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The Misconception of Threshold for Injury

The Misconception of Threshold for Injury – Where it Came From

In the early 1990s, a group of consultants who testify primarily for insurance companies and automobile manufacturers decided to run a set of safety optimized kinematic studies, using themselves as volunteers. Only individuals with a financial interest in the company were allowed to participate in order to limit the potential liability. Steps were specifically taken to prevent injury such as not impacting the researcher by surprise, the use of bite blocks and other safety measures.

The consultants reported their observations regarding their own injuries and then asserted that since they were not injured, they had determined a "threshold" for injury (1). It has not been established that these researchers did not have long term complications but regardless of this, it was asserted that the threshold for injury was a 4 to 5 m.p.h. change in velocity. (It is informative to note that there was reported to be no damage to the vehicles in the tests.) After the initial work of the consultants, several other groups of consultants who worked primarily for insurance companies and manufacturers ran tests using themselves as subjects. The fallacy of researchers using themselves as subjects was demonstrated by the assertion of the President of one company who stated when discussing their 1994 (2), that the researchers were hired to prove an injury would not occur. (It is informative to note that there was reported to be no damage to the vehicles in the tests.) However, even though this paper clearly cannot be used to establish a threshold, it is routinely cited as an example, of a case where an older woman with pre-existing radiographic findings was not injured. Omitted from the discussion is the fact that the woman was had a financial interest in not being injured and violated standard research protocol.

Since that time, it has become routine for an accident reconstructionist to assert that the change in velocity of a collision was 5 m.p.h. or less, regardless of the damage to the vehicles. This occurs despite the fact that the researchers failed to establish an actual threshold for the public at large. The researchers did not even establish that change in velocity is the relevant measure of injury potential in rear impacts. As an example, the differential movement of L3 versus L4 versus L5 is more important than the actual change in velocity or the peak acceleration.

Subsequently, some researchers have attempted to test non employees, although still in safety optimized conditions. This research has shown that symptoms can occur in collisions with a change of velocity of only 1 m.p.h.

Missing in this approach is a consideration of clearly identified aggravating factors which can increase the rate of injury. Also absent is the discussion of actual injury studies that have looked at actual collisions of the motoring population versus using pre-selected safety optimized volunteers. As discussed below, studies of actual people refute the claim of an identified injury threshold.

It has never been established that there is a minimum speed change value below which people are not injured in real collisions. This applies to all types of collisions including frontal impacts, rear impacts, side impacts and rollovers. For example, Professor Murray Mackay (i) has analyzed more than 2914 actual accidents reported in the U.S. National Accident Sampling System and showed that there is no threshold speed change value for an injury in real life (as opposed to staged) collisions. While Mackay concentrated on rear impacts, Kullgren and Kraft support the lack of injury threshold in rear impacts and demonstrated that there is also no injury threshold in frontal impacts (ii). Gabler (iii) demonstrated injuries in side impacts starting with a change of velocity as low as 2 to 3 m.p.h. However, this was not demonstrated to be the minimum threshold as was demonstrated by other NASS data (iv). Numerous other resources also support the lack of a threshold. (v, vi, vii, viii, xi)

1 SAE Paper 930889, Analysis of Human Test Subject Kinematic Responses to Low Velocity Rear End Impacts

2SAE Paper 940532, Human Occupant Kinematic Response to Low Speed Rear Impacts

i Low Speed Rear Impact Collision TOPTEC, SAE, 1994

ii Kullgren, Krafft, Tingvall and Lie, “Combining Crash Recorders and Paired Comparison Technique: Injury Risk Functions in Frontal and Rear Impacts with Special Reference to Neck Injuries”, Paper no 404,

iii Gabler, Fitzharris, Scully,Fildes, Digges, Sparke, “Far Side Impact Injury Risk for Belted Occupants in Australia and the United States, Paper No. 05-0420

iv NASS data provided to Dr. Zimmer from Ted Bloomquist.

v Minton, Murray, Stephenson & Gakasko, “A Study of Lower Back Strain Injuries Resulting From Road Accidents”, Transportation Research Laboratory, TRL532, 2002

vi National Automotive Sampling System, Department of Transportation.

vii Farmer. Wells, Werner, “Relationship of Head Restraints Positioning to Driver Neck Injury in Rear End Crashes”, Insurance Institute for Highway Safety, Arlington, VA

viii Elbel, Kramer, Huber-Lang, Hartwig, Dehner, “Deceleration During ‘Real Life’ Motor Vehicle Collisions – Predictors for the Risk of Sustaining a Cervical Spine Injury?”, Patient Safety in Surgery, 2009, 35 BioMed Cental, Ltd.

iv Smith, Smith, “The Lack of Correlation Between Spinal Injuries and Change in Velocity in Rear Impacts – An Evaluation of Spinal Strain, Proceedings of the 2007 International Whiplash Trauma Congress, October 2007