Simulation of Intramuscular EMG Signal Detection using Implantable MyoElectric Sensors

The volume conduction of intramuscular EMG signals recorded using implanted electrodes was examined using model simulation. Local effects due to electrode geometry, material properties including fibrocollagenous encapsulation tissue, and electrode orientation were investigated. Muscle fiber and motor unit action potentials were simulated using two different volume conductor models a finite element (FE) model that was used to explore the influence of the surrounding tissue properties, and an analytical infinite volume conductor model, used to explore the approximate pick-up volume of the electrode. The amplitude of simulated action potentials progressively increased as the conducting electrode poles, non-conducting electrode casing and highly resistive encapsulation tissue were added to the model. The pick-up volume of the electrode was estimated by simulating muscle fiber action potentials from 20,000 muscle fibers randomly located throughout the muscle. Changing the orientation of the electrode with respect to the fiber direction reduced the selectivity of the electrode and altered the shape of the pick-up volume. As the angle of rotation was increased from 0deg to 22.5deg and 45deg, the pick-up volume of the electrode and the shape of the surrounding isopotential contours became progressively wider and flatter. The estimated pick-up range of the IMES electrode, assuming a cylindrical muscle, was 4.8 mm, 6.2 mm and 7.5 mm for electrode orientations of 0deg, 22.5deg and 45deg, respectively