Low-Speed Accidents and Minimal Force Causing Bodily Injury
Patrick Sundby, Accident Investigator
Mark Studin DC, FASBE(C), DAAPM, DAAMLP
Citation: Sundby P., Studin M. (2019) Low-Speed Accidents and Minimal Force Causing Bodily Injury, American Chiropractor 41(7) 44, 46, 48-49
When considering bodily injury, too often rhetoric or false perception “rules the day” in spite of sound conclusions based upon the mathematics in physics. This is commonly seen in Independent Medical Examination, Defense Medical Examination and in the courtroom. When assigning causality in the clinical setting, most doctors experienced in the diagnosis and management of trauma cases have concluded their patient's bodily injuries are directly related to the specific trauma, but don’t have the tools to render an accurate rationale. To demonstrably conclude the transference of forces from the bullet car to the target car and then to the occupant, you must first understand and then apply the principles of the “forces” involved. There are several components to discussing the forces applied to the occupant in a collision and here we will discuss the two most important, the quantity of forces delivered and how the force is applied.
The quantity of the force? What do we mean when we say that? There are a lot of different scales one could use, so we need one which is reasonably universal and applicable. For this we use “g-forces.” G-Force is a relationship to gravity which can be easily quantified to any event of motion. The odds are good you (the reader) are sitting in a chair, the chair exerts a force on you to keep you from falling to the floor, this force is 1 g. You will experience this force for the entire time you are seated, which opens the second part of the discussion – time.
The g-forces you experience are one part of the issue, the time it takes to experience the force is the second part. Imagine flying in a military jet fighter and the pilot banks the plane into a turn. You will experience an increase in force on your body which is related to the angle of the bank and the radius of the turn – most importantly, you will experience this increase in force for as long as the plane stays in that flight path. If the force is 4 g’s and the plane maintains that path of travel for 10 seconds you will experience the force evenly over the 10 seconds. In most of this example the time doesn’t change.[1]
What happens when there is a time change? What happens when that same fighter jet lands on an aircraft carrier and the arresting wire take the plane from 200 miles per hour to zero in less than 4 seconds? The forces that are translated to the human body (what you feel) can be quantified in g-forces. The calculation is not quite as simple as multiplying the g forces against the time, rather we need to know the change in speed over the change in time. For the sake of discussion let’s say the slowest approach speed for the jet fighter landing on the carrier is 100 mph (147 fps) and it takes 4 seconds for the plane to come to a complete stop.
The math looks like this:
Although we commonly say g’s (meaning g force), there is no unit with this number, rather it’s a ratio of force acceleration against gravity (which is also acceleration) and the units divide out leaving us with just the 1.14. If we were in the plane in the scenario above, we would experience 1.14 times the force of gravity, 1.14 g’s.
We can apply this concept to starting to move from a complete stop. If were sitting in traffic, stopped, and we were struck from behind we would go from zero to a certain speed – let say 8 mph (11.76 fps). If the time to be accelerated took .1 or 1/10th of a second, we can also calculate the g-forces experienced by the occupant and then determine the injury potential. (See below)
The provided value of 3.65 g’s (in the calculation below) is the relationship experienced at the seat base and is not the same force experienced at the skull. We know the research shows the cervical spine and the skull experience approximately three times the force of the hip – why?
As the vehicle begins to move and so does the occupant’s hip, the skull however, isn’t moving just yet. After all the “slack” in the lumbar and thoracic spine is used the skull and cervical spine are all that’s left, and it takes time to use the slack in the lumbar and thoracic spine resulting in less time for the cervical spine and skull. As a demonstration of concept – if we said it takes 66% of the 1/10th of a second to load the lumbar and thoracic spine then 33% of the 1/10 is all that is left for the cervical spine and skull. This changes the calculations:
When we divide by 32.2 fps/s, we end up with 12.17 g’s at the cervical spine and skull. Notice this is almost exactly three times the initial 3.65 g’s at the hip.[2]
The graph below visualizes the forces experienced. The orange line is the force experienced at the cervical spine if twice the lumbar, the grey line is the force experienced at the cervical spine if three times the lumbar spine.
