The Effect of Obesity on the Restraint of Automobile Occupants

As obesity rates increase, the protection of obese occupants will become increasingly important in vehicle and restraint design. As a first step in this effort, this study seeks to compare the kinematics, dynamics, and injuries of obese post mortem human surrogates (PMHS) to (approximately) 50(th) percentile adult male PMHS in frontal impact sled tests with a force-limiting, pre-tensioning restraint system. Forty-eight km/h, frontal impact sled tests were performed with a sled buck representing the rear seat occupant compartment of a 2004 mid-sized sedan. The restraint system consisted of a 3-point belt with a pretensioner and a progressive force-limiter at the retractor. The test subjects were either obese PMHS or approximately 50(th) percentile adult male PMHS. Instrumentation included accelerometer packages on the spine. Deformation of the subjects' chests were measured using chestbands placed nominally at the superior-inferior locations of the 4(th) and 8(th) ribs. Tension in the restraint system was measured at the upper shoulder belt, lower shoulder belt, and the lap belt. Motion of the head, shoulder, pelvis, and knee were recorded using high-speed video. Two obese PMHS (average mass 137 kg, average stature 186 cm) and three approximately mid-sized male PMHS (average mass 68 kg, average stature 176 cm) were tested. The obese PMHS exhibited significantly greater forward motion of the head and the pelvis compared to the mid-sized PMHS. The obese PMHS also exhibited backwards torso rotation at the time of maximum forward excursion, whereas the mid-sized PMHS did not. The obese PMHS exhibited average maximum chest compressions of approximately 44% (+/- 9% standard deviation) of their initial chest depths, and exhibited 26 g (+/- 2 g) average 3 ms clip maximum chest resultant acceleration. In comparison, the mid-sized PMHS exhibited averages of 29% (+/- 9%) maximum chest compression and 35 g (+/- 4 g) maximum 3 ms clip chest acceleration. The obese PMHS exhibited 7 and 2 rib fractures, with maximum chest AIS scores of 3 and 2. The mid-sized PMHS exhibited 12, 2, and 17 rib fractures, with maximum chest AIS scores of 4, 1, and 4, respectively. This study is the first (to the authors' knowledge) to compare the kinematic, dynamic, and injury behaviors of obese and mid-sized PMHS in frontal impact sled tests with a force-limiting, pretensioning restraint system. The unfavorable kinematics observed with the obese PMHS highlights the difficulty of designing restraint systems to adequately restrain obese occupants, even with currently available advanced restraint technologies.