This illustration aids testimony by showing the mechanism of coup contrecoup injury in a front-end collision. On the left is the impact or hyperflexion stage. In this stage, the head snaps forward, the brain collides with the posterior skull, and muscles of the posterior neck (including the upper trapezius, shown in illustration) are stretched. On recoil or hyperextension, the head snaps backward and the brain bounces off the posterior skull and collides with the anterior. Additionally, anterior neck muscles, including the illustrated sternocleidomastoid, omohyoid, and sternohyoid, are stretched.