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

This study evaluated the biomechanical effectiveness of antirotation 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 to 1.28 (by 8%), and AIS ≥ 4 neck injury probability from 26% to 24% (p = 0.069); and forward CRS rotation by 6 deg-19 deg, with larger reductions under proper installation and loose seat conditions. In contrast, the load-leg use increased chest 3-ms acceleration by 5% (from 62 g to 65 g), and AIS ≥ 4 thoracic injury probability from 58% to 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.