###### Abstract
They are thin-walled, have cross-sectional openness and are subject to spatial force distribution. The relationship between the mechanical properties of the orthosis material and the pressures applied by the brace is debated in the literature [1]. Rigid orthoses are better at exerting corrective forces, while flexible ones tend to be more comfortable to use. There is an extensive literature on the subject of determining the distributions of corrective forces [3; 7; 8], but there is little information on the subject of the orthosis design's mechanical properties, which are indispensable for effecting these forces [11; 14]. Analysis of the literature indicates that the proper directions and amounts of the correction forces in an orthosis are a determining factor in its effectiveness. Therefore, the mechanical requirements that the orthosis must meet to ensure its ability to realise the required pressure fields appear to be equally important. It seems that an analysis of principal stress vectors and determination of force flow lines based on them can provide more information about the orthosis. This is one of the many directions used in issues concerning the optimisation of mechanical structures, but the literature lacks detailed information on the work of the brace itself. The influence of the brace structure on its functional properties, such as: weight, the level of the patient's thermal comfort, the level of gas exchange between the torso and the environment, and compliance to therapy, is not only an application issue, but also has a significant scientific importance. The problem of reducing the amount of brace material while increasing its functional properties and maintaining the unchanged corrective function is a big challenge. It seems that this task can only be solved with the use of advanced methods of analysis of the mechanical structure of the orthosis. The results of this analysis and the applied design improvements can obviously affect all medical aspects of the use of the orthosis and, most importantly, the assessment of the orthosis from the patient's point of view. It seems that the presented methods of designing braces have a varying impact on increasing their functional parameters, important from the patient's perspective. This article proposes a brace design analysis, which includes the possibility of preserving the material in places where it is necessary to ensure the unchanged corrective function of the orthosis. It seems that this type of analysis procedure should lead to the proposal of the lightest and the most functional brace structure for the patient. One may pose the hypothesis that the development of a reliable numerical model of a real orthosis, whose effectiveness has been proven in practice, is the best starting place to analyse the mechanical properties of orthosis designs. The results of in-depth modelling of such designs are rarely found in the literature. The goal of the numerical simulations and experimental tests presented in this article is to develop a reliable, experimentally verified FEM model of an exemplary orthosis. An in-depth model analysis will be used as the foundation for defining the performance scheme of the orthosis design, allowing potential opportunities for design optimisation to be identified.