Rehabilitation robotics is currently an open research area due to the necessity of having devices to support people with any impairment. According to the World Health Organization near 15% of world's population has some disability caused by accidents, chronic diseases, or other conditions. Rehab robots are aimed to simplify and improve people's quality of life and several characteristics are required, as safe human-robot interaction and more natural motions given by the actuation system. In this paper, we evaluate the design and implementation of a modular mechanism for knee rehabilitation, with soft actuation. The device can be used in sitting and lying positions for which the model changes, i.e. either a single pendulum or a five-bar linkage mechanism. The actuation will not be placed directly on the knee joint but at joints on the fixed link, to avoid undesired loading. We carry out in detail the latter kinematic model and give an insight of dynamics that in future will be used for controlling the structure. We validate the model and present results on simulation of the behavior of the links when giving a desired angular position and velocity to the actuated joints. Results suggest that the proposed system is feasible to be implemented with a proper control strategy or compensation for the link's position and stiffness.