## 7 Conclusions  
In this study, we successfully developed a seven-degree-of-freedom (7-DOF) model under vertical vibration and estimated 18 effective stiffness and damping coefficients using a new hybrid optimization method. The goal was to understand how vibration is transmitted to distinct human body segments, especially in the low-frequency region, by analyzing the estimated parameters by GBA, GA, and HOM using transmissibility responses from experiments and those calculated using the 7-DOF model. Results from experiments showed that the upper trunk and head are most susceptible to transmitted vibrations, with the thigh and leg showing lesser magnitudes. Also, the peak magnitude of transmissibility at each body segment reduced, and the resonance frequency became less observable and diminished in magnitude as the vibration exposure time increased.  
Our findings also showed that combining the 7-DOF model with the hybrid optimization method significantly accelerated the optimization process, enhanced numerical stability, and resulted in a significant reduction of the objective function value compared to using GBA or GA as standalone algorithms. Additionally, HOM was more suitable for the estimation of the unknown stiffness and damping parameters. Notably, we foundthat unknown human parameters, particularly stiffness, could be estimated using data measured from any of the body segments, with minor variations in optimized values across different excitation magnitudes. Finally, the'softening effect' was observed as the excitation magnitude increased from 0.5 to 2.0 m/s\({}^{2}\) rms., indicating the strong non-linearity of human body biodynamics.  
These results have practical implications for optimizing the comfort and safety of individuals exposed to whole-body vibration when riding in vehicles, as ergonomists and design engineers can use the developed model and HOM to identify the body segments most susceptible to vibration. Future work will focus on studying the correlation of transmissibility at different points on each body segment, the effects of subject variabilities, and the addition of lateral excitation.  
Conceptualization, A.O.A. and N.K.; methodology, A.O.A., B.-G.S., and N.K.; software, A.O.A. and B.-G.S.; validation, A.O.A. and J.-J.B.; formal analysis, A.O.A.; investigation, A.O.A.; resources, N.K.; data curation, A.O.A.; writing--original draft preparation, A.O.A.; writing--review and editing, N.K.; visualization, A.O.A.; supervision, N.K.; project administration, N.K.; funding acquisition, N.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MIST) (No. 2020R1A2C2013256).  
The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of Kyungpook National University (protocol code KNU-2020-0085 on 14 August 2020).  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
Data Availability Statement: Not applicable.  
Conflicts of Interest: The authors declare no conflict of interest.