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gdluca91
02-12-1999, 01:14 AM
Having recently read an article on the fundamentals of EMG, which was
advertised on this list server (at www.gcmas.org), I feel obligated
clarify a couple of statements which were made:

.."This means in general, one should be collecting in a range from 0 Hz to
600 Hz for surface electrodes and 0 Hz to 1,000 Hz for fine wire
electrodes. Using the Nyquest Theorem, this means that one must sample at a
minimum of 1,200 Hz for surface electrodes and 2,000 Hz for fine wire
electrodes in order to assure capturing the entire signal. Once the signals
have been recorded, then one could use a 10-15 Hz high-pass filter to
eliminate the movement artifacts and a 500 or 1,000 Hz (surface or fine
wire electrode respectively) low-pass filter as an anti-aliasing filter."...

While I don't have a problem with the bandwidths stated, I believe it is
much more advantageous to filter the signal *before* sampling it. This is
particularly important for the antialiasing filter. By definition, the
antialiasing filter is used to prevent the sampling of frequencies in the
signal that are higher than half the sampling frequency, as these
frequencies will be misrepresented if sampled. Any filtering applied after
the sampling process is technically not an antialiasing filter. Filtering
from 600 Hz to 500 Hz once the signal is sampled presents no obvious
advantage.

I suppose you can argue that no signals exist above 600 Hz and 1200 Hz for
surface and wire EMG, and therefore there is no need for a front end
filter. I don't believe this is a correct argument. Amplifier noise is a
function of bandwidth. By filtering the raw EMG signal before sampling, a
large portion of this noise can be eliminated. This is particularly
important when dealing with low-level EMG signals.

I think it is also advantageous to high pass filter the EMG signals before
sampling. It is possible for motion artifact to be orders of magnitude
larger than the EMG signal. These spike-like disturbances can easily
saturate the amplifiers and exceed the range of the A/D card. Saturated
amplifiers require time to settle, sometimes much longer than the duration
of the artifact. This settling time is dead time as far as EMG signals are
concerned. No amount of digital filtering will restore EMG signals in
these situations. By filtering the artifacts before sampling, a large
portion of these spikes and their associated problems, can be eliminated
from the system.

I understand that filtering signals before sampling complicates that
acquisition process, as digital filters are more convenient to implement
than analog filters. However, I think it is important for people to take
the time to thoroughly understand these issues, and to carefully consider
the pros and cons of both approaches to filtering before setting up an EMG
acquisition system. It may make an important difference.


Gianluca De Luca
Research Engineer






At 03:25 PM 2/10/99 -0500, you wrote:
>With the resent discussion on fine wire EMG, I thought it might be
>helpful to post some information on the fundamentals of EMG. I have
>posted a PDF file on the Gait & Clinical Movement Analysis Society
>(GCMAS) web page under the General Information Section. Any feedback
>is welcomed... The address is:
>
>http://www.gcmas.org
>
>
>Gregory S. Rash, EdD
>Chair, GCMAS Communications Committee
>Frazier Rehab Center
>Louisville, KY 40202 (USA)
>(502) 582-7657 (V)
>(502) 582-7617 (F)
>gsrash01@pol.net
>gsrash01@ulkyvm.louisville.edu
>http://www.gcmas.org/frazier/
>
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