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SUMMARY: EMG electrodes: sizes, shapes and frequency responses

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  • SUMMARY: EMG electrodes: sizes, shapes and frequency responses

    Dear BIOMCH-L,

    2. April I posted a request for information on the relationships
    between surface EMG electrode geometry and resulting frequency

    Enclosed you will find my original request (rather long, unfortunately)
    and an edited summary of the resonses I received. Thanks to Dr. Dan
    Antonelli and Dr. Peter Kyberd who contributed!
    (Peter Kyberd's contribution was accidentally deleted, and is therefore
    absent in the summary below.)

    Oyvind Stavdahl

    ****** ORIGINAL REQUEST ******

    Hello, BIOMCH-L!

    I'd like to add the following information - and question - to the
    recent EMG electrode discussion/info. exchange:

    We just bought a couple of preamplifier EMG electrodes (MYO 115 - EMG
    Research Electrodes) from Liberty:

    Liberty Mutual Research Center
    Prosthetics Group
    71 Frankland Road
    Hopkinton, MA 01748
    Phone: (508) 435-9061
    Fax: (508) 435-8369
    Orders: (800) 437-0024

    and we're about to buy some more in near future if they turn out to be
    OK. One of the reasons for our choice was the following publication:

    Hogan, N. & Mann, R. W., Myoelectric Signal Processing: Optimal
    Estimation Applied to Electromyography - Part II: Experimental
    Demonstration of Optimal Myoprocessor Performance. IEEE Transactions on
    Biomedical Engineering, Vol. BME-27, No. 7, July 1980.

    Here, Hogan and Mann predict and demonstrate that the distance between
    the electrodes (that is, a differential electrode pair) influences the
    bandwidth of the differential signal, with reduced interelectrode
    spacing resulting in a higher signal bandwidth (this will for certain
    myoprocessor algorithms, such as root-mean-square, result in higher
    signal-to-noise ratio). They also consider the size and shape of each
    electrode, which is also shown to influence the spectrum of the
    differential EMG observed. It seems to me that Liberty's engineers have
    been reading Hogans article....

    (A second reason for our choice was the small size of the
    electrode/preamp. unit: 26.5x17.2x4.8 mm., similar to those used for
    controlling the Boston elbow prosthesis.)

    >From the recent discussion I learn that Andrew W. (Drew) Smith at U. of
    Toronto uses Motion Control's electrodes, which I know have a
    relatively large distance between it's electrodes, and the electrodes
    themselves are also quite large. Furtermore, their ground electrode is
    placed right between the differetial pair, as opposed to Liberty's (and
    several other suppliers') which require an "extra" ground electrode
    with arbitrary placement.

    Can anyone provide expert opinions or personal experience on the

    1. The importance and implications of placing the ground electrode
    BETWEEN the differential pair. Does this influence the observed
    differential EMG signal in any significant (positive and/or negative)
    way? (The only positive result I can think of is a possibly improved
    Common Mode Rejection.)

    2. Larger electrodes cover a larger area and thus measure the EMG of a
    larger number of motor units, while they also apparently low-pass
    filter the signals (according to Hogan&Mann). Does anyone have opinions
    or experience regarding the tradeoffs that have to be made with respect
    to this? Does any formal or informal standard exist with respect to
    electrode geometry? (What were your specific reasons for choosing the
    Motion Control electrodes, Andrew?)

    As usual, I will post a summary (edited) of the responces.

    By the way, I have no commercial interests in either of the products
    mentioned above.

    Best regards,
    Oyvind Stavdahl


    From: Dr. Dan Antonelli, Research Director
    Motion Laboratory, Childrens Hospital Los Angeles.

    Placement of the reference electrode between the two active electrodes
    results in such a small impedance between each active electrode and the
    reference electrode that small differences become a larger percentage
    of the impedance, resulting in a larger imbalance and degrading the
    common mode rejection of the amplifier. Thus, placing the reference
    electrode far away from the active electrodes Ref. Hillbiber circa

    Wider spacing results in a longer time for the
    depolarization/repolarization wave to pass between the two active
    electrodes and, thus, you have a smaller bandwith with lower frequency
    signals. Coversely, closer spacing will result in higher frequencies
    because of the shorter wave transition time between electrodes.

    Electrode size affects the signal by dictating the volume from
    which the signal is obtained. The practical size is dictated by things
    like the availability of double sided tape attachment collars to
    practical considerations like the size of the muscles onto which the
    electrodes are to be attached without encountering crosstalk from other

    Childrens Hospital, Los Angeles uses surface electrodes from Motion
    Control and B & L Engineering. When the units are used with the Motion
    Control electrodes, the addition of the reference electrode required
    for the B & L Electrode improves the CMRR of the Motion Control
    electrode. Refer to the BIOMCH-L Yellow Pages for the address of B & L

    The last factor that should be mentioned is the method by which the
    signal is processed. Raw EMG signals should be normalized by some
    basis such as maximal volutional effort. This will make the
    utilization of EMG insensitive to day to day variations in placement
    and/or electrode size and spacing.