## 4 Discussion
It is possible to identify the areas of the orthosis that are most vulnerable to mechanical damage and those that do almost no work. Even if a similar \"three-point system\" was used, it might be hard to generalise the obtained results to orthosis cases with different applied load values and geometric parameters due to the quite irregular stress distribution in the orthosis body (Figure 7). As can be seen from von Mises stress calculations, the most stressed areas are the force application areas. In the study, very small force application areas were assumed in order to reduce the discrepancy between the experimental boundary conditions and the numerical model. In reality, the force application areas are larger and thus the stresses are much lower near force application areas. Nevertheless, the distribution of von Mises stresses, as shown in Figure 6 in areas other than the applied forces, obtain values significantly lower than the allowable stresses, indicating that the shell can be optimized. The principal stress distributions, which are shown in Figure 8 for different sections of the structure, seem to give more information in the general context of the performance of the orthoses. The outer surface of the front wall of the orthosis is in compression, while the inner face of the same wall is in tension. The values of absolute stresses on the interior side are approximately two times higher than those on the exterior layer. The rest of the shell of the brace works in a similar way. The characteristic stress distribution, characterised by a significant value of tensile stresses on the inside of the orthosis shell, is due to the superposition of stresses from two independently operating types of loads. The bracing shell is subjected to combined tension and bending states, as can be seen by comparing the signs and values of the stresses acting (Figure 10). Thisobservation is crucial to the proposal of the bracing optimisation method described later in this article, as it makes it possible to determine which site of the bracing wall is going to be subjected to the greatest tensile stress. Clear regularities shaping the obtained image of the lines can be seen from the determined vector fields of principal stresses and drawn maximum principal stresses \(\sigma_{1}\) trajectories in the regions in which these stresses take relatively large values (Figure 9). The trajectories of the principal stresses follow a characteristic pattern, connecting the correction load locations along paths of a relatively small sum of lengths. This image depicts the \"force flow lines\" concept that is currently being intensively developed in the theory of optimal design [26]. It is based on the insight that, in principle, the \"flow\" of forces across a mechanical structure among external load application points is strongest near the topographic lines connecting these locations in space, taking into account the imposed limits of the geometry of the structure. Although trajectories of principal stress are one way to practically apply the idea of \"force flow lines\", the literature offers alternative rival proposals [27]. On the basis of this idea, the orthosis may be characterised as a space-based mechanical system that transmits loads along comparatively brief trajectories between locations where correction loads are applied. It seems that analysis of principal stress vectors can provide more information about the orthosis. Principal stresses describe the stress state represented by normal stresses only, with zero tangential stresses. They are a useful way of presenting force flow lines. The force flow line is a concept used to visualize the path of an applied load through a load bearing member. This is one of many methods used in issues of the optimisation of mechanical structures, which are intensively developed, but the literature lacks detailed information on the work of the brace itself. The analysis in the proposed paper deals with one particular force application scheme.