## 3 Results and Discussion
### Neck Flexion Phase  
The head rebound phase begins when the head restraint releases its stored energy after the maximum indentation. This energy causes the head to rebound at a specific rate, and at the same time, the cervical spine straightens [26]. This rate is influenced by the seat design and its material properties. In Arrangement C, the rebound interval started when the seatbelt restrained the torso rebounding motion after the neck extension at 136 ms (Figure 3a). The head's relative velocity with the torso was lower than the other arrangement since the head restraint was not present to provide the head with additional energy. Simulated results were terminated at the end of the rebound phase for A (good), B (poor), and C (without head restraint) Arrangements at 120, 180, and 250 ms, respectively. Variations in the neck tension between the three head restraint arrangements were insignificant due to the absence of airbag deployment (Figure 3b). Overall, a maximum elongation of less than 19.6 mm was recorded within acceptable neck displacement boundaries. However, the peak head flexion angle of 32 degrees relative to the T1 was beyond the safe range of motion [11]. Consequently, the posterior cervical ligaments were the most load-bearing components, followed by the cancellous bone, CL, and IVDs. The results suggest that ISL and LF ligaments were strained at a high rate, especially in Arrangements A and B, and thus failed at multiple bottom levels (Figure 4, A&B-iv). Similar observations were reported from experimentation on intact biofdelic fresh-frozen cadaveric cervical spines [28]. The middle and lower cervical spine (C3-C4 through C6-C7) were the most vulnerable segments for ISL and LF failure at low impacts and throughout the entire spine at 10 g impact. In general, the ISL was at higher elongations than LF, possibly because they are located at a greater distance from the center of rotation during hyperflexion. There was no sign of a teardrop fracture of the anteroinferior aspect of the C2-C4 vertebral bodies since the hyperflexion motion was combined with tension rather than compression displacements. However, their corresponding IVD was exposed to pinching stresses from the anterior annular fiber layer [12] (Figure 4, C-iv). Those stresses were higher than the injury tolerance of 2.7 MPa [12]. Figure 7: Sagittal screens of spinal cord effective stress levels (MPa) during neck hyperextension phase for head restraint arrangement (**a**) A at 63 ms, (**b**) B at 73 ms, and (**c**) C at 124 ms.  
Failure of the posterior ligaments has added additional structural instability resulting in increased compressive loadings on the anterior IVDs and leading to disc and facet degeneration. As a result, chronic pain will appear from facet joint osteoarthritis, as demonstrated by radiographic studies [28]. Fortunately, those injuries were not accommodated by a displacement of the posterior portion of its body into the spinal canal. Therefore, injury to the spinal cord was not predicted in this stage. Stresses on the CL were the greatest at C4-C6 levels and infrequently exceeded the sub failure injury threshold at 3.5 MPa. Therefore, they are less likely to be injured. Those injuries do not lead to death, but the joints cartilage may bleed and get damaged, which will promote chronic long-term neck pain [12]. They are more vulnerable when the flexion motion is combined with an axial rotational moment, as shown at the end of Arrangement B in Figure 4, B-iv. These rotational moments most likely appear from the three-point restraining seatbelt, where one shoulder of the torso is fully restrained, unlike the other. Studies predicted that they would carry up to 70 percent of the total load, which leads to failure in its structures [11]. Since this stage was comprised of chronic injuries at multiple levels, an airbag deployment system is mandatorily equipped in vehicles.