IMPROVEMENT OF SPATIAL-RESOLUTION IN SURFACE-EMG - A THEORETICAL AND EXPERIMENTAL COMPARISON OF DIFFERENT SPATIAL FILTERS

The conventional bipolar surface electromyography (EMG) technique dete cts, due to its low spatial resolution, the superimposed electromyogra phic activity of a large number of motor units (MU's). In superficial muscles the isolated action potentials of the most superficial MU's ca n be recorded noninvasively by means of surface electrodes, if the met hod of spatial filtering, in connection with electrode arrays, is used , Up to now, only filters with an anisotropic transfer function have b een used, As the surface potential distribution generated by the excit ation of the MU's contains spatial frequencies in the anisotropic rang e of those filters, it can be assumed that isotropic spatial filters d etect the single MU activity more effectively, In the present study, d ifferent isotropic and anisotropic filters have been compared by means of theoretical field simulations and experiments in volunteers, A tri pole 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 resolut ion, For the experimental validation, the total number of single motor units was not directly investigated, but the signal-to-noise ratio (S NR) has been determined, Therefore, the potential distribution generat ed on the skin surface during maximum voluntary contraction has been s imultaneous spatially filtered with each of the investigated filters, The simulations show that an isotropic spatial filtering procedure red uces the spatial extension of the filter response and improves the spa tial resolution of the EMG-recording arrangement in comparison to anis otropic spatial filters up to 30%, In other words, the spatial selecti vity of the arrangement is increased, This improvement in the filter p erformance is more pronounced for MU's located close to the skin surfa ce than for MU's more distantly located, Additionally, this theoretica l improvement in selectivity depends on the direction of the excitatio n spread relative to the filter alignment, However, the investigations also show that isotropic filters offer an advantage, compared to anis otropic filters, only when the investigated MU is located extremely cl ose to the filter input, The results of the simulations can be confirm ed by the experimental investigations, An improvement of 11% in the SN R, relative to anisotropic spatial filters, can be established when us ing an isotropic spatial filter, This experimental improvement in sele ctivity is less than the theoretical improvement because the experimen tally investigated MU's have less portion in the anisotropic range of the filters than the simulated one at best.