Nonlinear Impact of Seatback Recline Angle and Crash Pulse Magnitude on Head Injury Risk During Rear-End Impacts

Out-of-position (OOP) testing is increasingly important due to the development of autonomous vehicles, innovative car seat designs, and the need to verify safety in various seating configurations. This study analyzes the impact of seatback recline angle and crash pulse magnitude on head injury risk during rear-end impacts, focusing on the Head Injury Criterion (HIC). Using a sled system and a Hybrid III 50th-percentile dummy, 12 crash scenarios were examined with crash pulses of 10 g, 15 g, and 20 g and seatback recline angles of 21°, 25°, 38°, and 55°. The results showed that increasing the seatback recline angle reduces peak head accelerations but extends their duration, which, based on the Wayne State Tolerance Curve (WSTC), may increase injury risk. The HIC increased nonlinearly with higher crash pulses, especially in upright positions. The study proposes the Pelvis-to-Headrest Transmission Effect as a newly observed dynamic mechanism affecting head and neck injury risk. Findings suggest that a more reclined posture may enhance biomechanical safety in rear-end collisions, although the effect is complex and depends on multiple factors. Video analysis and <i>Z</i>-axis acceleration data confirmed that certain reclined configurations can increase compressive forces on the cervical spine, highlighting the need for comprehensive safety assessment.