The Effect of the Head-to-Head Restraint Distance on Occupant Kinematics during Low-Speed Rear-End Crashes

The longitudinal motion of the head, thorax and lumbar spine of two test subjects was measured in low-speed rear-end collisions in order to understand the effect of the head-to-head restraint distance (backset) on the occupant kinematics. The two test subjects were exposed to three rear-end impacts at two crash severities, nominal changes in velocity (ΔV) of 1.11 (low ΔV) and 2.22 m/s (high ΔV). The backset was hypothesized to be an independent variable that would affect the head and neck motion and was set at 0, 5 or 10 cm. The x and z-axis accelerations of the impacted vehicle and the anatomical x and z-axis accelerations of each test subjects’ upper thorax and L5-S1 region were measured and then transformed to an earth-based coordinate system. Head accelerations were measured at the mouth and these accelerations were transformed to an earth-based coordinate system at the head center of gravity (CG). Along the earth x-axis the vehicle accelerations increased first, followed by the accelerations of the lumbar region, the thoracic region, and then the head CG. In the high ΔV crashes the peak earth x-axis accelerations of the lumbar and the upper thoracic region were similar to the vehicle peak x-axis acceleration, regardless of the amount of the backset. In the high ΔV crashes the peak head CG earth x-axis accelerations were approximately 1.6 times greater than the peak vehicle x-axis acceleration with no backset and 1.4 to 2.0 times greater than the peak vehicle x-axis acceleration when backset was present. The amount of head rearward rotation, the peak rearward head rotational velocity and the rearward displacement of the head relative to the thorax increased as the backset increased in the high ΔV crashes. While head and neck kinematics were found to be influenced by the amount of backset, the manner in which the subject created the backset, either through cervical flexion or through a combination of cervical and upper thoracic flexion, also affected the kinematics.