active electrodes increase the signal to noise
    (movement artifact) ratio, and so eliminate my problem of the baseline
    oscillation?
    2) What active electrodes are commercially available to be added to an
    existing system such as mine (i.e., feed a signal into my Grass amps).
    3) Any other comments or suggestions people have?


    *********************************************
    (2) THE RESULT I OBTAINED WHEN I TRIED THE DIFFERENT SUGGESTED SOLUTIONS.

    My conclusions are:

    (A) While I thought I was firmly attaching the electrodes, they were not
    firm enough. Increasing the tension on the tape & increasing tape length
    alone was sufficient to largely eliminate my problem (while using the 10 hz
    high pass filter I had in place).

    (B) The use of a small plate (very firmly attached) to keep the
    interelectrode distance constant, as suggested in some responses, was able
    to further reduce the low frequency noise component (as indicated by power
    spectrum analysis) compared to the result with firm tape alone.

    (C) Future data collection will include on line power spectrum analysis to
    check for lower freq noise so problems can be identified and corrected
    immediately.


    (3) THE RESPONSES I RECEIVED

    *********************************************
    Date: Wed, 27 May 1998 17:05:37 +0200
    From: Armin Kibele
    Organization: Institut fr Sport und Sportwissenschaft
    MIME-Version: 1.0
    To: Gordon Chalmers
    Subject: Re: Active Electrode Advice Needed
    Status:

    Before buying new expensive electrodes try the following:

    1) Tap the body of your electrodes and check the emg signal. Sometimes
    electrode paste does create funny artifacts. If you get a response
    without any muscular activity, something wrong with the attachment of
    the electrode via electrode paste or with the attachment of the cables
    to the skin.
    2) Attach all loose electrode cable firmly to the skin.
    3) Make sure, your system is properly ground connected.


    *********************************************
    Date: Wed, 27 May 1998 14:03:05 -0300 (ADT)
    From: Dave Grimshire
    To: Gordon Chalmers
    Subject: Re: Active Electrode Advice Needed
    MIME-Version: 1.0
    Status:

    Gordon,
    I'm no expert but this sounds like artifact noise. It occurs when wires
    move through the earths magnetic field. The solution is to make sure your
    wires move as little as possible. Also keep them away from magnetic fields
    produced by motors etc.
    Hopefully you are collecting using a differential amplifier. This means
    that signals generated on the two input wires will be ignored while
    signals only on one of the input leads will be amplified. Hope this helps.

    Dave Grimshire
    Technician
    School of Health and Human Performance
    Dalhousie University


    *********************************************

    I saw your message on EMG artifact and have a couple of comments. However,
    I'm not completely disinterested since my company makes and sells EMG
    preamplifiers. I've been installing gait analysis systems with EMG and
    force plates for quite a while so I've seen most of these problems from the
    users end...

    My guess it that there are a couple of possible causes for the artifact that
    you're seeing - both, as you guess, deriving from the use of passive
    electrodes:
    1. The relatively high impedance of the passive electrodes may change
    substantially as the subjects heel hits the treadmill and this change of
    impedance appears as artifact.
    2. The leads connecting the electrodes to the Grass Amplifiers are probably
    moving suddenly as the subject strike the treadmill. The leads are moving
    in an electro-magnetic field and generate EMF which appears as artifact.

    You ask "Will the use of active electrodes increase the signal to noise
    (movement artifact) ratio, and so eliminate my problem of the baseline
    oscillation?"

    In general you should find that an active electrode will perform better than
    the passive electrodes since the signal is amplified close to the source and
    the majority of the cable to the recording system is carrying a high level
    signal (~20-100mV) at a low impedance (400-1000 ohms). Currently your EMG
    cables to the Grass amplifiers are probably carrying a signal in the 1-2mV
    range with an impedance of 10-60,000 ohms.

    Note however that EMG pre-amplifiers are subject to a couple of constraints:
    Most EMG preamplifiers (a.k.a. active electrodes) come pre-packaged in a
    hard plastic shell with stainless steel sensor surfaces - it's very
    important that the pre-amplifier is well taped to the body so that the
    sensor do not come off the skin surface during movement. If they do come
    off the surface, or change the contact impedance, then you're no better off
    then before.
    You can attach passive electrodes (disposable EKG electrodes work well) to
    the sensor surfaces of many preamplifiers which gives you the best of both
    worlds (electrodes that stay put and preamplifiers) at the cost of a little
    more mess and complexity. This is really the only way to use pre-amplified
    electrodes on upper-body muscles since they're difficult to attach to the
    skin if you can't wrap tape around a limb.

