Does occupant age and arm position influence the pre-pretensioner repositioning effect in lateral-oblique low-speed...

Previous research has repeatedly shown pre-pretensioner (PPT) seatbelt effectiveness in reducing head and torso displacement in pre-crash maneuvers; however, in most of these studies the volunteers' hands were relaxed in their lap. Arm position variations can alter seatbelt interaction and influence the torso and head kinematics, particularly in far-side lateral maneuvers. This study's aim was to examine how arm position, age, and PPT activation influences head and spine kinematic in low-acceleration far-side lateral impacts.

Fifteen male human volunteers (five subjects per age group: 9-11 years, 12-14 years, 18-30 years) were enrolled in the study. A low-speed crash sled simulated a 60° from frontal low-speed lateral-oblique far-side pulse (2 g). A 3-point seatbelt equipped with a PPT triggered 200 ms before the sled onset. The following conditions were repeated twice: PPT vs noPPT and arms up (hands on knees) vs down (hands low on hips). Hands on the knees raised the upper arms, creating a clavicle pocket compared to arms down. Forward and lateral peak head and upper spine kinematics were collected with a 3D-motion capture system. Four-way repeated measures mixed model ANOVAs assessed the effect of Group, Arm, PPT, and Repetitions on the normalized kinematics. Tukey's HSD was used for pairwise comparison (p < 0.05).

A significant interaction of Arm and PPT (p < 0.02) indicated forward, and lateral head and spine displacement were similar between the two arm positions with the PPT (p < 0.9). In contrast, without PPT, arms up led to greater forward and lower lateral head and spine displacement than arms down (p < 0.05). A main effect of age (p < 0.02) showed greater forward displacement in children (9-11 y.o.) compared to adults (p < 0.03). A significant 3-way interaction of Group, Arm, and PPT (p < 0.03) showed that with the PPT, lateral C4 and T1 displacement was similar across arm positions in all age groups (p > 0.9) except in teens (12-14 y.o.) where the arms down led to greater lateral spine motion than the arms up position (p < 0.02).

The arms up position likely kept the shoulder belt engaged with the clavicle and reduced lateral head and spine motion than arms down, but arm position did not influence forward motion because the torso may respond to the shoulder belt load by flexing forward. The PPT overcame any influence that the arm position had on the kinematics by reducing motion similarly in both arm conditions. This was evident in all age groups, except for the teens, suggesting that different ages have different needs in terms of PPT force and onset.