Whiplash and whiplash-associated disorders describe a range of neck injuries related to sudden distortions of the neck that commonly occur in rear-end crashes. The most common symptom reported by whiplash victims is pain due to mild muscle strain or minor tearing of soft tissue. Other injuries include nerve damage, disc damage, and in the most severe cases ruptures of ligaments in the neck and fractures of the cervical vertebrae. Minor whiplash injuries generally are associated with pain and decreased range of motion in the head and neck. These symptoms usually last only a short time, but occasionally they last longer and include headaches, dizziness, and tingling in the arms. The physical injury to create symptoms of whiplash is uncertain. It is suspected that the biological cause of long-term whiplash symptoms is nerve damage while short-term pain may be a minor strain or sprain.

People can experience severe crashes with no neck injury if there is little or no movement of the head relative to the torso. Consequently, neck distortion resulting from sudden movement of the head relative to the torso probably explains most whiplash injuries. Hyperextension of the neck, or distortion beyond its normal range of motion, may explain many whiplash injuries, but experimental and field studies suggest that nerve damage and associated long-term symptoms can occur with milder levels of neck distortion. One hypothesis is that nerve damage is caused by motion of adjacent neck vertebrae during a crash.¹ Another hypothesis suggests that the nerve damage is caused by fluctuation in spinal fluid pressure arising from neck distortions.²

What many motorists refer to as a headrest is actually a head restraint. It is a common misconception that a restraint is a comfort feature. Head restraints are installed in vehicles for safety purposes and are an essential safety feature like lap/shoulder belts. Effective head restraints reduce the rearward motion of an occupant's head in a rear-end crash and decrease the likelihood of sustaining a whiplash injury. A 2002 Public Attitude Monitor survey asked participants what the main purpose of a head restraint was; 67 percent correctly identified safety, while 13 percent said comfort and another 15 percent did not know.³

While whiplash injuries are common in all types of crashes, motorists involved in rear-end crashes are more likely to experience whiplash. When a vehicle is struck from the rear it is accelerated forward, causing the seatback to push against the occupant's torso and propel it forward. The unsupported head lags behind the torso until the neck reaches its limit of distortion. Then the head is suddenly accelerated by the neck much like the tip of a whip; hence the term whiplash. Head restraints limit the neck distortion by pushing the head forward with the torso.

As the vehicle accelerates forward from the starting position (1), the seatback pushes the occupant forward (2). If the head is unsupported (top sequence), then the neck bends rearward (3). Eventually the seatback springs forward, pushing the body ahead of the neck during rebound (4).

Neck sprains and strains are the most frequently reported injuries in US insurance claims. In 2002, an estimated 66 percent of all insurance claimants under bodily injury liability coverage and 56 percent under personal injury protection coverage — the two most important insurance injury coverages — reported minor neck injuries. For 42 and 33 percent of bodily injury liability and personal injury protection claimants, respectively, neck sprains or strains were the most serious injuries reported. The cost of just those claims where neck pain was the most serious was about $8.5 billion, representing approximately 25 percent of the total dollars paid for all crash injuries combined.⁴

Whiplash injuries can be sustained in any type of crash but occur most often in rear-end collisions. Based on the National Analysis Sampling System data NHTSA has estimated that there were 805,581 whiplash injuries occurring annually between 1988 and 1996. Of these injuries, NHTSA estimates that 272,464 occurred as a result of a rear impact. A 1999 Institute study found that 26 percent of rear-struck vehicle drivers reported neck injuries to their insurance company.⁵ This was essentially the same as the 24 percent neck injury rate reported in a 1972 Institute study.⁶ Estimates of neck injury rates in other studies have ranged from 7 to 37 percent, depending on whether police or motorists reported the information.^7,8,9

The majority of studies on whiplash have focused on adult front-seat occupants. Unless otherwise noted, the following facts pertain to persons sitting up front.

Height: Height is a risk factor, particularly among females, according to recent German research.¹⁰ Shorter people often are protected by unadjusted head restraints. Stature may not play as big a role in the severity of injuries among males because many head restraints are too low to protect even shorter males. Taller motorists who do not adjust their head restraints are more likely to sustain whiplash injuries.

