## 2 Approaches
### Extraction of Output Data  
The retained indicators to interpret the results are the following.  
Local indicators:  
* Pressure map on intervertebral discs (MPa),
* Reaction forces and moments on vertebrae (N, N.mm).
* Global indicators:
* Variation of lordosis angle, defined as the angle between the upper plateau of L1 and the lower of L5 (\({}^{\circ}\)),* Max and min pressures on the spine (MPa),
* Mean pressure on the spine (MPa),
* Max and min of mean pressures on each disc (MPa),
* Max and min forces and moments obtained on the vertebrae (N, N.mm).  
It is important to note that the aforementioned pressure means the difference in pressure between the state without belt wearing and the state after belt wearing, while excluding all other physiological loadings, such as loadings due to gravity or movement.  
Each vertebra is modelled as a cylinder based on radiographies. Lumbar lordosis and thoracic kyphosis are constructed using two circle arcs obtained by the lateral inclinations of each vertebral endplate. It is assumed that the vertebra has parallel lower and upper endplates. The posterior segment of vertebra is not represented in this model since their action is assumed to be negligible for this study. The intervertebral discs are also modelled as a cylinder with two distinct parts: a central circular part corresponding to 30% of the volume and representing the nucleus, and a peripheral part representing the annulus [20].  
The tissues composing the trunk, except the spinal column, are divided into three zones in order to represent the abdomen, the iliac crests, and another part that includes the thorax and the soft tissues in the back zone (see Figure 2).  
Figure 2: Representation of modelling of trunk geometry and a meshed finite element model with boundary conditions: trunk geometry before modelling (**a**) from the back, (**b**) in profile; modelled trunk geometry (**c**,**e**) from the back, (**d**,**f**) in profile, (**g**) a meshed finite element model, and (**h**) its boundary conditions.  
The geometric model is meshed using ten-node tetrahedral second-order elements (C3D10H) in Abaqus. A static linear analysis is performed using the standard solver. To determine the appropriate number of elements, a convergence study was done with Abaqus. This study consists of determining the mean abdominal and intradiscal pressure in the lumbar region as a function of the mean size of elements [17].  
As it is shown that mechanical behaviour has a second-order influence [17], in order to ensure FEA simplicity and for the comparative purpose, all the structures have elastic linear mechanical properties and the same value for each part. Table 3 shows the mechanical properties of different tissues of the human body through mean values from the data taken by [PERSON] [19].  
Boundary conditions were determined in order to obtain realistic movements of the trunk when under belt application (Figure 2h). When the patient wears a belt, the pelvis can rotate slightly relative to the transverse axis. In addition, the patient tends to straighten up. This is why the upper plate of the trunk in translation is blocked in the transverse plane. The top of the trunk can translate in the frontal plane. The lower plate of the trunk is restricted in order to enable rotation of this plate only in the transverse axis. This restriction is obtained by embedding the two points at the intersection of the ellipse, thereby representing the extremity of the lower plate of the trunk and the frontal plane of the trunk. A meshed model of the trunk and its boundary conditions are represented in Figure 2g,h. Finally, the pressure field computed as described in the previous section was applied to the belt area.