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RE: RE: EMG in Biomechanics. Surface EMG freq. range.

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  • RE: RE: EMG in Biomechanics. Surface EMG freq. range.

    Ton van den Bogert gave the following description:
    > This is our standard method to obtain the EMG patterns:
    > 1. Surface electrodes, 20mm separation.
    > 2. Preamplifier (100x), carried on the subject. Connected by 25m cable
    > to recording equipment.
    > 3. Amplifier (up to 500x), bandpass filter (30-1000 Hz).
    > 4. Full-wave rectifier, lowpass filter (30 Hz).
    > 5. Analog to digital converter (12 bit), sampling interval 3 ms.
    > 6. Ensemble averaging with interactive graphics display for selection
    > of strides. Uses signal from hoof-mounted accelerometer for
    > automatic stride detection.
    > 7. Optional smoothing by a digital filter.

    I have no comment on points 1,2,6 and 7. However, there are two
    little problems with this set-up.

    o Why sampling the EMG envelope, which is low-pass filtered at 30 Hz,
    with such a high frequency (333.33Hz), more than 10 times f_Nyquist?

    o Why does the band-pass filter start with a high cut-off frequency
    of 30 Hz? In human surface EMG recordings, the band between 4 and 30 Hz
    contains a significant amount of (real myooelectric) power. In this
    range, the firing rate contributions of motor units may be expected to
    exert a predominant influence, whereas power at higher frequencies reflects
    motor unit action potential shapes.

    Boxtel, A. van, & Schomaker, L.R.B, (1983)
    Motor unit firing rate during static contraction indicated by the
    surface EMG power spectrum,
    IEEE Transactions on Biomedical Engineering, 30:601-609.

    Also, it might be interesting to know about acquisition and processing of
    the accelometer signal. In earlier studies I have used Entran miniature
    acceleration transducers to measure spectral coherence between EMG and
    accelerogram in tremor.
    Lambert Schomaker
    NICI Nijmegen Institute for Cognition Research
    and Information Technology
    University of Nijmegen, P.O.Box 9104
    6500 HE Nijmegen
    The Netherlands

    Telephone: +31 80 516029
    Telefax: +31 80 515938