The restraint of the pelvis by the lap belt is a prerequisite for occupant protection in a 3-point seatbelt system. If the lap belt slips over the anterior superior iliac spine (ASIS), the lap belt can penetrate the abdominal area during impact, leading to abdominal injuries. Many studies of this phenomenon (known as submarining) have focused on cases in which the lap belt is initially positioned correctly, but slips off due to the dynamics of impact. However, lap belt riding over the iliac bone can also occur when the lap belt is placed on the abdomen from the beginning, without overlapping the iliac crest. In this research, the relationship of the lap belt to the ASIS of seated occupants was investigated, first by statically measuring the initial lap belt-ASIS overlap in a group of volunteers and then by using FE analysis to assess the dynamic interaction of the lap belt with ASIS during test-sled crash simulations.

The lap belt-ASIS overlap of ten volunteers was measured as they sat in a small car’s rear seat (where the lap belt anchor is further back). The lap belt did not overlap with the ASIS for four volunteers:​ of these, three had a body mass index (BMI) of less than average (that is, <24.1 kg/m 2 ). Further measurements of 20 male volunteers sitting in a rigid seat were conducted to examine the factors which affect the lap belt-ASIS overlap. When volunteers sat in an upright posture, the overlap increased as the height of the ASIS relative to the thigh increased. When they sat in a slouching posture, for low-BMI volunteers the lap belt was located higher on the (flat) abdomen and the overlap of the lap belt with the ASIS tended to decrease.

FE analysis was carried out for rear seat occupants whose ASIS was located at the torso-thigh junction. For the occupant with a protruding abdomen, even though the lap belt did not initially overlap the ASIS, during impact the lap belt was pulled rearward and down;​ there was sufficient time for the lap belt to interact with the ASIS. However, for the occupant with a flat abdomen, since the abdomen fore-aft diameter and the flesh thickness on the ASIS was small, there was not enough time for the lap belt to interact with the ASIS even though the lap belt was pulled downward, and the lap belt penetrated into the abdomen. Thus, for low-BMI occupants in the rear seat both the initial lap belt-ASIS overlap and the dynamic interaction of the lap belt with the ASIS during vehicle deceleration tend to be insufficient for effective restraint of the pelvis.