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  • Linear enveloping of EMG

    I'm having trouble understanding some EMG signal analysis and was hoping
    someone could explain.

    We have EMG data from bipolar epimysial electrodes sampled at 500 Hz. A
    bandpass filter from 10 Hz to 1 KHz is applied online along with a
    full-wave rectification and a 5-point moving average (e.g. points 1-5
    become point 1, points 6-10 become point 2, etc.).

    Now we'd like to get the linear envelope of the EMG signal. So a 10 Hz
    low-pass filter is now applied.

    What I'm having trouble seeing is how I have anything left in the
    signal. If I bandpass from 10 Hz to 1 KHz and then low-pass at 10 Hz,
    what is left?

    I've been told two stories:

    (1) The bandpass filter will keep the relative timing of the EMG bursts
    so that the bandpass will actually pass the low frequency envelope
    (things less than 10 Hz).

    (2) The rectification is non-linear and this will prevent the
    bandpass/lowpass combination from filtering out everything in the
    signal.

    Thanks.
    -Tony Reina

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