## 4 Discussion  
In this study, a method was developed to adapt the stiffness of an individual insole using the FE method with boundary conditions from a musculoskeletal multibody model during walking. Afterwards, the insole was manufactured by means of additive manufacturing processes. It has to be noted that a FE model is always a simplification and, therefore, only an approximation of reality. The foot is a complex structure consisting of numerous ligaments, joints, and musculature. The more detailed the model is, the more computational time is required and the more difficult it becomes to achieve convergence [60]. Therefore, to make the FE model as efficient as possible, in this work, only the most important structures, the foot skeleton and the foot surface, were used [28, 61]. However, this is one limitation of the present study. Recommendations exist to adapt insoles based on plantar pressure distribution during gait [13, 17]. Therefore, a static analysis of two-time points of gait was simulated; i.e., the  
Figure 12: Influence of stiffness adjustment of the insole in the forefoot area (view from below) with Young’s moduli of 1.5 MPa, 1.0 MPa, and 0.5 MPa on the plantar pressure distribution compared to the plantar pressure distribution without stiffness adjustment at the time of the low point of the vertical ground reaction force. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Percentage Reduction in Peak Plantar Pressure [\%]**} \\ \hline
**Area** & **1.5 MPa** & **1.0 MPa** & **0.5 MPa** \\ Heel & \(-8.16\) & \(-13.01\) & \(-22.96\) \\ Lateral Midfoot & \(-4.44\) & \(-8.15\) & \(-16.30\) \\ Lateral Forefoot & \(-6.78\) & \(-10.17\) & \(-16.95\) \\ Toe & \(-11.30\) & \(-17.39\) & \(-26.09\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Percentage reduction of the peak plantar pressure at the heel and lateral midfoot at the time of the first high point of the vertical ground reaction force, as well as in the lateral forefoot and toe at the time of the low point of the vertical ground reaction force depending on the Young’s moduli 0.5 MPa, 1.0 MPa, and 1.5 MPa of the corresponding soft insole used compared to the peak plantar pressure without stiffness adjustment of the insole. early and late middle stance phases were used. However, it should be noted that the time points chosen for the FE model do not correspond to the phases that exhibit the highest peak plantar pressures. These occur at the time of the heel strike and during the push-off phase [62]. However, this would require MRI scans of two different foot or foot skeletal positions if relative motions of bone structures are not considered, as in this FE model. In contrast, the foot is flat on the insole during the early and late mid-stance phase, which is why these time points were chosen. By simplifying the foot skeletal geometry, the cushioning properties of the foot, through arch, ligaments, as well as joints, were not considered. The neglect of the relative motions of the bones has also been described in some other studies and may be the cause of some of the observed deviations with respect to their validation [34; 63; 64]. The reason for this is the challenging modelling of joints in FE analysis [22]. Furthermore, most studies have equally relied on MRI or CT data to obtain patient-specific foot skeletal geometries. However, these procedures are very costly or, in the case of CT, involve radiation exposure. In addition, reconstruction of geometries from MRI cross-sectional images is usually difficult and often has to be manually conducted, which increases the workload. The dependence on these imaging techniques is a major problem, which has been previously addressed [22; 23; 65]. Therefore, attempts have been started to generate individualized geometries using ultrasound images [61; 66]. Another conceivable option would be to use a CAD programme to customize a standardized skeleton. The contact between the bone and the soft tissue was neglected by merging and meshing them together.