    Since the preamplified electrodes boost the EMG signal considerably you're
    likely to have problems connecting them directly to the Grass amplifiers.

    There's a list of various EMG preamp and other gait related manufacturers on
    our links page at www.emgsrus.com

    Good Luck,
    Edmund Cramp,
    Motion Lab Systems, Inc.
    4326 Pine Park Drive,
    Baton Rouge, LA 70809 USA
    +1 504 928-4248 (voice, 2 lines)
    +1 504 928-0261 (fax)
    My email address is eac@emgsrus.com
    For information about Motion Lab Systems please visit our web site at
    http://www.emgsrus.com
    *********************************************
    From: Brian Davis
    To: "'Gordon Chalmers'"
    Subject: RE: Active Electrode Advice Needed
    Date: Wed, 27 May 1998 13:28:57 -0400
    Mime-Version: 1.0
    Status:

    Hi Gordon

    When Jeannie McCrory was doing her Ph.D. at Penn State university (in
    the Center for Locomotion Studies) she had problems with static
    electricity building up on the treadmill belt. This interferred with her
    EMG data. I think they solved the problem why lightly wetting the
    belt.

    Regards, Brian Davis

    *********************************************
    Date: Wed, 27 May 1998 13:29:46 -0400 (EDT)
    From: James Dowling
    To: Gordon Chalmers
    Subject: Re: Active Electrode Advice Needed
    MIME-Version: 1.0
    Status:

    Hello Gordon,
    Active electrodes will increase the signal to movement artifact ratio
    but it may not increase it enough for your application. I would suggest a
    highpass filter of at least fifth order with a cutoff frequency of 10 Hz
    as the best initial solution to your problem. I have very good active
    electrodes (DELSYS) and I sometimes have to filter the data anyway. If
    you have trouble implementing a suitable highpass filter for EMG, let me
    know and I can send you a copy of mine.

    *********************************************
    Mime-Version: 1.0
    X-Sender: glg@acs-mail.bu.edu
    Date: Wed, 27 May 1998 13:39:17 -0400
    To: Gordon Chalmers
    From: "Gerald L. Gottlieb"
    Subject: Re: Active Electrode Advice Needed
    Status:


    >1) Will the use of active electrodes increase the signal to noise
    >(movement artifact) ratio, and so eliminate my problem of the baseline
    >oscillation?
    >
    In principal they should be increasing the input inpedance and improving
    common mode rejection.

    >2) What active electrodes are commercially available to be added to an
    >existing system such as mine (i.e., feed a signal into my Grass amps)?
    >
    I have been useing Delsys (formerly liberty mutual) and am pleased with
    their performance.

    >3) Any other comments or suggestions people have?
    >
    The key question is whatis the bandwidth of your amplified signal? It
    sounds like you have the high-pass cut-off too low. It should be above 20
    hz and 50 hz is better. There is no EMG signal below there, just
    mechanical artifact of various sources. getting the right band pass might
    solve the problem without changing electrodes.
    __________________________________________________ ___
    | Gerald Gottlieb (617) 353-8984 beauty |
    | NeuroMuscular Research Center 353-9757 is |
    | Boston University fax 353-5737 truth |
    | 44 Cummington St. etc, etc, etc. |
    | Boston MA 02215 /\ http://nmrc.bu.edu/MCL/glg.html |
    |_______________________/\ / \ /\_______________
    |

    *********************************************
    X-Sender: wdarling@blue.weeg.uiowa.edu
    Date: Wed, 27 May 1998 13:17:10 -0500
    To: Gordon Chalmers
    From: Warren Darling
    Subject: Re: Active Electrode Advice Needed
    Mime-Version: 1.0
    Status:

    I have some suggestions after your questions below. Good Luck.

