On the one hand, a modern sedentary lifestyle, on the other hand, excessive activity, both accompanied by improper performance of movements lead to a number of problems and impaired health of the human locomotor system. The locomotor system of man is complex, and as one of its most important parts, and thus one of the most sensitive, the knee joint stands out. Considering that almost every injury leads to reduced mobility of the knee joint and that the supply of products on the market that would cure this problem is weak, there is a need for development physiotherapy aid that would facilitate the implementation of therapeutic actions that are used for rehabilitation of this problem, which is the goal of this paper. After the introduction, where the problem and the goal of the paper are explained, the whole knee joint is described. In addition to introducing it to the entire topic and issue, the description of anatomy, biomechanics and possible joint injuries leads to a series of rehabilitations where the sense of making this type of device is proven. After getting acquainted with the problem and introduction to the topic, the problems, requirements and goals are defined and the market is analyzed. Based on the above, development guidelines were derived from which concepts were generated and evaluated. Finally, the best solution was selected from the evaluation of concepts, the shortcomings of which were corrected by solutions from other concepts. The final solution is presented in 3D models, where the principle of operation of the device and its advantages are clearly seen. The solution is manifested in the simplicity of manufacture and use, and the potential for further product development in terms of a device for strengthening the quadriceps muscle, especially m. Vastus medialis, crucial for the last 15° extension in the knee joint, is also noticable.