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  • EMG filtering

    Hello to all,

    Since Dr. Rash suggested opening up the issue of EMG filtering to
    discussion, allow me to add my two cents.

    In my opinion, both Dr. Rash and Mr. DeLuca are correct to a certain
    extent. Dr. Rash stated, "I say collect 1st, then filter as it is often
    difficult to detect artifact in the signal when it is raw, and almost
    impossible if you do any manipulation of the data on the front end." I
    agree with this statement with one important proviso: aliasing error cannot
    be eliminated after the fact. On this point I wholeheartedly agree with
    Mr. DeLuca. I want to make sure I am emphatic on this point. Eliminating
    aliasing error on the front end should not be considered an option, it's a
    necessity. Aliasing occurs when a high frequency signal is inadequately
    sampled (i.e., sampled at less than the Nyquist frequency). Inadequate
    sampling will not make the higher frequencies go away. Rather, it will
    cause them to appear as lower frequencies -- frequencies that are very
    likely to be within the spectrum of the muscle being studied. Frequencies
    outside the Nyquist range can occur for a number of reasons, such as
    capacitance artifacts in the electrodes or lead wires, as well as
    electromagnetic interference in the atmosphere. If these sources are not
    effectively addressed they can obscure the true signal. And there is
    simply nothing you can do about it after the fact.

    In all other respects I agree with Dr. Rash. Different muscles have
    different fiber compositions, and therefore different frequency spectra. A
    hardware device that filters too tightly may be inappropriate for certain
    situations. I have heard recommendations that frequencies below 50 Hz can
    be effectively ignored(!). Well, that may be approximately true if one is
    talking about the large muscles of the leg, or muscles mostly composed of
    fast fibers. But that is definitely not true of other muscles. In my
    experience, there are situations where even 20-25 Hz can be too high a
    cutoff. Another important point is that the hardware filters employed in
    many EMG instruments possess broad rolloff characteristics -- usually no
    more than about 5-6 db/octave. This can allow high amplitude frequencies
    beyond the recording instrument's intended pass-through range to "bleed
    through". Using software one can construct extremely efficient filters with
    exceptional rolloff characteristics, making them much more effective.
    Moreover, the cutoff frequencies of software filters are easily adjustable
    as well. For these reasons, my advice always is, use your hardware to get
    rid of aliasing, then let your software take care of the rest.

    Thank you all for your time.

    Regards,

    Rick Lambert

    Richard W. Lambert
    Dir. of Product Development & Marketing
    RUN Technologies
    25622 Rolling Hills Road
    Laguna Hills, CA 92653 USA
    Phone/Fax: (949) 348-1234
    email: Rick@runtech.com
    web site: http://www.runtech.com

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