Effect of electrode shape on spectral features of surface detected motor unit action potentials

A method for simulating the effect of electrodes of any shape on surface detected motor unit action potentials (MUAPs) is described. The method is based on an approach we have recently proposed according to which the spatial integration of an electrode with physical dimensions is described as a two-dimensional spatial filtering of the electric potential over the skin. The effect of circular, square and bar electrodes on the frequency characteristics of surface detected MUAPs is investigated from the theoretical point of view. In particular the appearance of characteristic frequencies at which the power spectrum of the detected surface EMG signal is zero (spectral dips) is analyzed in relationship with electrode size and shape. It is shown that the physical dimensions of the electrodes may determine spectral dips in the surface EMG power spectrum but only for particular electrode shapes. Simulation results are shown to validate the theoretical considerations.