Musings of a Compulsive Thinker

"Whiplash" injuries: a car safety lecture

Today we're going to learn about one of our friendly vehicle safety features, the "head restraint" (apparently, it is not properly referred to as a "head rest"), how it can save us from nasty "whiplash"-type neck injuries, and how to properly adjust it so that it will be more effective in the event of a crash.


In a rear-impact car crash, as the vehicle is effectively given a shove forward, a poorly protected occupant will undergo three primary phases of movement, as illustrated in the three-part diagram above [IIHS, 1997]. Initially, the torso is forced against the seatback and is pushed forward; the head, prior to contacting the head restraint, initially remains level and lags behind the torso. This results in a characteristic "S"-shape of the neck, where the upper portion of the neck is in flexion while the lower part is in extension (above, left). Following the "S-phase" is the extension phase, in which poor head support will allow the neck to transition into full extension (above, center). Here the torso may "ramp" up the seatback, causing the head restraint to provide even less head support. The forces on the head then accelerate it to catch up with and pass the torso, giving rise to the full flexion or "rebound" phase (above, right). So-called "whiplash" injuries may occur in one or more of these phases. In general, it is thought that the risk of whiplash injuries may be reduced or even completely eliminated if we can minimize relative motion between the head and torso. Controlling this relative motion involves various safety features, but head restraint geometry is especially important to reducing the risk of whiplash injuries in rear-impact crashes.


The two common measurements of head restraint geometry are vertical offset and horizontal backset. Vertical offset is commonly measured as the distance of the head’s center of gravity above the top of the head restraint; in U.S. federal safety regulations, this measurement is replaced by that of height above the so-called "seating reference point" (SRP). Horizontal backset is measured as the horizontal distance between the back of the head and the head restraint. The Insurance Institute for Highway Safety (IIHS) and the Research Council for Automobile Repair (RCAR) commonly define these as shown in the diagrams above [RCAR, 2001]. Studies over the last four decades have generally agreed that whiplash will be reduced if the head restraint is positioned sufficiently high and close behind the head. Independently from federal regulation, IIHS evaluates head restraints according to guidelines set by RCAR. IIHS recommended that head restraints have a vertical offset of less than 90 mm (3.5 inches), so as to make the top of the head restraint at least level with the head’s center of gravity, and a backset of less than 100 mm (4 inches) [IIHS, 1997].

As of January 1, 1969, Federal Motor Vehicle Safety Standard (FMVSS) No. 202 required that all passenger cars manufactured for U.S. sale must have a head restraint in the front outboard seating positions that could achieve a specified height above the SRP, providing adequate protection for a 50th-percentile male [Kahane, 1982]. This standard has not changed since then. According to IIHS, the standard is weak; two major deficiencies are the lack of a minimum height requirement for head restraints in the down position, and the lack of a backset requirement. FMVSS 202 states only that head restraints must achieve a certain minimum height above the SRP when in the fully-extended position; when adjustable head restraints meeting this requirement are left in the down position, the occupant will be inadequately protected. In addition, the required height is only minimally protective. An IIHS evaluation of 1997 model year cars rated over 50% as having poor head restraint geometry, and less than 3% were rated "good" [IIHS, 1997].

To address the deficiencies, the National Highway Traffic Safety Administration (NHTSA) has revised FMVSS 202 in recent years [NHTSA, 2000]. Effective Sept. 1, 2009, head restraints must achieve a new, greater height above the SRP and lock in this position, with a specified minimum lowest height; the head restraint's backset must also fall within a specified range in any adjustment position. According to NHTSA, the new front outboard standards will provide adequate protection for 99.7% of the male population and all females, where "adequate protection" is defined as the head restraint reaching at least as high as the head’s center of gravity.

IIHS and RCAR support these revisions, but nonetheless there is still concern that the revised standards are not what they should be [IIHS, 2001]. RCAR recommended, and IIHS has adopted, stricter testing standards consisting of both geometric and dynamic tests [RCAR, 2006]. By these standards, an IIHS evaluation of 2004 model year cars rated 80% as having "good" or "acceptable" head restraint geometry; however, once seats passing this static geometry test were subjected to RCAR dynamic tests, ratings dropped significantly. By the combined static and dynamic testing standards, only about 33% (24 out of 73) passed with a rating of "good" or "acceptable." Including the 24 car seats that did not pass the static test, a total of about 56% (54 out of 97) were rated "poor" [IIHS, 2004].

With the average level of current technology, it's important to promote education among vehicle drivers and occupants alike. Most occupants don't properly position their adjustable head restraints; in many cases this may be due to mere ignorance and/or indifference, but it is likely that many other people leave their head restraints down for reasons of comfort. Ford Motor Company, for example, reported a recent increase in customer complaints pertaining to head restraint comfort; it is believed that these complaints correlate with reduced backsets in head restraints [NHTSA, 2004].

Next time you hop in the car, check your head restraint. The first time I did so, I was dismayed to realize that mine was positioned far too low to provide any real protection against rear-impact whiplash injuries. Now, however, it's adjusted as it should be. Remember: horizontal backset within 4 inches, and vertical offset within 3.5 inches, or such that the top of the head restraint is at least level with your head’s center of gravity. If you fail to comply with these recommendations, I will feel free to call you stupid.

References and further reading:
>> IIHS (1997): "Special Issue: Head Restraints" [PDF, 575 KB], Status Report Vol.32, No.4.
>> RCAR (2001): "A Procedure for Evaluating Motor Vehicle Head Restraints" [PDF, 471 KB].
>> C.J. Kahane, NHTSA (1982): "An Evaluation of Head Restraints."
>> NHTSA (2000): FMVSS 202: Head Restraints, Code of Federal Regulations, Title 49, Part 571.202.
>> NHTSA (2004): "Final Regulatory Impact Analysis, FMVSS No. 202 Head Restraints for Passenger Vehicles" [PDF, 4.3 MB].
>> IIHS (2001): "New head restraint rule would prevent many whiplash injuries, but proposed dynamic tests could compromise safety" [PDF, 230 KB], Status Report Vol.36, No.4.
>> RCAR (2006): "RCAR-IIWPG Seat/Head Restraint Evaluation Protocol" [PDF, 3.2 MB].
>> IIHS (2004): "Special issue: protection against neck injury in rear crashes" [PDF, 504 KB], Status Report Vol.39, No.10.

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Labels: engineering, everyday, travel

posted by the alygator @ 9:52 AM