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jbdingwell38
12-30-2005, 08:15 AM
Dear Yuniarto:

If you are analyzing raw EMG signals, then I would expect these signals to
be high dimensional. The signal itself is an interference pattern
generated by the interference of the electrical output of a large number of
individual motor units (which may be from a few to a few hundred, depending
on the muscle you're looking at and the level of activation). If one can
think of each motor unit as a "degree of freedom" in the underlying system,
then the true dimension could be quite high. This could cause you
problems, as the correlation dimension algorithm is not very good at
assessing such high dimensional systems (i.e., d > about 4 or 5). You
should be looking at the entire correlation integral (not just the slope of
the curve) to see if you really are obtaining results that will allow you
to compute a valid correlation integral to begin with. You should also see
if/how these curves change as you vary the embedding dimension. If they do
change substantially, this could be problematic.

Also, I don't know how you are filtering your data, but standard frequency
domain linear filters (e.g. Butterworth low-pass filter) can substantially
alter the nonlinear properties (including correlation dimension) of
inherently nonlinear signals.

There's an excellent text that deals with these issues, recently published
in it's second edition, by Holger Kantz & Thomas Schreiber. The book is
called "Nonlinear Time Series Analysis" and is published by Cambridge
University Press (2nd Ed., 2004). These authors also offer their analysis
software (the "TISEAN" package) via a well-documented web site:
http://www.mpipks-dresden.mpg.de/~tisean/

I hope this is helpful.

Good luck!
Jon Dingwell



At 11:45 PM 12/26/2005, you wrote:
>Dear Experts,
>
>I am doing Chaotic analysis to EMG by taking the
>Correlation Dimension, and found in most case that the
>Corr. Dimension increased as the increasing of the muscle
>contraction.
>
>However in some case, it was observed that even the
>contraction supposed to be weak (at least showed from the
>EMG power), the Corr. Dim is relatively high (higher than
>the relax stage). As noise has been removed, We consider
>that there is underlying mechanism cause the high Corr.
>Dimension in the relatively weak contraction.
>
>Can anyone advise on such phenomenon and to share idea on
>what happening behind.
>
>Thanks for any opinion.
>
>Regards
>Yuniarto Swie
>
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Jonathan Dingwell, Ph.D., Assistant Professor

University of Texas at Austin
Department of Kinesiology and Health Education
1 University Station, D3700
Austin, TX 78712-1204

Phone: 512-232-1782
Lab: 512-471-4017
Fax: 512-471-8914
Web: http://www.edb.utexas.edu/faculty/dingwell/

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