    Warren Darling

    At 09:14 AM 5/27/98 -0700, you wrote:
    > I have been collecting EMG from the Soleus muscle while subjects walk on
    >a treadmill. On some subjects there is an oscillation in the EMG baseline
    >immediately around the time of heel strike, which is large enough to make
    >the EMG level useless. The electrodes are firmly attached and sway of the
    >wires is minimized as much as I see is possible. I have not used active
    >electrodes before, but I assume that one of their purposes is to boost the
    >EMG signal well beyond this noise level.
    > I use standard passive disc electrodes, Grass Amplifiers, and Cambridge
    >Electronic Designs A/D system.
    >
    >So here are three questions:
    >1) Will the use of active electrodes increase the signal to noise
    >(movement artifact) ratio, and so eliminate my problem of the baseline
    >oscillation?

    It should definitely help reduce the problem, if not eliminate it.

    >
    >2) What active electrodes are commercially available to be added to an
    >existing system such as mine (i.e., feed a signal into my Grass amps)?
    >
    There are a number of active electrodes available, some do not even need
    your Grass amps because they provide an onsite amplification of 1,000
    (e.g., Delsys electrodes).

    >3) Any other comments or suggestions people have?

    You can try twisting together the wires of the two electrodes over soleus.
    This sometimes helps reduce such movement artifact.


    *********************************************
    Date: Wed, 27 May 1998 14:23:42 -0500 (CDT)
    X-Sender: runtech@popd.ix.netcom.com (Unverified)
    Mime-Version: 1.0
    To: Gordon Chalmers
    From: RUN Technologies
    Subject: Re: Active Electrode Advice Needed
    X-Attachments: D:\RUNCUST\CHALMER1.GIF;
    Status:

    Dear Dr. Chalmers,

    RUN Technologies produces a software package named Datapac III that may be
    able to help you. Datapac III is a collection of software modules designed
    for many different applications. One of them is a signal processing module
    that, among other functions, offers a high pass filter that can eliminate
    low frequency artifact from your signal. The filter has adjustable cutoff
    and rolloff characteristics, so you can shape it to exactly suit your needs.
    To illustrate its use, I have attached a gif graphics file that shows an EMG
    signal containing a significant amount of low frequency movement artifact,
    both before and after the application of a 20 Hz high pass filter with a
    steep rolloff (approx 50 dB/oct).

    In addition to its own acquisition capabilities, Datapac III can accept
    almost any form of data file, whether it is in ASCII, binary, or floating
    point format.
    Thus, it may offer a very cost-effective solution for you. If you like, I
    would be happy to demonstrate what it can do with your data files. Please
    let me know if you would like to set something up. Additional information
    about Datapac III is also available on our web site (http://www.runtech.com)
    Thank you.

    Sincerely,

    Richard W. Lambert




    Attachment converted: Macintosh HD:CHALMER1.GIF (GIFf/JVWR) (000203CD)
    RUN Technologies
    25622 Rolling Hills Road
    Laguna Hills, CA 92653 USA
    Phone/Fax: (949) 348-1234
    email: Main@runtech.com
    web site: http://www.runtech.com

    Please visit our web site for product information and prices, as well as the
    latest program updates and demonstration software.

    *********************************************
    Date: Thu, 28 May 1998 09:12:36 +0800 (WST)
    Mime-Version: 1.0
    To: Gordon Chalmers ,
    From: iallison@info.curtin.edu.au (Garry Allison)
    Subject: Re: Active Electrode Advice Needed
    Status:

    >Dr Gordon Chalmers - re low frequency artefact around heel strike

    It still seems that there is a low frequency artefact in the signal. In the
    simplest sense it could be movement artefact yet you have noted is unlikely
    from observation of the leads etc - it could be skin movement (or the
    muscle under the skin).
    I'm not clear on how you are collecting & processing the data.- here are
    some things to consider.
    If you have access to the raw data then you may wish to consider the power
    spectrum to see if it is truely a low frequency problem - then it may be
    advantageous to increase your high pass cut off filter frequency - Lower
    your low pass to 1/3rd Sampling Freq.

