Boundary element analysis of the directional sensitivity of the concentric EMG electrode

Employing the boundary element method, the authors improve earlier models of the concentric electromyography (EMG) electrode by including an accurate geometric representation of the electrode, as well as the mutual electrical influence between the electrode surfaces. A three-dimensional sensitivity function is defined from which information about the preferential direction of sensitivity, blind spots, phase changes, rate of attenuation, and range of pick-up radius can be derived. The study focuses on the intrinsic features linked to the geometry of the electrode. The results show that the cannula perturbs the potential distribution significantly. The preferential directions of sensitivity are determined by the amount of geometric offset between the individual sensitivity functions of the core and the cannula. The sensitivity function also reveals a complicated pattern of phase changes in the pick-up range. Rotation of the electrode about its axis was found to alter the duration, the peak-to-peak amplitude, and the risetime of waveforms recorded from a moving dipole.