In rear end collisions, the movement of the occupant can be subdivided into three phases:​ first, a relative rearward movement (towards the seat) occurs. Second, due to seat elasticity and deceleration of the car after the collision, the occupant reverses the direction of movement. Third, the forward-moving occupant is caught by the seat belts. The latter two phases are generally termed "rebound phase". It has long been assumed that the rebound phase in low speed rear end impacts is of a minor biomechanical significance where soft tissue neck injuries (also termed whiplash associated disorders or WAD) are concerned. In view of the increasing elasticity and stiffness of modem seat back designs, which lead to an increase of the biomechanical loads imposed on the occupant during the rebound phase, it is deemed necessary to re-examine this assumption.

Results from a total of 2.5 sled tests with a standard crash pulse, various car front seat models, and using a Hybrid IIUTRID anthropomorphic test device are examined with respect to relative displacements and relative displacement velocities of the c.g. of the head and the uppermost thoracic (Tl) vertebra. These values are assessed for the different phases of the occupant relative motion, and brought into correlation with mechanical properties of the various seats tested. Furthermore, we propose ways of quantifying "elasticity" of the seat back, and discuss possible injury mechanisms occurring during the rebound phase.