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Re: Correlation Dimension of EMG

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  • Re: Correlation Dimension of EMG

    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:

    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
    >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.
    >Yuniarto Swie
    >Yahoo! JAPAN Redribbon Campaign 2005
    >Information about BIOMCH-L:

    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

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