View Full Version : Re: Linear enveloping of EMG

08-30-2000, 09:02 PM
Dear all,
In the discussion of EMG postprocessing Drew Smith advocates a low
cut-off frequency for the low-pass filter after rectification, as
advocated by Winter. The filter is then chosen to match the twitch characteristics of
isometric muscle. My opinion on this:

1) This is not a bad idea, when the muscle is isometric. It may be
good to check the time constant by comparing the isometric force with
the filtered EMG, when possible. The filter should be second order,
crotically damped (not Butterworth, that is somewhat different).

2) For many applications, i.e. when things move, muscles are not
isometric, not even "almost isometric". In those cases I use a much
higher cut-off frequency. For the lower leg muscles I recommend
either a 3rd order Butterworth filter of 24 Hz, or a moving average
of 25 ms. (These are practically equivalent.) I have tried out with a
muscle model (J. Biomech 14:749) that a higher cut-off is
unnnecessary, but a lower one filters out useful information.

3) When 2) is chosen, for the processed signal a sampling frequency
of 100 Hz is sufficient.

4) People who are interested in reflexes, reaction times etc., may
prefer an even higher cut-off frequency.

5) To be sure in all cases, apply recipe #2, and when for cases when
the muscle was guaranteed isometric, filter this signal afterward
according to #1. Both filters will hardly influence each other.

6) For example, in walking at normal speed, processing of triceps
surae EMG with the 'Winter-filter' gives results markedly deviating
from the recorded ankle moment. In running both signals are
completely different.

7) For anyone wrestling with this kind of problems, I can recommend
the booklet "Signal Processing and Linear systems for the movement
Sciences" By DA Winter and AE Patla, Waterloo Biomechanics, 1997
For details and orders:

I hope this will help,

At Hof
Department of Medical Physiology &
Laboratory of Human Movement Analysis AZG
University of Groningen
A. Deusinglaan 1
room 769

PO Box 196
Tel: (31) 50 3632645
Fax: (31) 50 3632751
e-mail: a.l.hof@med.rug.nl

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