View Full Version : EMG filtering

Run Technologies
02-12-1999, 09:07 AM
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.


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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