The appropriate ergonomic design of a wearable robotic device is critical for the effectiveness of the device itself. In this paper we identified two key requirements for a structural ergonomics: the correct kinematic compatibility with the human limb and a comfortable and adaptable physical human-robot interface. We then show how the aforementioned requirements have been faced and implemented in the mechanical design of two wearable devices for elbow and hand rehabilitation, both developed at The BioRobotics Institute of Scuola Superiore Sant' Anna.