With an ageing population with increased needs of mobility, special attention to the safety of senior car occupants is becoming more important. As seniors in fatal crashes primarily die of chest injuries there is need of understanding how to reduce the risk of rib fractures. Recently new types of belts have been introduced on the market including inflatable and supplemental.

It has been suggested, one key to protect more ribs in frontal impacts is by optimizing the force distribution. In this study the role of kinematics or more specifically the orientation of the torso in relation to the belt loading, is evaluated.

The aim of this paper was to further understand the protection role of a supplementary belt. The hypothesis was that the upper body rotation, the twisting of the torso is critical in saving ribs. We conducted simulated frontal tests in three configurations by using a human FE model (THUMS) representative of an American 50 percentile male adult. The three configurations were a reference 4 kN three point belt and a driver airbag, an added 1 kN two point shoulder belt restraining the shoulder not restrained by the three point belt (the single) and two added 0.5 kN two point belts restraining each shoulder (the double).We compared the kinematics of the upper torso with the chest deflections and rib strains.

Adding a 1 kN belt load, single or double, resulted in reduced chest deflection and excursion as well as rib strain. The single belt as opposed to the double reduced the upper body rotation considerably. The greatest chest deflection reductions were found at the lower part of the chest for the single belt and at the upper for the double.

As a conclusion, the kinematics of the occupant may contribute to the loading on the chest. The paper is relevant for understanding how to optimize belt systems for minimal occupant loading and excursion.