Feedback control methods for task regulation by electrical stimulation of muscles

Three feedback control algorithms of varying complexity were compared for controlling three different tasks during electrical stimulation of muscles. Two controllers use stimulus pulse width (PW) (or recruitment) modulation to grade muscle force (the fixed-parameter, first-order PW controller and the adaptive controller). The third controller varies both stimulus pulse (SP) width and period simultaneously for muscle force modulation (the PW/SP controller). The three tasks tested were isometric torque control, unloaded position tracking, and control of transitions between isometric and unloaded conditions. The comparative evaluation was carried out in an intact cat ankle joint with stimulation of tibialis anterior and medial gastrocnemius muscles. The simplest PW controller demonstrated robust control for all tasks. The PW/SP controller improved the performance of the PW controller significantly for control of isometric torque and load transition, but only slightly for control of unloaded joint position. However, the adaptive controller did not consistently achieve a significant improvement in performance compared with the PW controller for any task.