Gender: The same German report found females to be 1.8 to 2.2 times more at risk of neck injury in all types of collisions, a finding consistent with research by the Institute and other organizations. A Swedish study found that females with whiplash injuries were more likely to develop long-term symptoms. Fifty-five percent of females who sustained whiplash injuries went on to develop long-term symptoms compared with 38 percent of males.¹¹ One possible explanation is that males have more neck musculature than females for about the same size head.

Seating position: It is uncertain which seating position exposes an occupant to a greater chance of neck injury. One study concluded that drivers regardless of gender have a higher risk of injury than passengers. It was hypothesized that drivers are prone to move forward and away from the seatback as they reach for the steering wheel and observe traffic around them, whereas passengers usually are more relaxed and lean further back in their seats, with their heads closer to the restraint. Rear-seat occupants also are less likely than front-seat occupants, regardless of gender, to sustain neck injuries.¹²

A recent study indicates a higher risk of disability from neck injury among females than males in any seating position. The risk for females is lowest in front passenger seats and is significantly higher in rear seats than in front seats. For males, the risk differs only a little between front-seat positions but decreases from front seats to rear seats. The risk of disability for drivers is three times higher for females than for males; in rear seats, the risk is four times higher for females than for males.¹³

Yes. Since January 1, 1969, the National Highway Traffic Safety Administration (NHTSA) under Federal Motor Vehicle Safety Standard (FMVSS) No. 202 has mandated head restraints in the front outboard seats of all new passenger cars. On September 1, 1991, head restraint standards were extended to pickups, vans, and SUVs. Manufacturers may install either of two types of restraints. The first is an integral head restraint with a seatback that is high enough to meet the head restraint height requirement. The second type is an adjustable head restraint consisting of a cushion attached to the seatback by sliding metal shafts. Adjustable restraints must be adjustable to the same height as fixed restraints. Adjustable restraints can be moved and sometimes locked at different heights; some also can be adjusted horizontally to change the distance between the back of the head and restraint.

In December 2004, NHTSA upgraded FMVSS 202 to require head restraints that are higher and closer to the back of people's heads. The new height requirements for front and rear seat head restraints are similar to the ones mandated in the European Union.

FMVSS 202 establishes a minimum of 29.5 inches from an occupant's hip to the top of a head restraint. This compares with 27.5 inches under the previous rule. Adjustable restraints in their lowest (down) position must be at least 29.5 inches from an occupant's hip; the previous rule did not specify a minimum height for adjustable restraints. Another change is that all adjustable restraints must lock once in position. Under the previous rule, the amount of space between the back of an occupant's head and the head restraint (backset) was not regulated. Now the backset must be 2.2 inches or less. Manufacturers will have to comply starting September 1, 2009, under a phase-in schedule that requires all new vehicles to meet the rule by September 2011.

Head restraints will not be required in rear seats, but if they are voluntarily installed they must meet a height requirement. Fixed restraints in rear seats must be at least 29.5 inches from an occupant's hip, and adjustable restraints cannot be adjusted below 29.5 inches. There will not be a backset requirement for head restraints installed in rear seats. The phase-in schedule for rear head restraints begins September 1, 2010, with all vehicles complying by September 2012.

To reduce the likelihood of sustaining a whiplash injury in a crash, head restraints should be positioned high enough to protect the head so as to minimize neck distortion. Ideally the top of the head restraint should be positioned even with the top of the head and at least level with the top of the ears or about 3.5 inches below the top of the head. Because people differ in height, the amount of adjustment varies. For some occupants, no adjustment from the lowest position is required. The distance from the back of the head to the restraint should be as small as possible, preferably less than 4 inches. On seats without horizontally adjusting head restraints, this can be adjusted by adjusting the seatback recline angle.

Since 1995, the Institute has evaluated the geometry of head restraints in hundreds of new passenger vehicles. Each restraint is classified into one of four geometric zones defined by its height and backset (distance from the back of an occupant's head to the front of the restraint). Restraints are rated good, acceptable, marginal, or poor. Measurements are made with a dummy representing an average-size adult male with the seat set at a typical seatback angle. Restraints rated acceptable or good can protect motorists of various statures, while head restraints with marginal or poor geometry cannot begin to protect many taller people from neck injury. Head restraints have improved since the surveys began. In 1995, only 3 percent of measured head restraints were rated good compared with 51 percent in 2005. The number of poor restraints decreased from 82 percent in 1995 to 6 percent in 2005. Since 2004, head restraints have been rated not only with regard to geometry but also with a dynamic test if their geometry is good or acceptable.

