A co-op student of mine was collecting surface EMG data. We rectified these data

Question

A co-op student of mine was collecting surface EMG data. We rectified these data and then low pass tiltered. This provides a signal A(t) that varies in proportion to the (absolute) amplitude of the EMG, which varies in sympathy to the contractile force. When I examined the signal, I noted that even when the muscle was relaxed, A(t) had an unusually large value (I would have expected something closer to a value of zero). Going through the experimental methods I discovered that the co-op student was using two different types of electrodes when collecting the EMG in a bipolar configuration; one electrode was a Ag/AgCI electrode, while the other was a stainless steel electrode (there was a ground electrode used that was Ag/AgCI). I suspected that the measurement was highly contaminated with power line interference. a) Provide an explanation why there would be a lot of power line interference (around 5 sentences), using a sketch/diagram as required. b) Explain what you would do to confirm that power line interference is indeed this issue (around 5 sentences)

Explanation / Answer

Electromyogram (EMG) recordings are often corrupted by power line interference (PLI) even though the skin is prepared and well-designed instruments are used. This study focuses on the analysis of some of the recent and classical existing digital signal processing approaches have been used to attenuate, if not eliminate, the power line interference from EMG signals. A comparison of the signal to interference ratio (SIR) of the output signals is presented, for four methods: classical notch filter, spectral interpolation, adaptive noise canceller with phase locked loop (ANC-PLL) and adaptive filter, applied to simulated multichannel monopolar EMG signals with different SIR. The effect of each method on the shape of the EMG signals is also analyzed. The results show that ANC-PLL method gives the best output SIR and lowest shape distortion compared to the other methods. Classical notch filtering is the simplest method but some information might be lost as it removes both the interference and the EMG signals. Thus, it is obvious that notch filter has the lowest performance and it introduces distortion into the resulting signals.

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