Now that we have explored the quantification of forces applied, let's look at how the forces act on humans. Below is a graph which depicts the forces experienced in everyday events as well as the collision we discussed earlier in this writing (8 mph at .1 seconds).
Consider how the forces on the bottom of the slide can act on a human, is coughing a natural act? Why is it then that the cited reference, (Brault et al 1998) can establish injury to the cervical spine and we can quantify that value at almost the same as coughing? By this comparison coughing and a rear-end collision at 2.49 mph should result in almost the same injury every time. Why then are doctors and hospitals everywhere not overrun with patients who have cervical spine injuries from coughing?
The answer is HOW the forces are applied to us! Walking, sneezing, coughing, hopping, sitting in a chair, etc. are actions we, as humans, are biomechanically designed to do. We do these things every day with no negative sequelae. However, when you sit in a vehicle and you are struck from behind nothing about that action mimics an activity which is normal to us. Being accelerated from behind in a short amount of time, such as a car collision, is not a natural action and not something we are designed to do.
When considering traumatic bodily injury to the human spine, advanced knowledge of spinal biomechanical engineering and spinal function at both the global and regional scale is a necessary requirement. Advanced knowledge is inclusive of the resistive forces of connective tissue attachments, bony stabilizing mechanisms and central nervous system (brain) innervation for both the guarding and the compensatory aspects of the body’s response to injury. Additional application of the principles of physics regarding the forces applied to the occupant in trauma, gives the provider a scientific rationale for causation and bodily injury devoid of false perception and rhetoric. The combination of spinal biomechanical engineering knowledge and an understanding of the physics of the forces applied will resolve most questions of fact and provides a demonstrable answer when assigning the cause of bodily injury.
References:
2. Brault J., Wheeler J., Gunter S., Brault E., (1998) Clinical Response of Human Subjects to Rear End Automobile Collisions, Archives of Physical Medicine and Rehabilitation, 79 (1) pgs. 72-80
[1] There is a change at the beginning and end of the maneuver, good for you if you recognized this!
Image Credit: Wikipedia Commons
[2] There are some variances in the results and the graphs, this is a prime example of rounding and/or truncating throughout the calculations.
Low Speed Crashes
and Missed Vehicle Damage
Standards to Demand in a Vehicle Inspection
By: Patrick Sundby, Accident Investigator
Specializing in Low Speed and Catastrophic Crashes
Mark Studin DC, FASBE(C), DAAPM, DAAMLP
One of the most common problems with low speed collisions is determining the extent of the damage. The common real world practice is to visually examine the exterior of the vehicle and document any damage. The problem with this is not knowing the extent of the damage behind the exterior panels. Very few cases have had through and complete vehicle examination. The question is why?
The crash Reconstructionist has a tedious job ahead of him when facing a collision with what appears to be minimal damage at first glance. Happer et al (2003) acknowledges different vehicles will have different damage, or appearance of damage, at the same speed due to different designs. The goal of the paper was to provide a sound method for determining the severity of a collision. Happer et al (2003) also states the physical evidence remaining after the impact must be reviewed and this process begins with dividing bumpers into three categories. In this writing we will focus on the second one, reinforcement beams with a polymer absorber. These bumpers are categorized as having a metal reinforcement beam with a polymer absorber behind a plastic or urethane cover; this is the bumper to focus on as the vast majority of the vehicles on the road today are constructed in this manner.
Below is a picture of one such polymer structure. In this instance a vehicle struck a guardrail (during a training event) in a glancing motion and the polymer structure was pushed out of the bumper cover.
