Current user-centered software development approaches make special emphasis on the characteristics of the user and the tasks that must be accomplished because it affects greatly the way the user interacts with the system. Most research works propose some user characteristics; however, there is not a model that integrates aspects such as psychological, cognitive, and physical. In this paper, we propose a user model integrating all the user's characteristics, with the purpose of achieving an adaptive software interface improving the overall usability of any system. We have integrated this model gathering information from different disciplines such as medicine, sociology and their practitioners. In order to validate our model, we have implemented user interfaces for various real projects taking into account characteristics from real users. Also we present here an example of how a user interface can be adapted for a specific set of user capabilities.