Over the years, a number of engineering studies have shown that head restraints are effective at reducing the head motion experienced during rear-end collisions—a properly adjusted restraint is considered to be a reliable way to reduce injuries from such impacts.
Unfortunately, another body of literature shows that head restraints in modern vehicles are virtually useless. They are either designed improperly by the manufacturer or adjusted improperly by the car owner. Head restraints that are not there to restrain the head do little good.
The fundamental problem with whiplash-type injuries, according to the latest studies with test subjects, is that there is a large difference in how the torso moves in comparison to the upper neck and head. It is this discrepancy in velocity change that is believed to be responsible for whiplash trauma.
Engineers at SAAB believe they have the solution. Known as the SAAB Active Head Restraint (SAHR), their invention is designed to prevent such injuries in collisions over 5 mph. The following is a description of the device:
"The active head restraint as such is mounted to a pressure plate in the seat back by means of a spring-resisted link mechanism. When the seat pushes the occupant forward with more force than the spring can resist, the plate moves rearward into the seat. This forces the head restraint to move upward and forward, thus supporting the head before the relative motion between the head and the torso becomes significant. When the load from the occupant diminishes, the spring returns the plate to its original position. The deflecting plate in combination with structural measures also softens the push from the seat back on the torso and allows occupant penetration into the seat back. In addition to further decreasing the gap between the head and the head-restraint on impact, this reduced force lowers the risk that the kyphosis of the thoracic spine is straightened out during impact, which may be important to reduce the S-bending of the neck."
In this current study, carried out by SAAB Automobile, researchers1 examined the effectiveness of this SAHR on impact tests on a crash test dummy. The researchers tested the dummy in a normal SAAB seat and one equipped with the SAHR system. They also compared the effects of "out-of-position" posture—when the dummy was leaning forward in the seat at the time of impact. They tested the new seats at Delta Vs of 7.5, 10, 12.5, and 16 mph; the standard seats were tested only at 7.5 and 16 mph.
The authors report that the SAHR seat "significantly reduces neck retraction and extension as well as neck loads compared to a baseline seat." And, their numbers support that finding—to a point. There are some issues, however, that have not been addressed.
First, there are significant differences between a Hybrid-III crash dummy and a live person. While dummies may be efficient in demonstrating gross motions of the spine, they are totally incapable of duplicating the complex motions of the human spine.
Second, the study doesn't provide enough data to make a determination of whether the seat really prevents the S-bending motion discussed above and in other studies. Ono and Kaneoka2 (STR Volume 3, Number 2) reported that the S-bending motion occurs between 50 and 75 milliseconds after impact—at least 43-68 milliseconds before the head normally touches a properly adjusted head restraint. The data is sparse in this current study, but according to one graph included showing the accelerations of the dummy during the "out-of-position" test, the head still seems to contact the head restraint at about 110-120 milliseconds. If this is indeed the case, then it would still be possible for injury to occur.
The bottom line? The results from this study are encouraging and it would indeed be a good thing if the SAHR can actually reduce the chance of injury. However, it is premature for SAAB to declare victory over whiplash, and, indeed, the authors write at the conclusion, "Thus it can potentially reduce the risk of whiplash..." [emphasis is added]
What really is needed is human testing with the new restraints before any determination can be made.