## 4 Results and Discussion
### ISO 10328 Equivalent Static Test Results  
Once the stiffness optimization of its elastic components was performed, a prototype of MyFlex-\(\gamma\) was built. The composite elements were manufactured using the lamination sequences for the upper blade and the middle blade listed in Table 8. In addition, the other components were designed with the same safety factor. The assembled physical prototype was tested with the ISO-10328 equivalent static test setup, as presented in Section 2.3. The results are plotted as _rotation-reaction force_ in Figure 24 and as _displacement-reaction force_ in Figure 25.  
The dorsiflexion curves, which are reported as _rotation-reaction force_ (Figure 24), or as _displacement-reaction force_ (Figure 25), are quite similar. Therefore, it can be said that both the 2D FE and 3D FE models were validated with the mechanical tests carried out on the physical prototype of MyFlex-\(\gamma\).  
If the curves of dorsiflexion are very similar to each other, the same cannot be said for those of plantarflexion. In fact, 2D FEA and 3D FEA curves diverge from the test curve obtained for the physical prototype. The reasons for the divergence could be the following: (i) the not-perfect mounting of the foot prosthesis on the test set-up; (ii) the not perfect CAD and FE modeling of the foot shell; (iii) manufacturing defects that can cause slight variations in the geometry of the physical prototype; (iv) the joints, such as the ankle joint, hinge joints between the link and tube connector, and between the link and spring holder, modeled without friction--thus, approximated; (v) the heel of the foot shell not fully covered by the platform, as depicted in Figure 27.  
The plantarflexion _displacement-reaction force_ curve obtained from the 3D FEA resulted stiffer than the curve obtained from the mechanical test after around 6-7 mm of the vertical displacement of the platform. After 6-7 mm of vertical displacement, the heel was not  
Figure 26: Upper blade and middle blade evaluated with the Tsai–Wu criterion; the most critical areas, when the foot was loaded with the 220% of body weight of intended users, presented an _inverse reserve factor_ of 0.5, which means a _safety factor_ of 2.  
Figure 27: Plantarflexion test: the heel of the foot shell was not completely covered by the platform.  
fully supported by the heel platform. The vertical support caused this situation (Figure 27), which was not considered in the 3D FEA. During the heel test, the contact area between the heel and the platform shifted forward. The distance between the point of application of the force and the center of the ankle joint was reduced. This condition did not occur in the mechanical test after 6-7 mm; the contact between the foot and the heel platform did not evolve as it should have. With the same torsional stiffness of the foot prosthesis in the sagittal plane, the force necessary to generate a plantarflexion was higher in the 3D FEA due to the shorter lever arm. It has to be specified that the platform was built according to standards. However, the space for the heel portion of the foot was not enough. For a more appropriate assessment of the plantarflexion stiffness, the heel platform (see Figure 27) should provide more space, or the vertical face should be removed. In the case of the dorsiflexion test, the test setup did not present a space issue. It is interesting to note that the two dorsiflexion stiffness curves are in good agreement. This condition should provide confidence in the 3D FE model presented in the _design phase_ (Section 3.1.2).