In this thesis we study human joint forces, and especially the hip joint force, in order to understand the cause of hip joint deformation in Perthes disease. In this regard, first a three dimensional dynamic model with 7 links and 18 degrees of freedom is proposed to obtain the human joint forces and the ground reaction force while walking. In the double support phase of walking, in which both feet are on the ground, we are faced with an undetermined problem. Thus, in order to develop a dynamic model which appropriately resembles the actual walking, iired by experimental results, we consider extra assumptions for the distribution of the force between the two feet in the double support phase. Within the framework of these assumptions, inverse dynamics is solved to obtain the joint forces (and specifically the hip joint force) in both single and double support phases. The validity of the proposed theoretical model (which is independent of experimental data) is then verified through some experiments. Then a parametric study is carried out to see the effects of changing the parameters of the orthosis on the hip joint force, avoiding the time and cost requirements of producing different orthoses and carrying out clinical tests. In particular, we investigate how and to what extent the orthosis reduces the hip joint force by keeping the two legs far from each other. Moreover, since it seems that the main reason of hip joint deformation in Perthes disease is the impact applied to the hip joint, we further study the hip joint impact forces while walking and jumping. In this vein, by developing dynamic models, we see how some parameters can affect these impulsive forces. In particular, we look into four parameters: the elasticity of the shoe, the leg configuration before impact, the stiffness of the joint, and the angle between the two legs. Keywords: Perthes disease, Hip joint, Scottish rite orthosis, Inverse dynamics, Single and double support phases, Impact dynamics, Shoe elasticity, Leg configuration before impact, Joint stiffness.