## 4 Discussion
### Limitations  
Given the extremely complex structure of the foot, a compromise had to be reached between the need to represent the phenomena of interest and the need for simplification. However, there is a need to improve the knowledge of the mechanical properties of specific components of the foot structure and the computational capacity of the simulation software in order to increase the complexity of the model, make it more realistic and improve the validity of its predictions. In this study, we were interested in the movement and foot configurations produced by the TA and EDL. Hence, we deemed it necessary to represent the main joints as accurately as possible. This was obtained by modelling the interaction of joint surfaces (the tibialolar joint), the predefined 3D oriented rotation axes (the latoclancal and the cuboid-calcancal joints) and by applying spherical constraints when necessary (the latonavalicular joint)--see Supplementary File. Instead, for the sake of convenience, the navicular, cuneiforms and the first three metatarsi were considered to be rigidly connected, forming a single unit for the medial forefoot, while the lateral forefoot included the cuboid and fourth and fifth metatarsi. For the toes, only the metatarsal-phalangeal joints were represented by revolute joints. These were the main structural limits of our model. These were not supposed to influence the effects of TA and EDL activity on the ankle joint. Other somewhat questionable choices were the positioning and the mechanical characteristics of the foot ligaments. We tried to represent the ligaments according to the anatomical descriptions in specific biomechanical studies [52; 53; 54; 55; 56]. However, because of discrepancies between different authors on ligament mechanical characteristics, we had to adapt the literature data to our specific model. Nevertheless, since the movements obtained under different loading conditions were quite realistic and within the physiological range of motion, we are confident that the adopted parameters were mostly correct. For the muscles, we considered the muscle-tendon complex as one (i.e., without separation between contractile and tendon components), since it did not change nor alter the force transmitted at the insertion point. The direction of that force is important, though, and for this reason we have represented the via-points constituted by the retinaculum, which forces the tendons of the TA and EDL to remain close to the tibial bone. As to the plantarflexors, both the SO and the gastrocnemius muscles were taken into consideration, but their contracture was simulated simply by limiting the length excursion of the SO. Since these two muscles act on the same tendon (the Achilles tendon), the limits imposed on just one of them are representative of the contracture of the whole group. Other calf and foot muscles could have been con sidered, but the complexity of their interaction with synergistic and antagonistic muscles would have required a more in-depth investigation, and this was beyond the scope of the present study. For the same reason, the EHL was not analysed separately. It works synergically with TA but has a smaller cross-sectional area and lever arm. This results in smaller dorsiflexion and inversion moments. Moreover, it has additional effects on the first metatarsal-phalangeal joint. Tibialis posterior was not included in our model. Hence, its contribution to EVFD was not tested. Moreover, we did not model any reflex circuit and, consequently, TS spasticity. Rather, we modelled the limited extensibility of the calf muscles that can result from TS spasticity. It is worth noting that we presented the results based on the anatomical characteristics of a single subject. A sensitivity analysis on the effects that a variation in each moment arm value has on the computed angle, as in [57], is missing. Future studies should analyse a wider range of scenarios or input conditions, thus being representative of the samples of subjects instead of one single healthy adult.