# Behavior Analysis of Active and Proactive Headrest during Low-Velocity Rear-End Collisions
## 4 Comparison of Neck Injury Indices for the Two Headsets  
Based on the sled test results, the head restraint contact (HRC) time, T1 acceleration, upper neck shear and tensile force (F\({}_{\mathrm{x}}\), F\({}_{\mathrm{z}}\)), head rebound velocity (HRV), neck injury criteria (NIC), and neck force criterion (Nkm) were derived. The measurement range of the neck injury was from t = 0 when the acceleration waveform began to be transmitted through the sled machine until the time when the contact between the head and headrest was lost due to recoil of the seat. Table 3 presents the values of these parameters derived from the headrest test results. The test data for the active headrest and the proactive headrest are compared here. In the experiment, the HRC measurement was performed by attaching a contact switch to the head support to electrically measure the time of the contact with a rear cover on the head of the human body. As shown in Table 3, the minimum and maximum HRC times of the active headrest were 58.9 ms and 99.7 ms, respectively. For the proactive headrest, these values were 52 ms and 64.3 ms, respectively, which are shorter than those for the active headrest. This result is consistent with the previous trajectory analysis. T1 acceleration is an injury index that examines the acceleration in the lower part of the neck based on the acceleration acting on the T1 node of the thoracic spine. The T1 acceleration of the active headrest ranged from 3.56\(\times\)_g_ to 7.27\(\times\)_g_, and that of the proactive headrest was relatively smaller and ranged from 1.83\(\times\)_g_ to 3.28\(\times\)_g_. In the active headrest, the dummy did not move initially due to inertia, but later moved slowly in the rear direction, and the chest of the dummy was supported by the seatback support. However, the head of the dummy was not supported by the headrest and, thus, continued to move. In this process, T1 acceleration showed an increasing tendency due to the increased relative movement between the head and torso. Shear force (F\({}_{\mathrm{x}}\)) at the top of the neck was initially negative when the head was restrained to the headrest, and, when the dummy was recovered by the rebound of the seat, the chest moved farther in the forward direction than the head and F\({}_{\mathrm{x}}\) became positive. The maximum value of F\({}_{\mathrm{x}}\) was attained when the contact with the headrest was lost due to the rebound of the head, and it ranged from 4.21 N to 166.07 N. The tension force (F\({}_{\mathrm{z}}\)) at the top of the neck was negative due to the compressive load \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Criteria**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**Active Headrest**} & \multicolumn{2}{c}{**Proactive Headrest**} \\ \cline{3-6}  & & **Minimum** & **Maximum** & **Minimum** & **Maximum** \\ \hline HRC & ms & 58.9 & 99.7 & 52.0 & 64.3 \\ T1 acceleration & \(g\) & 3.5561 & 7.2691 & 1.8361 & 3.2810 \\ F\({}_{\mathrm{x}}\) & N & 11.9289 & 166.0741 & 4.2053 & 64.9093 \\ F\({}_{\mathrm{z}}\) & N & 109.6478 & 873.4311 & 212.9865 & 407.8819 \\ HRV & m/s & 1.3417 & 3.0693 & 1.2398 & 2.0175 \\ NIC & m\({}^{2}\)/s\({}^{2}\) & 10.4189 & 24.1186 & 9.7396 & 22.1362 \\ Nkm & - & 0.1882 & 0.4855 & 0.1217 & 0.2430 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of sled test for active headrest and proactive headrest. acting along the axial direction of the chest when the head was restrained to the headrest, and, as the rebound increased the load in the axial direction, it attained a positive value ranging from 109.65 N to 407.88 N.  
Head rebound velocity (HRV) is the difference between the absolute velocity at the center of gravity of the dummy's head and the absolute velocity of the sled. HRV can be obtained by integrating the measurements from an accelerometer on the head model and the sled acceleration pulse values to convert to the velocity value. The calculated values ranged from 1.24 to 3.07 m/s.