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Re: Linear enveloping of EMG

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  • Re: Linear enveloping of EMG

    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:
    http://www.health.uottawa.ca/biomech/watbiom/index.htm

    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
    NL-9700 AD GRONINGEN
    THE NETHERLANDS
    Tel: (31) 50 3632645
    Fax: (31) 50 3632751
    e-mail: a.l.hof@med.rug.nl

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