Evaluation of static belt fit and belt torso contact for children on belt-positioning booster seats.

Previous studies have indicated that gap between the seatbelt and torso (reduced belt torso contact) for children on belt-positioning booster seats (BPBs) may lead to less torso engagement and increased likelihood of shoulder belt slip-off during evasive vehicle maneuvers, potentially increasing injury risk during crashes. However, current BPB belt fit measures do not quantify belt gap and may not be able to fully discriminate between designs which provide good vs. poor dynamic outcomes. The goal of this study was to evaluate both novel (belt gap characteristics) and conventional measures of seatbelt fit for BPB-seated children. Ten BPBs and three seatbelt anchor locations were investigated. Fifty volunteers (4-14 years) were recruited and each evaluated on six unique combinations of BPB and seatbelt anchor location on a vehicle rear seat in a laboratory setting. A 3 D coordinate measurement system quantified positions of anatomic, seatbelt, BPB, and vehicle reference points. Novel belt gap (gap size, length, location, and percent torso contact) and conventional belt fit (position of belt on shoulder and pelvis) metrics were calculated using anatomic and seatbelt landmarks. Variation in belt fit and belt gap outcomes due to BPB, seatbelt anchor location, and anthropometry were investigated. BPBs produced significantly different outcomes, while seatbelt anchor location did not. BPBs with features that directly routed the lower portion of the shoulder belt more forward on the buckle side produced the largest (29.3 ± 12.6 mm) and longest (106.9 ± 68.2 mm) belt gap on average, while BPBs that pulled the belt less forward or did not directly route the belt produced the smallest (13.9 ± 6.7 mm) and shortest (16.9 ± 33.9 mm) gap on average. Belt gap outcomes were not strongly correlated with conventional belt fit metrics, indicating that evaluation of belt gap may provide additional insight when attempting to discriminate between BPBs which provide good vs. poor seatbelt engagement during vehicle maneuvers and crashes. This is the first study to evaluate belt gap characteristics for BPB-seated children. Results suggest that belt fit and belt gap are influenced by BPB design, particularly lower shoulder belt routings, and may have implications for belt engagement during dynamic events.