Impedance control of an electrohydraulic actuator interacting with a human ankle joint : a new experimental setup for studying ankle dynamic stiffness during voluntary movements

A position-based impedance controller was designed and implemented to regulate the interaction between forces produced at the ankle joint and the position of an electrohydraulic actuator. The goal was to provide a test environment suitable for studying ankle dynamic stiffness during voluntary movements. The new experimental setup allows: (1) subjects to perform voluntary movements, where both the length of the muscles associated with the ankle joint and their activation level change, (2) the simultaneous application of position perturbation to the joint (maximum bandwidth of 50 Hz), (3) evoking and responding to reflex activity at the ankle joint.The setup was validated by demonstrating that it can be used to correctly estimate ankle dynamic stiffness when the subject maintains a steady voluntary contraction