## 3 Results and Discussion
### Head Retraction Phase  
The overall cervical spine kinematics were divided into three distinct phases; the S-shape, hyperextension, and rebound phases [8]. The human body interactions with the accelerating seat, restraining seatbelt, and head restraint support are illustrated in Figure 3a. At the beginning of the impulse, the torso gradually absorbs kinetic energy while indenting into the seatback. Consequently, the torso begins to accelerate upward, depending on the passenger's seatback angle and material properties [9]. Meanwhile, the unsupported head does not move accordingly and lags behind the torso due to its inertia. This, in turn, causes the neck to endure combined flexion and extension moments at the upper and bottom levels, respectively, to form what is known as an S-shape posture.  
Figure 3: (**a**) Side view of simulated occupant response for A (good), B (poor), and C (without head restraint) with (**b**) their corresponding peak relative neck retraction, compression, tension, and extension measured between the occipital condyle (OC) and the first thoracic vertebra (T1).  
Figure 2: Comparison between the developed model results and [PERSON] et al. [22] reported data of eight male volunteers in terms of (**a**) head retraction in horizontal axis relative to the first thoracic vertebra and (**b**) head angular displacement.  
In comparison between the head restraint arrangements, the head retraction increased linearly with the head restraint-to-head gap distance (Figure 3b). Besides, the S-shape curvature becomes more severe when the head retracts further behind the torso. Arrangement A was more responsive in providing head support at an early stage before excessive head retraction (<12 mm). Therefore, the cervical structure sustained without failure against the stresses, as shown in Figure 4, A-ii. On the contrary, the poorly adjusted head restraint in Arrangements B and C has permitted the head to retract beyond 52 mm and 76 mm, respectively.  
The cervical spine elements started to collapse and tear due to the vertebra's excessive relative rotational movements. Firstly, the upper flexion moment forced the posterior ligaments (LF and ISL) to stretch beyond their physiological limit and fail in the craniovertebral region (Figure 4, B&C-ii). Accordingly, higher stresses were shifted to the CL and ITL ligaments due to increased S-shaped curvature. The ITL ligament was able to withstand those stresses due to its elasticity and strength [23]. Unlike the ITL, however, the CL ligaments were disrupted at C1-C2 facet joints. Secondly, the ISL ligament undertook significant shear stresses at the bottom spinous process (C6-C7-T1) owing to high neck contraction displacement due to extension motion. It is a very delicate ligament and will require long-term treatments to recover [14]. Lastly, shear stresses induced on the bottom IVDs have reached a significant level of more than 2.4 MPa (Figure 4, C-ii). Those will most probably produce damage to the outer annulus' neural arch and delamination [12].  
Figure 4: Effective stress levels (MPa) at the sagittal view for cervical cancellous bones and ligaments. Responses were captured for **A**, **B**, and **C** Arrangements at (i) initial posture, (ii) S-shape, (iii) hyperextension, and (iv) head rebound phases.