## 4 Conclusions  
An FE model of a prosthetic foot was presented and validated against actual measured values on a physical prosthetic foot. The model can be used to predict mechanical reactions to forces, moments, and displacements. It also proved to be a useful tool to analyze the functional characteristics of the prosthetic foot. Furthermore, the model can be used to simulate the response of the device when modifications are implemented in comparison to the original design. The work showed how the stiffness of a system of series and parallel springs can be altered by introducing a pure damping element. Stiffness change of 51% was demonstrated. Although the change achieved is less than demonstrated in previous work [11; 12] it is considerably higher than the reported user perception of 8% [13]. Furthermore, for the given system and boundary conditions, it was presented that the dissipation of a damping element will reach a maximum at a given value of damping coefficient. For higher values, the energy dissipation decreases due to decreased velocity, causing the element to be better described as force coupling, becoming more efficient at transferring the forces as the coefficient is higher. The dissipation in the damping element has been shown to reduce the recoverable energy by over 20% for the utilized load profile. The benefit of being able to change or adapt the foot stiffness during loading, needs to outweigh the effect of this lost energy. For the load condition of constant displacement, change in the system function such as the increased damping in the case presented, will have more impact on work on the boundary than how the foot deforms. The strain energy does therefore not vary as much as might be expected in gait analysis of a user walking on the foot, where it is expected that the reaction would be more complex. It is anticipated that users will adjust their gait to the force response of the foot, affecting both deformations, as well as, reactive forces and moments. As stated previously, the force and rotation input functions for mechanical testing [28] differ from what is seen in clinical gait analysis when investigating amputee gait at normal walking speed. This is apparent when the strain energy curves are compared to earlier work. The strain energy results presented here, compares quite well with results for fast walking speed presented by [PERSON] et al. [18], while it differs for normal walking speed in the same work. The load in early stance phase is higher  
Figure 9: Ankle moment vs. ankle angle results from the FE model. Comparison between different activation times of the functional element. The actuation times correspond to 50%, 58%, and 67% of the stance phase. Damping coefficient of the activated element is 300 Ns/mm. than for normal walking speed and the mid-stance minimum occurs later in the stance phase. Further work will include simulations with changed boundary conditions. Force vector data from gait analysis will be used to dictate vertical force curves and tilt table rotations in order to better imitate normal walking. Furthermore, data will be gathered to produce different gait patterns, such as ascent and descent of ramps and stairs. Conceptualization, H.T. and F.S. ; Methodology, H.T. and F.S. ; Validation, H.T. and F.S. ; Formal Analysis, H.T. ; Investigation, H.T. and F.S. ; Resources, F.S. and C.L. ; Data Curation, H.T. ; Writing--original draft preparation, H.T; Writing--review and editing, all authors; Visualization, H.T. ; Supervision, F.J.; Project Administration, C.L. ; Funding Acquisition, F.J. All authors have read and agreed to the published version of the manuscript. This research was funded by the Technology Development fund at the Icelandic Center for Research, grant number 163805-0613. All materials and facilities for measurements were supplied by Ossur hf. The authors declare no conflict of interest.