## 4 Discussions
In free-free boundary conditions, [PERSON] [41] identified modal frequencies at 449, 524 and 1109 Hz, while [PERSON] [42] determined frequencies of 722 Hz for bending mode and 1582 Hz for double bending. In fixed-fixed conditions, the extracted frequencies are 1211, 1269 and 2818 Hz [41]. These studies identified modes on a particular bone with boundary conditions that are completely different from those depicted in the present paper. Our methodology is dependent on the mass of the subject and the whole anatomical environment, which accounts for the lower values and corroborates previous studies on the influence of anatomical parts. [PERSON] [43] shows from cadaverous specimens that the resonance frequencies are uninfluenced by the skin, increased with the dissection of the muscles and foot and decreased with the dissection of the femur and the fibula. This is confirmed by the work of [PERSON] [35], who adds that the joint has no influence on the modal frequencies of the tibia, but alters modal damping by 10-16%. These results are in agreement with the present work, where the values of frequencies are much lower than the values determined in vitro. In addition, the results of the sensitivity of the parameters show that a variation of 50% in the parameters of soft tissues, including the trunk and hard tissues, leads to a variation of about 10% in the frequencies. The proposed FE model is a preliminary model that requires many developments, but the values identified experimentally are very similar to the numerical ones, even without updates. Several perspectives are possible. The model does not integrate the fibula, which may actually be necessary: the works of [PERSON] and [PERSON] show that the fibula makes the lower limb more rigid. Moreover, the work of [PERSON] [44] proves that the fibula has modal properties close to those of the tibia (173.5, 249.3 and 516.2 Hz): there is no denying that it has a given influence on the dynamic behavior of the limb. Joint cartilage has also not been modeled: its influence, especially on damping, may be investigated in future works. A possible source of error is also the choice of a linear elastic constitutive law for soft tissues. Switching to hybrid models featuring Ogden-type hyperelasticity and exponential anisotropic behaviors [45] may improve the model. Then, it would be interesting to calculate the resonance modes at different knee angles. The work of [PERSON] [8] shows that the resonance frequencies are a function of the knee angle during a squat. The identified frequencies could be located in the excited frequency range 6-8 or 9-20 Hz, matching that of running activities [46]. Finally, upgrading to a 3D model would most certainly provide a greater number of modes in the range 0-150 Hz, two-dimensional modeling only allowing extraction of the modes in the mid-lateral plane. Reading the standards on human exposure to vibrations, the frequency of 52.54 Hz seemed to be the most harmful frequency for lower limbs. Indeed standard ISO 2631-1 [23] defines a frequency weighting filter: the gain at 52 Hz is 0.87, but drops below 0.10 beyond 150 Hz. In the field of public transport and sports, the development of specific materials able to absorb these specific frequencies will decrease the amount of vibrations received by the human lower limbs. For example, a ground made of this material in public transport or in industries could prevent or at least significantly decrease diseases such as arthritis among workers. It could also drastically reduce the tiredness of lower limbs for passengers and make travel far more comfortable. We can note that [PERSON] [47] determined the vibration responses of anti-fatigue mats to reduce the pain associated with vibratory exposure by considering these standards and defined the optimal geometry and design for risk prevention. Author Contributions:[PERSON] conceived of and performed the experiments and analyzed the data. [PERSON], [PERSON] wrote the paper and interpreted the data with the first author.