In the above photograph the damage is obvious, what we need to focus on are the collisions where damage appears minimal. The purpose of the polymer structure is to crush under a predetermined load to reduce the damage to a vehicle during lower speed collisions. This structure eliminates or reduces the damage to structural parts and thus will reduce the cost of repair as well. The energy it takes to deform the polymer structure also reduces the likelihood of injury to the occupant – IF it deforms. When the polymer structure doesn’t deform what are the consequences? Consider the photograph below:
The vehicle in the photograph was the striking vehicle in a rear end collision. The bulge & paint scraps in the bumper cover, the misalignment of the front fascia to the hood and fender (next to the headlight), and a small crack in the edge of the fascia near the grill are all external signs of damage. In this case, the vehicle was deemed to have “minor” damage but no further structural analysis was completed. How can one be sure the damage is limited to just what we see?
Let’s take a minute and draw a comparison. Dr. Studin has often spoke of strain/sprain. As a quick recap, there are three levels, primary stretches the tissue & fibers, and secondary begins to tear the tissue & fibers, and tertiary is a complete tearing of the tissue and fibers. When a patient has a complaint of pain and there are no outwards signs of trauma, i.e.: no scrapes, bruising, or other wounds some form of medical imaging is ordered. The imaging is ordered to see inside of the patient and determine if there is any internal injury. Trauma to a disc in the cervical spine (neck) is an example of an injury which would not be expected to show up on any external physical exam but should be easily seen in a good medical imaging process and with correct image reading or interpretation.
The concept of examining a patient completely to determine the source of the problem is the same template which should be applied to a vehicle inspection. A complete inspection demands measuring all the structural components against known factory specifications, this process could entail the removal of the bumper cover, grill, headlights, and other parts to ensure accuracy. Further, as time passes secondary systems can elude to undisclosed damage. As an example, if the geometry of the suspension changed due to a collision the alignment could be off resulting in uneven tire wear. It would take some time for the wear patterns in the tires to change.
The concern for this level of detail can be summarized by saying any energy which is absorbed by the vehicle, but not accounted for, will reduce the final calculated speeds. If you want to be sure about the results, you need to be sure about the facts and must have all the internal protective structures analyzed, not just the “skin of the car.” A perfect understructure can indicate significant energy transferences to the occupant, where “crushed” understructures can mean the car absorbed or deflected the energy and protected the occupant.
Not knowing is leaving the final answer to rhetoric vs. the truth, with physics to back it up.
Reference:
Happer, A., Hughes, M., Peck, M., and Boehme, S., "Practical Analysis Methodology for Low Speed Vehicle Collisions Involving Vehicles with Modern Bumper Systems," SAE Technical Paper 2003-01-0492, 2003, doi:10.4271/2003-01-0492.
Patrick Sundby has decades of experience in the automotive industry including several years in law enforcement collision investigation. He has also been a driver training and firearms instructor in law enforcement and a police officer for 9 years before specializing in accident investigations. He has had the privilege of participating in both learning and teaching at Prince William County Criminal Justice Training Academy in Virginia and studied at the Federal Law Enforcement Training Center in Georgia. His specialty is low speed and catastrophic crashes and has testified over 500 times at various level. He can be reached at 571-265-8076 orpatrick.sundby@gmail.com
Dr. Mark Studin is an adjunct associate professor of chiropractic at the University of Bridgeport College of Chiropractic, an Adjunct Professor of Clinical Sciences at Texas Chiropractic College and a clinical presenter for the State of New York at Buffalo, School of Medicine and Biomedical Sciences for postdoctoral education, teaching MRI spine interpretation and triaging trauma cases. He is also the president of the Academy of Chiropractic, teaching doctors how to interface with the legal community (www.DoctorsPIProgram.com). He teaches MRI interpretation and triaging trauma cases to doctors of all disciplines nationally, and studies trends in health care on a national scale (www.TeachDoctors.com). He can be reached atDrMark@AcademyofChiropractic.comor at 631-786-4253.
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