Automatic determination of the optimal shape of a surface electrode: selective stimulation

We present a method for automatic determination of the shape and position of the surface electrode for selective control of fingers extension and flexion by means of electrical stimulation. The multi-pad electrodes used in the experiments comprised 24 pads (1cm diameter) distributed over an area (7 cm x 10 cm) positioned over dorsal and volar aspects of the forearm. The four-channel stimulation system for grasping comprised also an oval reference electrode over the carpal tunnel, and two oval electrodes over the thenar and thumb extensor muscles. We measured seven angles: proximal inter-phalangeal and metacarpal phalangeal index and ring finger joint rotations, wrist extension/flexion and ulnar/radial rotation, and pronation/supination of the forearm. The optimal electrode was determined as the combination of pads that led to fingers, wrist and forearm rotations being similar to the trajectories of healthy individuals when grasping. The similarity of trajectories was assessed by analyzing the aggregate error defined as the sum of squares of differences between the angles measured when stimulating the forearm in tetraplegics and the angles measured in healthy individuals. The aggregate errors were determined from measurements during sequential stimulation of each of the 24 pads. The analysis comprised hand opening and closing for palmar and lateral grasps. The time for determining the optimal electrode was about 10 min. The optimal electrodes had different branched shapes in each of the six tetraplegics; however, once determined they remained unchanged when tested on different days.