EVALUATION OF ISOMETRIC ANTAGONIST COACTIVATION STRATEGIES OF ELECTRICALLY STIMULATED MUSCLES

The performance of various coactivation strategies to control agonist- antagonist muscles in functional electrical stimulation (FES) applicat ions was examined in a cat model using the tibialis anterior and soleu s muscles to produce ankle isometric dorsiflexion and plantarflexion t orques, respectively, Three types of coactivation strategies were impl emented and tested, The first strategy was based on coactivation maps described in the literature as consisting of decreasing antagonistic a ctivity as the input command to the agonist was increased, The second type of strategy was based on the physiologic coactivation data collec ted from normal subjects exhibiting joint stabilization during the ful l range of contractions. These strategies included scaled increasing a ntagonist activity and therefore joint stiffness with increasing agoni st input command, A third strategy was devised which at low force leve ls mimicked the strategies described in the literature and at high for ce levels resembled strategies exhibited by normal subjects, The three strategies were evaluated based on their ability to track a linear or sinusoidal input command and their efficiency of torque transmission across the joint, Coactivation strategies using increasing antagonist activity resulted in decreased maximal joint torque and efficiency, de creased signal tracking capability for linear inputs, and increased ha rmonic distortion for sinusoidal inputs, Peak efficiency and tracking ability appeared when a moderate degree of antagonist activity was eng aged near the neutral joint position, Signal tracking quality improved with earlier engagement of the antagonist muscles, Our results sugges t that strategies combining low-level coactivation as described in the physiological literature and previous FES studies could satisfactoril y address the issues of controllability, efficiency, and long-term joi nt integrity.