The importance of head injuries to restrained far seat occupants has been previously documented. Control of the kinematics leading to these injuries can likely be achieved by improved torso lateral restraint. In adults, seat belt pre-tensioning reduced lateral head displacement by approximately 200 mm in far-side impacts. Children, however, may demonstrate greater lateral movement as previous studies have shown greater spine flexibility in the pediatric population relative to adults. The objective of this study was to investigate pediatric and young adult far-side head kinematics in low-speed lateral and oblique impacts and explore the effect of pre-tensioning.

Thirty male human volunteers, ages 9-14 years (n=20) and 18-30 years (n=10), were tested on a lowspeed, sub-injurious crash sled at either 60° or 90°. The safety envelope of the crash pulse was defined by an amusement park bumper car impact. The acceleration pulse was provided by a customdesigned hydro-pneumatically-driven sled system composed of a cart on a set of low friction rails (max pulse:​ 1.91 g;​ rise time:​ 53.8 ms;​ pulse duration:​ 146.5 ms). Each subject was restrained by a customfit automotive three-point belt system with an electromechanical motorized seat belt retractor (EMSR). The EMSR activated 200 ms prior to initiation of the crash pulse and provided a pretensioning load of approximately 300N, with a rise time to peak load of 100 ms. The restraint system was designed such that the EMSR could be active or inactive. Photo-reflective targets were attached to a tight-fitting head piece on each subject and adhered to skeletal landmarks on the spine, shoulders, sternum, and legs as well as along the shoulder belt. A 3-D near-infrared target tracking system quantified the position of the targets throughout the event. Subjects participated in a set of 8 randomized trials, four with EMSR activation and four without EMSR activation. Maximum head and spine excursions were measured.

EMSR activation significantly reduced the magnitude of head and spine kinematics. With EMSR activation, lateral head excursion decreased by an average of 96 mm and 114 mm, and T1 excursions were reduced by an average of 105 mm and 106 mm for oblique and lateral impacts, respectively.

Although EMSR activation to reduce seat belt slack is primarily indicated as a frontal impact countermeasure, these data demonstrate its efficacy in reducing head excursion in far-side impacts. Lowspeed human volunteer tests provide insight into occupant motion at these impact angles in the presence of active musculature. These results are useful for the development of rear seat countermeasures.