The effect of reclined seatback angles on the motion of booster-seated children during lateral-oblique low-acceleration impacts

Belt-positioning boosters (BPB) may prevent submarining in novel seating configurations such as seats with reclined seatbacks. However, several knowledge gaps in the motion of reclined child occupants remain as previous reclined child studies only examined responses of a child anthropomorphic test device (ATD) and the PIPER finite element (FE) model in frontal impacts. The aim of this study is to investigate the effect of reclined seatback angles and two types of BPBs on the motion of child volunteer occupants in low-acceleration far-side lateral-oblique impacts. Six healthy children (3 males, 3 females, 6–8 years, seated height: 66±3.2 cm, weight: 25.2±3.2 kg) were seated on two types of low-back BPB (standard and lightweight) on a vehicle seat and restrained by a 3-point simulated-integrated seatbelt on a low-acceleration sled. The sled exposed the participants to a low-speed lateral-oblique (80° from frontal) pulse (2 g). Three seatback recline angles (25°, 45°, 60° from vertical) with two BPB (standard and lightweight) were tested. A 10-camera 3D-motion-capture system (Natural Point Inc.) was used to capture peak lateral head and trunk displacements and forward knee-head distance. Three seat-belt load cells (Denton ATD Inc) captured peak seatbelt loads. Electromyography (EMG, Delsys Inc) recorded muscle activation. Repeated Measure 2-way ANOVAs were performed to evaluate the effect of seatback recline angle and BPB on kinematics. Tukey’s post-hoc test for pairwise comparisons was used. P-level was set to 0.05. Peak lateral head and trunk displacement decreased with the increasing seatback recline angle (p < 0.005, p < 0.001, respectively). Lateral peak head displacement was greater in the 25° compared to the 60° condition (p < 0.002) and in the 45° condition compared to the 60° condition (p < 0.04). Lateral peak trunk displacement was greater in the 25° condition than the 45° condition (p < 0.009) and the 60° condition (p < 0.001), and in the 45° condition than the 60° condition (p < 0.03). Overall peak lateral head and trunk displacements and knee-head forward distance were slightly greater in the standard than the lightweight BPB (p < 0.04), however these differences between BPBs were small (∼10 mm). Shoulder belt peak load decreased as the reclined seatback angle increased (p < 0.03): the shoulder belt peak load was statistically greater in the 25° condition than the 60° condition (p < 0.02). Muscle activation from the neck, upper trunk, and lower legs showed great activation. Neck muscles activation increased with the increase in seatback recline angle. Thighs, upper arms, and abdominal muscles showed small activation and no effect of conditions. Child volunteers showed decreased displacement suggesting that reclined seatbacks placed the booster-seated children in a more favorable position within the shoulder belt in a low-acceleration lateral-oblique impact, compared to nominal seatback angles. BPB type seemed to minimally influence the children’s motion: the small differences found may have been due to the slight difference in heights between the two BPBs. Future research with more severe pulses is needed to better understand reclined children’s motion in far-side lateral-oblique impacts.