Load-Leg Supports as a Biomechanical Intervention in Rear-Facing Child Restraint Misuse

This study evaluated the biomechanical effectiveness of anti-rotation load-leg support in rear-facing child restraint systems (CRSs) under proper and improper (misuse) installation conditions. We conducted eight FMVSS 213 sled tests using a CRABI 12-month anthropomorphic test device under four installation configurations—proper installation, loose seat, loose harness, and both loose seat and loose harness—with and without the load-leg support. The use of the load leg reduced head and neck injury metrics in all installation conditions: mean HIC36 decreased from 682 to 571 (by 16%) and AIS≥4 head injury probability from 33% to 21% (p=0.049); mean Nij decreased from 1.39~1.28 (by 8%), and AIS≥4 neck injury probability from 26%~24% (p=0.069); and forward CRS rotation by 6~19degree, with larger reductions under proper installation and loose seat conditions. In contrast, the load-leg use increased chest 3-ms acceleration by 5% (62~65g), and AIS≥4 thoracic injury probability from 58%~63% (p=0.049), suggesting that because a 3-ms metric captures only short-duration loading, these findings should be cautiously interpreted. Force-plate measurements showed that the load leg transferred 90% (242 N.s) of the estimated CRS-ATD forward momentum under proper installation and 93% (249 N.s) under loose seat conditions, compared to 70% (189 N.s) under loose-harness conditions. These findings show that load legs provide effective head and neck protection, especially when the harness maintains strong child—CRS coupling.