## 4 Discussion  
The excitation and measurement locations were chosen at the implant neck, at the extremity of the femur-implant structure, in order to obtain FRFs with numerous and high amplitude peaks. When the artificial femur is inserted in the artificial soft tissue, as shown in Figure 2, it is considered to be suspended, in an analogous fashion to a femur in the human body. The free-free boundary condition is, therefore, assumed valid for the artificial femur in this situation. In addition, when the structure is excited at the excitation locations on the implant, the excitation energy is dissipated due to the damping imposed by the artificial soft tissue before reaching the surrounding cylindrical container. This is important because the obtained measurements should be independent of the mass and stiffness characteristics of the cylindrical container that holds the artificial soft tissue. The validity of the free-free condition assumption for the femur in this configuration was checked by placing an accelerometer on the container, exciting the artificial femur and verifying that the accelerometer did not pick up any signal. As a result, since the free-free boundary condition is adequately realized by the artificial soft tissue, the cylindrical container could be fixed to the environment via a rigid fixture, as shown in Figure 3, without affecting the obtained measurements. Overall higher PCC and FRAC values were obtained when the insertion was performed with soft tissue than when it was performed without soft tissue. These higher PCC and FRAC values are a result of the effect of the soft tissue-induced damping that renders subsequent FRFs more similar by damping or eliminating fixation-sensitive peaks, as shown in Figures 10 and 11. As a result, the fixation-sensitivity of the VA measurement technique is reduced by the surrounding soft tissue. This sensitivity limitation in the presence of soft tissue is also reflected in the early appearance of the insertion endpoint. The FRAC values appear overall lower than the corresponding PCC values, except for Figure 9b. This difference can be explained by comparing Equation (1) to Equation (2); while the FRAC considers the FRF amplitude and phase, the PCC considers the FRF amplitude solely. This could explain why the FRAC can show a higher sensitivity than the PCC. As an insertion endpoint detection criterion, a threshold value for the PCC of 0.99 for either the ML or AP direction was used. This is an adaptation of [PERSON]'s method [20], who used the same threshold of 0.99 for the PCC, but only considered measurements in a single direction. The use of this threshold indicates that the endpoint was detected at the ninth step, after the eighth hammer hit, when there was no artificial soft tissue, while it was already detected at the seventh step, after the sixth hammer hit, when the artificial soft tissue was present. Reaching the insertion endpoint means that the FRFs of two subsequent insertion steps are nearly identical, so the last hammer hit did not affect the fixation level of the implant. During the insertion with soft tissue, after reaching the insertion endpoint, the PCC and FRAC values fell. This fall suggests that further hammering after the endpoint is reached can compromise the stability of the implant. During the comparative test (Section 2.4), the soft tissue caused the previously observed sharp peaks to become damped and shift to lower frequencies as seen in Figure 9. These changes appear to be in agreement to the observations made by [PERSON] et al. in a cadaveric study where the biological soft tissue around the tibia was gradually removed [12]. The observed resonant frequency decrease seems to be the combined result of mass loading and damping caused by the soft tissue. Despite most peaks shifting to lower frequencies, two peaks appear to exceptionally shift to higher frequencies.