## 4 Validation and Discussions
Acceleration transmission investigation on human lumbar spine [47] found the frequency to be in-between 4 Hz and 5 Hz. Seat-head transmissibility analysis [48] reported the frequency value of 5 Hz. Acceleration transmission and mode shapes of human body were investigated [13; 49], and the first two natural frequencies for the entire human body were reported as 5 Hz and 8-9 Hz. ISO 5892 (2001) defined the frequency value of 5 Hz for seat-head transmissibility in seated posture without the presence of car seat backrest. Hence, it can be stated that the results from this simulation methodology showed justified levels of average frequency values, compared to the allowable ranges for human organs as explored in different studies. In this simulation work, to obtain the frequency data at different locations of a human inside an automotive structure, forced vibration formulations were implemented and three-dimensional human shape-specific stiffness values were applied. The frequencies of the waist and chest shown in Figure 7 were almost identical and this occurrence because of the consideration of waist and chest within a single ellipsoidal body. Human thigh was under the effect of highest level of frequency, and that was justified as the fixed connection had been applied at the thigh underneath instead of in contact interaction to the polyurethane seat foam cushion. Peak value of each curve of the frequency plot was above the permissible limit. It was obviously understandable that association of automotive seat cushion with hyperelastic and viscoelastic material properties to the developed human model in this unique finite element technique will provide more realistic outputs, possibly with lower peak magnitudes. Consideration of real shaped anatomical geometry of the human being will also add some advantage to obtain more accurate results. Because of the limitation of computer hardware, the simulation running time was set to 10 s for this research work. Operating the complete simulation work in a highly configured computer system \begin{table}
\begin{tabular}{c c} \hline \hline
**Human Segment** & **Average Frequency (Hz)** \\ \hline Head & 4–5 \\ Jaw & 6–8 \\ Eye & 60–90 \\ Chest organs & 5–9 \\ Upper limb & 3 \\ Stomach organs & 4.5–10 \\ Bladder & 10–18 \\ Pelvis & 5–9 \\ Muscles & 13–20 \\ Liver & 3–4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Frequencies of human organs and parts ([PERSON], 1989). will help the simulation to run more than 10 s. Bearing in mind the uneven and non-uniform surface of the road terrain, it is recommended that the analysis be run for at least 60 s. From the curve in the frequency plot, we observed that the frequency values gradually declined and stabilized over time. Thus, running the simulation for a longer period would eventually yield more accurate frequency and vertical acceleration values for the seated human portions. This unique simulation methodology is useful in predicting the vertical accelerations and frequencies at various segments of seated human body inside a moving car to optimize human health, comfort, and safety, regardless of the justified amount of gaps between the collected test data and simulation outcomes.