This paper presents a two-dimensional dynamic analysis for a human leg, a model was developed using a Lagrangian approach to describe the body-leg interaction. This model could be useful analyzing the human gait for people with disabilities in a lower limb, and also helps to design and improve therapy procedures. The aim of this research is restricted to a specific scenario: working out with a step machine-common machine usually found in fitness centers. The movement of raising and lowering the leg was modeled, which is the most relevant motion in this activity. In the process were identified the different anthropometric parameters for the human body in relation with the weight and height of a specific individual. A set of partial differential equations relate the force generated in the foot with three variables: The angle between the thigh and the vertical, its variation with respect to time (angular velocity) and the angular acceleration. The results showed that the dynamic condition increases the reaction forces about 16% in this activity. The developed sequence of analysis could be used for optimization processes based on anthropometric and movement conditions. These results are useful for detailed designs of prosthesis and also to find the best economical options for manufacturing aid devices that meet the dynamic requirements. The mathematical formulation process confirms the advantages of the Lagrangian over Newtonian approach for these types of systems.