Whiplash Biomechanics: Latest Findings

This study¹ was published at the 1998 Stapp Car Crash Conference, and was done by the TNO Crash Safety Research Centre. TNO is a Dutch organization funded by a group of European auto manufacturers who seem genuinely concerned about understanding whiplash injuries.

The primary value of this study is that it confirms what other recent studies have found in regard to the early phase of whiplash biomechanics—the phase that is now thought to be at the root of the whiplash problem.

The researchers in this study performed test collisions on 3 women and 14 men. The researchers carefully tracked head and torso movements, with specific attention to the relative motions of the head in comparison to the first thoracic vertebrae (T1). This is important, as it is the differential in velocity between the head and torso that is believed to be responsible for whiplash injury.

The most important findings are summarized as follows:

The head does seem to experience initial flexion, as has been written about in other studies. This is important, as it indicates that the entire spine experiences a straightening and compression in the very early phases of impact—in fact, long before the head even touches the head restraint. (These findings are summarized in STR Volume 2, #9 & 11, and Volume 3, #2.)
This study also found compression of the cervical spine during impact—and at potentially hazardous levels. The first researchers to write about the hazards of compression of the spine were Yang et al² (STR Volume 2, #9). They found that when the cervical spine was compressed, it lost its ability to withstand shear forces. For example, they found that a 40-pound load on the spine would reduce the stiffness of the facet joints by 73%. This current study reported that the cervical spine experienced a compressive load of approximately 50-pounds at the exact same time that the spine was undergoing shear strain of about 22-pounds. These significant forces were produced in collisions of just 9.5 km/h, or 6 mph.
The study also reported differences between men and women. "For the same values of seat back angle (25 degrees) and head restraint position ('high')...all values for peak head acceleration were larger for the females compared to the males."

The medical literature consistently reports that women are more likely to suffer long-term pain from whiplash injuries than are men. Unfortunately, test collisions rarely include women subjects. This study is unusual in that the researchers carefully examined the differences between the biomechanics of men and women. This examination confirmed what other authors have only suggested: that women may be at higher risk because of the smaller circumference of the female neck.

"Head peak accelerations can be used as a general qualitative indicator for head/neck injury. Based on this assumption, it can be concluded from this study that a smaller value for neck circumference is a risk factor for injury from rear end impacts. From real accident databases, it is already known that women are at higher risk for neck injuries from low severity rear impact crashes. Although this study did not use enough females to perform any statistical tests, the results of this study confirm that women are at higher risk. The fact that women generally have smaller values for neck circumference suggest that this may be the actual risk factor. It is possible that due to a smaller, less 'resisting' neck, the impact velocities of the female heads to the head restraints are higher, resulting in higher head accelerations."

As we can see from this figure, a 20% reduction in neck circumference (from .4 meters (15.7 inches) to .32 meters (12.6 inches)) results in peak head accelerations nearly 2.5 times higher! This higher acceleration occurs in the exact same impact speed and seating situation with proper head restraints.
The researchers, however, also provide another theory that must be investigated in relation to women and whiplash.

"However, another possible explanation for the higher peak x-accelerations for the female volunteers could be that the heavier males deflected the seat back more, and thus produced lower T1 accelerations, resulting in lower head accelerations. Indeed, we found higher mean values for the T1 and torso accelerations for the females compared to the males...Finally, we observed that the female volunteers generally demonstrated more rebound motions with larger maximum torso flexion angles compared to the males."

This is a new idea in the whiplash literature—that women are more likely to be injured in a whiplash accident because the seat back is too stiff. This stiffness results in a localized, fragile part of the body—the head and neck—taking the brunt of the collision, rather than a more whole-body experience for the heavier, male occupant, where the forces of the impact are more evenly distributed.
This study stresses the need for more tests on female subjects, to examine these and other issues.

van den Kroonenberg A, Philippens M, Cappon H, et al. Human head-neck response during low-speed rear end impacts. 42nd Stapp Car Crash Conference Proceedings (P-337), 1998. SAE 983158.
Yang KH, Begeman PC, Muser M, et al. On the role of cervical facet joints in rear end impact neck injury mechanisms. Society of Automotive Engineers 1997; SAE 970497.

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