## 4 Discussion
It seems that this apparent frequency increase is actually the result of two individual peaks merging into a single peak, as a result of two modes overlapping under the increased damping. The presence of these overlapping modes is further supported by the fact that these peaks exhibit exceptionally higher amplitudes compared to other peaks in the FRF. In the cadaveric study of [PERSON] et al., a similar phenomenon of overlapping resonant frequencies was encountered. More specifically, asingle high amplitude peak was replaced by two peaks after the dissection of the medial collateral ligament [12]. Adding the soft tissue seems to almost completely damp the amplitudes of the FRF at the frequency range above 3 kHz. The damping imposed by the added artificial soft tissue decreased the amplitudes of all peaks. The two peaks, that seem to have shifted to a higher frequency, are the only ones whose amplitudes did not reduce drastically, but only by a small amount, as displayed in Figure 9.  
In the present study, an attempt was made to imitate biological soft tissue in order to investigate its effects on VA measurements. Nevertheless, while the artificial soft tissue exhibits reasonably similar material properties to fat and muscle, the actual biological soft tissue features a complex anatomical configuration of the muscle, ligaments and fat. The dynamic effects of this complex anatomical configuration are not carried over to the artificial soft tissue, which is a simpler isotropic uniform material. Consequently, a cadaveric or clinical study is needed to evaluate how closely this material simplification relates to the actual biological soft tissue encountered in intraoperative conditions. It is hypothesized that non homogenous biological soft tissue would influence the modal mass, stiffness and damping of each individual mode in a different manner. This is due to the attachment sites of the tissues on the bone and the uneven distribution of tissue mass that create a non-homogeneous distribution of mass, stiffness and damping which overlays the femur-implant structure. Although the physical properties are not identical to those of human tissue, the artificial soft tissue presents certain advantages over the use of cadavers, such as lower cost, easier preservation, and a lesser degree of regulation involving its use in experiments. The second limitation is that the physical properties of the materials used in this study represent average values, thus do not cover the whole span of the individual properties for each anatomic component (e.g., bone, muscle and fat) which are heavily subjected to inter-individual variability. A promising solution to mitigate the negative effects of the soft tissue damping on the system's sensitivity has been proposed by [PERSON] et al. [21]. This solution involves structural modifications that can be applied to the exposed part of the femoral implant so that less damped, sharper peaks are obtained. In addition, instead of using a modal hammer, the experiment could be repeated using alternative excitation methods, like, for example, a shaker. The shaker as an excitation method can present certain advantages over the modal hammer, for this application. More specifically, the shaker can be fixed on the structure and can, therefore, produce an excitation signal in a more repeatable manner than the manually operated modal hammer, in the case of which the repeatability depends on the operator. Furthermore, using the modal hammer can require multiple time-consuming excitations for a single measurement in order to obtain enough samples that are averaged to increase the signal to noise ratio to an acceptable level. In contrast, the shaker can obtain the multiple averaged samples automatically as it produces a continuous excitation signal. This difference potentially renders use of the shaker more practical for intraoperative use.