The Mid Back: The mid back or thoracic spine is most commonly injured as a result of the asymmetry of most seat belt designs. As the torso moves forward the seat belt locks and the torso is forced to stop at the end of the shoulder strap. Because the shoulder strap crosses only one shoulder (usually the left), the side without a strap (usually right) is able to travel forward further which results in a forward flexing and twisting injury to the mid back.

The Lower Back: The lumbar is also frequently injured, but the mechanism differs from the above. As the torso and pelvis move forward the pelvis is stopped by the lap belt. However, the torso continues to move forward which producing a shearing force in the lumbar.

Pain Sources: When the back is injured, pain is experienced. Damage to these tissues also results in inflammation, edema, microscopic hemorrhage will seen. The release of noxious chemicals such as histamine, prostaglandins, substance P, and kinins which further hypersensitize already painful and injured tissues. In addition, when spinal injury occurs spinal muscles often reflexively spasm to protect the area from further injury - whether or not the muscles are injured. This spasming can lead to muscle ischemia (lack of oxygen) due to increased energy demands.