Five physiotherapists organised a user-centric design process of a knowledge-based support system for promoting exercise and preventing falls. The process integrated focus group studies with 17 older adults and prototyping. The transformation of informal medical and rehabilitation expertise and older adults' experiences into formal information and process models during the development was studied. As tool they used ACKTUS, a development platform for knowledge-based applications. The process became agile and incremental, partly due to the diversity of expectations and preferences among both older adults and physiotherapists, and the participatory approach to design and development. In addition, there was a need to develop the knowledge content alongside with the formal models and their presentations, which allowed the participants to test hands-on and evaluate the ideas, content and design. The resulting application is modular, extendable, flexible and adaptable to the individual end user. Moreover, the physiotherapists are able to modify the information and process models, and in this way further develop the application. The main constraint was found to be the lack of support for the initial phase of concept modelling, which lead to a redesigned user interface and functionality of ACKTUS.