Nowadays, “user-cooperative” control strategies are commonly used in robot-assisted motor (re-)learning. User-cooperative strategies enable compliant haptic interactions between robot and user: the robot only intervenes as needed, instead of forcing the user to follow a fixed predefined movement. However, the effectiveness of user-cooperative control is contro-versially discussed. Recent studies indicate that the effectiveness of user-cooperative control strategies will be enhanced when every user is individually provided with an optimal amount of assistance or challenge. In conventional motor (re-)learning, such an optimal amount of assistance or challenge is successfully applied by physiotherapists and trainers.