The head restraint dummy (above left) is fully assembled for measurements. A score is assigned from the measurements using the table (above right). Manufacturers have been improving their head restraints over time, as shown in the graph below.

After years of neglect, improved head restraints are beginning to appear. Some auto manufacturers include head restraints that automatically adjust position when the seat is adjusted. Tall occupants who adjust a seat rearward to gain leg room also gain the added protection of a higher head restraint. Other manufacturers have incorporated active systems that automatically improve head restraint position during a crash. For example, the Saab active head restraint is attached to a lever arm in the seatback that moves the restraint up and forward as an occupant's back pushes into the seatback in a rear-end crash. Volvo's WHIPS, or whiplash injury prevention system, seat combines good head restraint geometry with a special seatback hinge that absorbs energy and thereby moves the body forward more gently until the head restraint catches the head. Mercedes-Benz has installed spring loaded active head restraints that move forward in response to a rear impact.

Good geometry is necessary but not sufficient for good protection. The relative stiffness of a seatback also helps determine effectiveness. Assessing seatback stiffness and other characteristics of head restraints designed to reduce whiplash, such as active head restraints, requires dynamic testing. A dynamic test simulates a crash and activates the whiplash protection system. Seats with head restraints that have good or acceptable geometry are being tested dynamically by the Institute. The test simulates a rear-end impact at 20 mph by another vehicle of the same weight and uses a dummy with a realistic spine and neck. Very few seats tested dynamically performed well even though they had good or acceptable geometry. IIHS head restraint ratings

Yes. The Institute analyzed more than 5,000 insurance claims and determined that, all other factors being the same, drivers with restraints rated good were 24 percent less likely than drivers with poor head restraints to sustain neck injuries in rear-end crashes. In vehicles with restraints rated acceptable compared with vehicles with poor restraints, insurance claims for neck pain were lower for females but not for males. This may be because females generally are shorter, allowing a lower head restraint to offer some protection.⁴ A second Institute study conducted in Rochester, New York, surveyed 585 drivers who had been in rear-end crashes. Measurements were taken while each driver assumed a normal driving posture with their restraints positioned as they were in the crash. The findings showed that head restraints positioned at or above the head's center of gravity reduced reported neck pain after a rear-end crash. Drivers with such restraints reported 40 percent fewer instances of neck pain than those with poorly positioned restraints.¹⁴ Finally, a 2005 Highway Loss Data Institute report found that personal injury protection claims were reduced by an estimated 11-22 percent for passenger cars with head restraints with good geometry compared with restraints with poor geometry.¹⁵ The Institute's current ratings which include a dynamic test have not yet been evaluated in comparison to real-world crashes.

The Institute recently gathered data from Nationwide, Progressive, and State Farm insurance companies to look at improved seat design in rear-end crashes. More than 2,000 property damage liability claims were identified involving selected vehicles: Ford Taurus and Mercury Sable models with improved head restraint geometry, Volvo S70s with WHIPS (whiplash injury prevention system), Toyota and Lexus models with the WIL (whiplash injury lessening) system plus a number of Buick, Nissan, Pontiac, and Saab models with active head restraints. The rates of insurance claims for driver neck injuries in rear-end crashes were compared before and after the seat and head restraint design changes. A 43 percent reduction in neck injury claim rates was found for the Saab, General Motors, and Nissan models with active head restraints, compared with similar models before such restraints were introduced. Before/after comparisons of Volvos and Fords also showed reductions in claim rates — 49 percent reduction for Volvos and an 18 percent reduction for Fords with improved restraint geometry. The Toyota WIL system did not reduce neck injuries.¹⁶ Further evidence from a 2005 Highway Loss Data Institute report showed that personal injury protection claims were reduced by an estimated 14-26 percent for passenger cars equipped with active head restraints struck in the rear.¹⁵

Minimizing the backset during a crash was recently shown to minimize whiplash injury. The recommended 6cm backset can be achieved either with a well positioned head restraint or with the incorporation of an active head restraint.¹⁷