Natural Control of Triceps Stimulation Using FES

Individuals with C5/C6 spinal cord injury have paralyzed elbow extensors, yet retain weak to strong voluntary control of elbow flexion and shoulder function. Previous studies have shown that functional electrical stimulation (FES) of the triceps provides sufficient elbow extension strength and control to greatly improve function. With triceps stimulation applied at a constant level, elbow angle is controlled naturally by voluntary flexion opposing the stimulated extension. An alternative is a reciprocal control scheme employing biceps EMG to modulate triceps stimulation. With reciprocal control, increasing biceps EMG proportionally reduces triceps stimulation. A PC based lab system was designed to test the feasibility of reciprocal control. Reciprocal control increased the range of elbow moments, was stable during maintained elbow angle or isometric moment, and used less stimulation. Reciprocal control of triceps stimulation using biceps EMG is an effective method for restoring elbow extension to C5/C6 spinal cord injury patients. Introduction/Background Functional electrical stimulation (FES) has restored motor function to individuals with spinal cord injuries [1-3]. Individuals with C5/C6 tetraplegia are left without voluntary control of upper and lower extremity muscles. Voluntary movements include elbow flexion, some shoulder movement, and in C6 subjects wrist extension. Elbow extensor muscles, important for reaching during activities of daily living, are paralyzed. Although mechanical orthoses can compensate for the elbow extensor muscles [4,5], FES provides a cosmetically acceptable solution using the patient’s own natural muscles. Two FES systems used mechanical sensors to modulate triceps stimulation [6,7]. In one without sensors, an implanted triceps electrode was stimulated at a constant level [2]. Using voluntary antagonist control, the user controlled elbow angle by voluntary elbow flexion opposing the stimulation. Good functional restoration was achieved. However, constant stimulation decreases the maximum flexion moment about the elbow since a user must flex against a stimulated extension moment. An alternative control method varied stimulation [7]. However, it required mechanical sensors and did not provide for maximal stimulation in many different arm positions. Electromyograms (EMG) from voluntary muscles have proportionally controlled stimulation to paralyzed muscles [8,9]. The biceps is under voluntary control in C5/C6 spinal cord injury. Reciprocal control proportionally reduces stimulation to paralyzed muscles in response to EMG from a voluntary muscle. Biceps and triceps are antagonists, providing natural control with little training. EMG occurs naturally, detectable with implantable devices for a cosmetic solution [10]. Reciprocal control of elbow extension has three main goals: increase the total range of elbow moments compared to no or constant stimulation, reduce the total stimulation delivered compared to constant stimulation, and be at least as stable as constant stimulation.