    Another point that may not be raised by others and could be worth an
    outside guess...
    Some digital processing packages offer the ability to increase the order of
    the filter - this has the advantage of making the cut-off frequency profile
    much sharper. The disadvantage is that they become unstable and develop a
    ripple effect in the processed data. This is easily seen if you run a
    square pulse through the processing program and is exaggerated with "high
    impulse" data.
    The fact that the ripple is just before heal strike is a little more
    difficult to explain - however the digital processes pass the data in both
    directions so it is possible to have it before and after the actual
    impulse.

    Finally, if you are creating a linear envelop (leaky integrators) before
    the AD conversion the raw data may be clipping (too high a gain) which is
    then mis-interpreted as (intermittent - corresponding with expected high
    amplitudes) low frequency noise of large amplitude wiping out a signal with
    was previously seen to be normal.

    Hope this helps
    cheers
    Garry.

    ------------------------------------------------------------------------
    Dr Garry T Allison Lecturer in Functional Rehabilitation,
    School of Physiotherapy, email:iallison@info.curtin.edu.au
    Curtin University of Technology, Tel. +61 8 9266 3648
    Selby Street, Shenton Park, Fax. +61 8 9266 3636
    Western Australia 6008
    http://www.curtin.edu.au/curtin/dept/physio/pt/staff/allison/
    ================================================== ======================

    *********************************************
    Date: Sat, 04 Jan 1997 09:34:25 +0800
    From: JerJunn Luh
    Reply-To: d81068@me.ee.ntu.edu.tw
    MIME-Version: 1.0
    To: Gordon Chalmers
    Subject: Re: Active Electrode Advice Needed
    Status:

    >

    Dear Sir:

    I used the active electrodes which made by the Motion Control Inc. before.
    Now I am developing a new type of active electrodes in our Lab.
    The followings are my opions about active electrodes:


    > So here are three questions:
    > 1) Will the use of active electrodes increase the signal to noise
    > (movement artifact) ratio, and so eliminate my problem of the baseline
    > oscillation?

    Yes, the active electrodes can improve the S/N ratio. The EMG signals canbe
    amplified by the pre-amplifier which attached on the electrodes. That means
    the noise from cable movement could be reduced. The passive electrodes can
    apply on the skin more firmly. The motion artefact between the skin and the
    passive electrode disc
    is less than the artefact between the skin and the active electrode. This
    effect could
    be reduced when you apply the conductive jelly between the electrode and skin,
    and apply a constant
    pressure on the electrodes.


    > 2) What active electrodes are commercially available to be added to an
    > existing system such as mine (i.e., feed a signal into my Grass amps)?
    >

    You may contact with the Motion Control Inc.http://www.utaharm.com

    > 3) Any other comments or suggestions people have?

    The noise from the cable moving is hard to eliminate. Generally we have to
    used a high pass filterto reduce the effect of this problem. Some frequency
    band (DC-10 or 20) shoud not be used in your
    study. I am sorry that I do not have the useful reference on my hand now.


    Sincerely yours
    Jer-Junn Luh
    Ph.D. candidate,
    Dept. Electrical Eng.
    National Taiwan Univ.
    Taipei, Taiwan.


    *********************************************
    X-Sender: tiburon@pop.flash.net
    Mime-Version: 1.0
    Date: Wed, 27 May 1998 23:19:55 -0500
    To: chalmers@cc.wwu.edu
    From: James Carollo
    Subject: EMG Active Electrodes
    Status:

    The low frequency base-line shift you are experiencing is motion artifact
    induced by a mechanical displacement of one recording electrode relative to
    the other in the bipolar pair, as a direct result of the impact wave
    propagating up the posterior calf. The secret to eliminating this is to
    link your two electrodes together (like a bar electrode), and securely
    fastening the entire low mass bar assembly to the calf. The electrodes
    should be close together (2 to 4 cm, center-to-center), have small active
    area (1cm or less, to reduce crosstalk), and be fixed rigidly to the bar.

    Linking the electrodes together increases the probability that when the
    impact wave arrives, both electrodes will be displaced together and by the
    same amount. If you use a differential recording amplifier with a high
    Common mode rejection (CMRR), the unavoidable displacement of the bar will
    produce the same voltage change in both electrodes simultaneously, and the
    amplifier will ignore it.

