Knee injuries are frequent in all the population. In recent years there has been an increasing interest in the research of assistive devices with multiple scopes, that can be used to improve people's quality of life. In this context, we have designed a knee rehabilitation device to assist the physiotherapy process when there is an injury. We use variable stiffness actuators to achieve more natural motions and keep safety in the interaction of the patient with the device. This kind of soft-actuated systems requires control actions that do not affect the intrinsic desired dynamics. In this paper, we present a combined feedback-feedforward strategy for a one-Degree-of-Freedom (DoF) system with a variable stiffness actuator, to gain an insight of the controller behavior. Nevertheless, without lost generality, the results obtained here can be later applied to the designed rehabilitation device. The proposed control strategy takes into account the dynamics behavior for changing the joint stiffness when the desired angular position changes.