A post-stroke rehabilitation based on therapeutic serious game can be a useful approach to increase rehabilitation volume by maintaining patients' motivation and engagement. In particular, a dynamic difficulty adaptation technique of therapeutic tasks allows increasing patients' motivation by adapting the game difficulty to the patients' capabilities. This paper presents a difficulty adaptation technique dedicated to a family of therapeutic games for upper limb rehabilitation. The proposed technique provides a generic real-time adaptation module to dynamically adjust the game difficulty to each patient according to his profile and observed performances. A pre-pilot experiment is presented to demonstrate how this technique has been implemented and could be integrated to post-stroke therapeutic games.