[Context and motivation] In the last years motion-based games have achieved an increasing success. These games have great potential to support physiotherapeutic programs, as they can guide the patients in performing the right movements for their rehabilitation. [Question/problem] However, on the one hand, existing games performed on commercial systems (e.g., Wii, Kinect) are not suitable for people affected by motor pathologies. On the other hand, the design of games for physiotherapy is hard, as they should meet the "physiotherapy requirements" of the medical staff, provide an enjoyable experience to the patients, and overcome the technical limitations of the systems that support their execution. [Principal ideas/results] These limitations can be addressed by defining a standard process, independent from the considered pathology and that starts from the requirements collection and representation, to support the development of motion-based games for physiotherapy [Contribution] For this reason, this paper proposes RE-FIT, a methodology to elicit and model the RE-FIT extends existing requirements elicitation (brainstorming, surveys, and direct observation) and modeling techniques (FLAGS goal model). RE-FIT was developed in collaboration with the Spinal Unit of Niguarda Hospital and the Respiratory Medicine Section of Policlinico in Milan. Our experience demonstrated that RE-FIT is not only suitable to develop new physiotherapeutic games, but also to evaluate the adequacy of existing games for people affected by a specific pathology.