Improvement of spatial resolution in surface-EMG: a theoretical and experimental comparison of different spatial filters

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Abstract
The conventional bipolar surface electromyography (EMG) technique detects, due to its low spatial resolution, the superimposed electromyographic activity of a large number of motor units (MU's). In superficial muscles the isolated action potentials of the most superficial MU's can be recorded noninvasively by means of surface electrodes, if the method of spatial filtering, in connection with electrode arrays, is used. Up to now, only filters with an anisotropic transfer function have been used. As the surface potential distribution generated by the excitation of the MU's contains spatial frequencies in the anisotropic range of those filters, it can be assumed that isotropic spatial filters detect the single MU activity more effectively. In the present study, different isotropic and anisotropic filters have been compared by means of theoretical field simulations and experiments in volunteers. A tripole model for an excited MU was used as the basis for simulating the spatial extension of the filter response for each of the investigated filters. The spatial extension is an indicative of the spatial resolution. For the experimental validation, the total number of single motor units was not directly investigated, but the signal-to-noise ratio (SNR) has been determined. Therefore, the potential distribution generated on the skin surface during maximum voluntary contraction has been simultaneous spatially filtered with each of the investigated filters. The simulations show that an isotropic spatial filtering procedure reduces the spatial extension of the filter response and improves the spatial resolution of the EMG-recording arrangement in comparison to anisotropic spatial filters up to 30%. In other words, the spatial selectivity of the arrangement is increased. This improvement in the filter performance is more pronounced for MU's located close to the skin surface than for MU's more distantly located. Additionally, this theoretical improvement in selectivity depends on the direction of the excitation spread relative to the filter alignment. However, the investigations also show that isotropic filters offer an advantage, compared to anisotropic filters, only when the investigated MU is located extremely close to the filter input. The results of the simulations can be confirmed by the experimental investigations. An improvement of 11% in the SNR, relative to anisotropic spatial filters, can be established when using an isotropic spatial filter. This experimental improvement in selectivity is less than the theoretical improvement because the experimentally investigated MU's have less portion in the anisotropic range of the filters than the simulated one at best.

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