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) applications was examined in a cat model using the tibialis anterior and soleus muscles to produce ankle isometric dorsiflexion and plantarflexion torques, respectively. Three types of coactivation strategies were implemented and tested. The first strategy was based on coactivation maps described in the literature as consisting of decreasing antagonistic activity as the input command to the agonist was increased. The second type of strategy was based on the physiologic coactivation data collected from normal subjects exhibiting joint stabilization during the full range of contractions. These strategies included scaled increasing antagonist activity and therefore joint stiffness with increasing agonist input command. A third strategy was devised which at low force levels mimicked the strategies described in the literature and at high force 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, decreased signal tracking capability for linear inputs, and increased harmonic distortion for sinusoidal inputs. Peak efficiency and tracking ability appeared when a moderate degree of antagonist activity was engaged near the neutral joint position. Signal tracking quality improved with earlier engagement of the antagonist muscles. Our results suggest that strategies combining low-level coactivation as described in the physiological literature and previous FES studies could satisfactorily address the issues of controllability, efficiency, and long-term joint integrity.