(eng): This work has been motivated by the need to implement the previously developed sensor and control algorithms for the real-time movement of the laboratory walking biped robot, designed and built at the Department of Control Theory of the Institute of Information Theory and Automation of the Czech Academy of Sciences. Consequently, our focus shifts to a comprehensive examination of models describing underactuated mechanical systems featuring legged locomotion. In this study we elucidate the process of mathematical modeling of a simple locomotion system with hybrid dynamics, we scrutinize the well-known mechanical system of\nthe stepladder model. This analysis encompasses scenarios with and without an autonomously moving upper body, being a decorator.