This paper presents an assist-as-needed method for ankle rehabilitation based on adaptive impedance control strategies. First, the dynamic contribution of the patient during the motion is estimated from robot's torque and kinematic information. Then, two robot assistance control strategies (complementary and optimal) are proposed to calculate the stiffness parameter of the impedance controller. In addition, the robot level of assistance is adapted based on patient's performance in a video game. The proposed strategies were implemented using the Anklebot and evaluated in four post-stroke patients for dorsi/plantarflexion and inversion/eversion movements. Results indicate that the patient's stiffness estimation is a valid method to determine the amount of the assistance and to help the patient to complete the tasks. Finally, the results confirm that increasing the participation and performance of the patient generates a decrease in the robotic assistance, and vice versa.
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