Essentials: Biomechanical Engineering in Low-Speed Rear End Collisions

September 10, 2009

There are several basic biomechanical engineering principles that need to be understood before discussing low speed rear end collision scenarios.

First, when objects collide with each other, momentum changes. The vehicle being struck experiences an increase in its momentum, while the striking vehicle generally experiences a loss of momentum. There is an exchange of energy. Generally this is known as conservation of momentum, which is an important concept in both accident reconstruction and biomechanics.

Linked to the idea of momentum change is the concept of “reaction force.” By way of illustration, if a large force resulting from a vehicle striking a concrete barrier is applied to the striking vehicle over a short period of time, momentum changes quickly and often drops to zero. This type of collision results in a large “reaction force” with concomitant consequential injuries to the vehicle occupants. Put into human terms, if the vehicle occupants brace themselves prior to impact, the occupant bracing serves to extend the time of momentum change allowing for human joints to bend, etc., thereby decreasing joint reaction forces.

A common antidotal example comes from cats that jump from high places. At landing, when the cat lands on all four feet with its back raised and its four limbs in full extension, joint bending in the cat’s extremities and spine extension increase the time over which the cat’s linear momentum is brought to zero. Extending the time over which momentum changes, even though it may only be milliseconds, helps to reduce injury potential.

When an automobile collides with another vehicle, energy is transferred from the striking vehicle to the vehicle that is struck and is absorbed. During the sequence both vehicles may deform and crush. As the vehicles move through the collision event, with crush defamation taking place, transferred energy is dissipated. Modern cars have been designed with crush zones to lower injury thresholds to vehicle occupants. The kinetic energy that is not absorbed or extracted from the collision event by vehicle defamation results in energy loading to the vehicle occupants as abrupt changes in vehicle velocities occur, either through acceleration (the struck vehicle) or deceleration (the striking vehicle).

Delta V

In understanding biomechanical engineering, an adequate understanding of delta velocity, also commonly called “Delta V” is important. Delta V is a change in velocity. One methodology for describing the severity of a collision is to analyze the change in velocity that occurs. By way of example, where two vehicles of equal weight, traveling at an equal speed, i.e., 30 miles per hour, have a head-on collision with each other, each of the vehicles will experience a change in velocity (Delta V) of 30 miles per hour. If one of the vehicles weights twice as much as the other vehicle, the heavier vehicle will experience half of the Delta V speed change (15 mph). It is the Delta V coupled with the average and peak accelerations applied to the struck vehicle that creates the potential for human injury.

Most of the biomechanical engineering studies have focused upon low to moderate rear end impact scenarios. This is so because the risk of injury in using live human test subjects is low. Because there have been numerous low speed rear-end tests, the recurrent analysis and ultimate conclusion of no significant injury oftentimes survives evidentiary challenges at trial. Where the actual accident event is less common and there has been little human testing, the biomechanical analysis may be subject to a successful challenge against admissibility. See e.g., Hallmark v. Eldridge, 189 P.3d 646 (Nev. 2008) (side impact at significant speed).

Low Speed Rear Impact Collisions

The likelihood of significant injury arising from a low speed rear impact collision is the subject of scholarly debate. There is reasonable support in the literature for the following: At impact the receiving vehicle moves forward (assuming that there is some appreciable change in forward velocity). If this occurs, the vehicle motion has the effect of moving the seatback/headrest into the posterior torso and head of the occupant. Subsequent forward acceleration of the head and torso may occur as the head and torso rebound off of the seatback and headrest. Generally, the forward acceleration of the head and torso is similar to but generally less than the forward vehicle acceleration. Loading reductions of 40-60 percent of the inertial forces has been observed. (Strother and James, 1987, Evaluation of Seat Back Strength and Seat Belt Effectiveness in Rear-End Impacts)

The period of occupant acceleration is relatively short. A vehicle with seat headrests will restrain the head and prevent the head from going into extreme hyperextension. Under this scenario the cervical spine ligamentous tissues do not reach a point where the stretch (i.e., mechanical strain) is sufficient to produce trauma induced injury according to biomechanical engineering analysis.

Indeed, biomechanical studies have been performed using actual human volunteers, placing them in scenarios where a low velocity rear impact occurs at Delta-V velocities between 2.5 and 5mph. (See e.g., McConnell W., et al., 1993, Analysis of Human Test Subject Kinematic Responses to Low Velocity Rear-End Impacts) In these tests the cervical spine extension and flexion displacements were found to fall within the test subjects physiological limits (i.e., normal range of motion). Some of these tests involve digitized high speed cinematography data which indicated that there was minimal head link motion through the 400ms of torso and head movements.

Follow up human subject tests were performed using a Delta-V of 6.8mph which confirmed the previous observations. (McConnell W., et al., 1995, Human Head and Neck Kinematics After Low Velocity Rear-End Impacts – Understanding “Whiplash.” See also Szabo T., et al., 1994, Human Occupant Kinematic Response to Low Speed Rear-End Impacts) These tests found that the subjects of the test did not receive injury.

Overall, the test data involved subjects being exposed to delta velocities ranging from 1-10.3mph and the test subjects age ranged from 22-63 years. In the majority of these tests no injury symptoms were reported by the test subjects. The most severe symptom reported in these tests was minor neck pain lasting one week. A comparative look at the low impact literature can be found in Hannon & Knapp. Hannon, P. and Knapp, H. (2003) Chapter 18: A Review of the Low Impact Literature. Chapter in Watts, A. Low Speed Automobile Accidents.

Rear End Impact Collisions

In a rear-end impact scenario, the lumbar and sacral spine are protected by the energy-absorbing pad of the seatback as it initially moves forward into the torso resulting in absorbing energy and damping the impact to the torso. The torso link may subsequently rebound and move forward after initial deformation of the seatback.

In situations where the occupant is wearing a three point restraint, the safety restraint arrests the forward bounce back torso movement as the body link separates from the seatback. When the torso moves forward and impacts the shoulder belt — in those instances where the impact is large enough to produce this result — the torso of the occupant experiences greater negative accelerations. However, the negative accelerations load the anterior chest. The belt impact upon the chest does not significantly load the lumbar or thoracic spine in compression or bending. The lap belt secures the pelvis and does not produce a bending moment upon the sacral or lumbar spine because the shoulder belt prevents forward torso excursion.

Most claim adjusters have a general understanding that low velocity rear end accident scenarios do not produce significant injuries and at best might produce some short term transient muscle stiffness and perhaps mild aches and pains. However, most claim adjusters do not have an understanding, biomechanically, that there intuitive sense regarding minimal injury potential is actually supported by scientific biomechanical engineering principles and tests.

Plitt is a national legal expert on insurance law and insurance agent issues. Web site:

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