Robotic exoskeletons are wearable robots coupled to the human body, being the human an integral part of the human-robot system. Specifically, the control plays an important role, and stability must be guarantee. Both actors form a close loop with two control systems interacting dynamically. In this regard, a relevant issue is to understand how humans and robots could physically (kinesthetically) interact and communicate better. This paper deeps in the control of systems interacting dynamically and presents an analytical discussion on the controller design for exoskeletons and the coupled stability problem. Finally, a particular study is described in order to demonstrate the human cognitive system adapting under functional compensation of tremor.