    Active electrodes generally show lower motion artifact, but it is usually
    because they include the 2 EMG electrodes in the assembly, not necessarily
    because they are preamplified. The amplifiers do increase the signal/noise
    ratio, but the net effect is to reduce high frequency or 60 cycle noise
    induced from the environment. This type of noise can generally be removed
    just as well by twisting the 2 lead wires together soon after they leave
    the electrode, continuing all the way back to the main amplifier.

    I'd try these tips before investing in active EMG electrodes that may not
    be necessary.

    Best Regards,
    -JJC

    +----------------------------------------------------------------------+
    | James Carollo, Ph.D., P.E. james.carollo@email.swmed.edu
    | Senior Research Scientist
    | Mobility Research and Assessment Laboratory
    | The University of Texas
    | Southwestern Medical Center at Dallas
    | 9705 Harry Hines Blvd., Suite 105
    | Dallas, TX USA 75208 (214) 351-2041
    +----------------------------------------------------------------------+

    *********************************************
    Subject: Re: Active Electrode Advice Needed
    Date: Thu, 28 May 98 13:41:58 -0000
    From: Ippei Akiya
    To: "Gordon Chalmers"
    Mime-Version: 1.0
    Status:

    Hello Dr. Gordon Chalmers
    I have a experience that record EMG from soleus during jumping. In that
    time, I did not seem the problem of baseline oscillation. Explaining my
    method, I used a passive disc electrodes (5mm diameter) and 10mm inter
    electrodes distance. I think that EMG in narrow inter electrodes distance
    condition include smaller artifacts, since a changing of relative
    electrode positions is little.
    And I was setting a filter of 0.03sec time constants and 500Hz high cut
    filter. It is standard setting, I think you know it.

    Thanks
    Ippei Akiya
    Nagoya University

    *********************************************
    X-Sender: gdeluca@90.0.0.1
    Date: Thu, 28 May 1998 10:26:31 -0400
    To: chalmers@cc.wwu.edu
    From: Gianluca De Luca
    Subject: Active Electrodes Needed
    Mime-Version: 1.0
    Status:

    In response to your recent posting on the Biomch newsgroup:

    You are quite correct in hypothesizing that active electrodes increase the
    signal to noise ratio. It is important to used effective electrode-skin
    attachment methods. There are several commercial interfaces which are
    specifically designed to remove motion artifact. Another point to consider
    is the bandwidth of the system you are using. Motion artifact tends to
    appear in the lower frequencies. If you limit the lower corner of your
    bandpass to something like 20 Hz, much (not all) of this noise will be
    eliminated. Naturally, this would be a problem if you are interested in
    viewing the spectrum below 20 Hz.

    I suggest that you look at the Delsys Inc. home page (www.delsys.com). You
    may find what you are looking for. The electrodes advertised at this site
    can be used with the amplifiers you described. Adhesive interfaces
    designed specifically for these electrodes are also available.

    Hope this helps.

    Gianluca De Luca

    *********************************************
    From: GSRASH01@ULKYVM.LOUISVILLE.EDU
    Date: 28 May 1998 13:23:19 EDT
    To:
    Subject: EMG
    Status:


    The active electrode should help your signal to noise & decrease your movement
    artifact, but it doesn't eliminate it. Use a high pass filter at 15Hz & the
    movement artifact do to movement of the electrode will go away. If you
    don't have software to do it, we have some shareware my lab programer wrote
    which will do this & much more with the EMG.

    *********************************************
    > So here are three questions:
    > 1) Will the use of active electrodes increase the signal to noise
    > (movement artifact) ratio, and so eliminate my problem of the baseline
    > oscillation?

    No. Active electrodes can greatly reduce noise and mains (60 Hz)
    interference, not electrode artifacts. But see 3)

    > 2) What active electrodes are commercially available to be added to an
    > existing system such as mine (i.e., feed a signal into my Grass amps)?
    ?>
    > 3) Any other comments or suggestions people have?
    Electrode artifacts are low frequency. The main solution is to
    raise the high-pass cut-off frequency of the unprocessed EMG, try 10
    Hz. Myself I use 20 or 30 Hz, which gives really artifact free EMGs,
    but the reviewers of some papers don't like it.

    At Hof
    Vakgroep Medische Fysiologie
    University of Groningen
    Bloemsingel 10 |----/\/\/\/----| |\
    NL-9712 KZ GRONINGEN
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