What does a “consistent with” finding actually assert in injury biomechanics?
A “consistent with” finding in injury biomechanics asserts that the loading a body experienced and the loading pattern associated with the observed injury overlap, so that the two do not contradict one another. Mechanism-consistency analysis compares two independently derived pictures. One is the loading the body experienced, reconstructed from physical evidence. The other is the loading pattern the observed injury type is associated with in the literature: direction, magnitude, rate, anatomical site. Where the two overlap, the injury is consistent with the event.
A “consistent with” finding does not assert that the loading produced the injury, that no other loading would have produced it, or that anything has been established about a particular person.
The asymmetry between the two possible outcomes of mechanism-consistency analysis matters. A finding of inconsistency is strong, because it identifies a contradiction — the injury pattern requires loading in a direction or of a magnitude the reconstruction does not support. A finding of consistency is far weaker, because a great many events are consistent with the same injury, and on its own a consistency finding excludes almost nothing.
How is a vehicle occupant’s body motion reconstructed from physical evidence?
A vehicle occupant’s body motion, or occupant kinematics, is not observed; it is inferred from marks. Contact damage to interior surfaces, headliner and pillar deformation, glass fracture patterns, footwell intrusion, seat track position, seat back deformation and the character of loading marks on restraint webbing together constrain where a body went and in what order.
Vehicle interior evidence of occupant motion is perishable in a way exterior damage is not. Interiors are cleaned, belts cut, seats repositioned, vehicles salvaged. One inspection before that happens is worth several after.
What can seat belt and airbag evidence show about an occupant’s position and restraint use?
Restraint use, which is frequently disputed, is one of the few occupant-position questions physical evidence can answer directly rather than by inference, and airbag deployment state, airbag module records and contact residue add to the picture of where an occupant sat relative to the restraint at the moment it acted.
Seat belt webbing carries evidence of loading in the marks left at the D-ring, latch plate and retractor, and in the condition of any pretensioner or load limiter.
What does a vehicle’s event data recorder hold, and what does it not?
A vehicle’s event data recorder typically holds a longitudinal velocity change over a recorded interval, together with pre-crash parameters and belt and airbag status flags. In the United States, event data recorder output is regulated as to content and format for vehicles that record it. Event data recorder output is valuable because it is independent of anyone’s recollection.
Event data recorder output is also bounded. The recorded interval may not span the full event, lateral and rotational components may be limited or absent, records can be truncated in a multiple-impact sequence, and data can be overwritten. The practical advice is to image the event data recorder early, with documented tooling and chain of custody.
Does delta-V alone tell you how much loading a vehicle occupant experienced?
No — delta-V, the velocity change in a collision, is an input, not a conclusion about how much loading a vehicle occupant experienced. Delta-V is a collision reconstruction’s main handle on occupant loading. It can be derived from crush measurement, momentum analysis, or recorded data, and where more than one method is available they are worth running against each other. Principal direction of force matters as much as magnitude, since a given delta-V loads a body differently head-on than obliquely.
Occupant loading is not delta-V itself. It is delta-V mediated by restraint, seat, posture, occupant size and the shape of the acceleration pulse. Two collisions with the same velocity change and different pulse durations expose an occupant to different accelerations.
How is a fall analyzed in injury biomechanics?
Fall analysis in injury biomechanics constrains fall height, body position at contact, surface stiffness and the resulting distribution of ground reaction force, a different set of variables from a vehicle occupant analysis. Fracture location and orientation carry directional information, and the geometry of a collapse differs from that of a fall onto an extended limb.
Scene evidence in a fall — surface condition, contact marks, disturbed items, final rest position — bounds the fall reconstruction the way vehicle interior evidence bounds an occupant analysis, and fall scene evidence disappears faster.
How much weight can an occupant simulation or human body model carry in an injury analysis?
The value of a multibody occupant model or finite element human body model depends entirely on validation: whether the model reproduces instrumented physical tests under comparable loading. Multibody occupant models and finite element human body models can be configured to a specific person’s size, seated position and restraint use, which no physical dummy can be adjusted to match exactly.
A simulation run for the matter, with no stated validation basis and no sensitivity analysis over inputs that were assumed rather than measured, is an illustration rather than evidence. Naming the assumed inputs and showing how far the simulation output moves when they change is what turns an illustration into evidence.
How do exclusion findings compare with affirmative findings in injury biomechanics?
Exclusion is the stronger use of injury biomechanics: the most durable biomechanical findings tend to be negative, while affirmative statements that an event produced a particular injury in a particular person carry a heavier burden and, as a matter of discipline, belong partly to medicine.
Examples of exclusion findings are that the reconstructed loading direction cannot produce the observed fracture orientation; that an occupant could not have reached the surface said to have caused an injury, given the seat track position as found; and that the recorded velocity change and the claimed mechanism do not fit together. These exclusion findings are falsifiable propositions, and they can be tested.
Where are injury biomechanics opinions typically challenged?
Injury biomechanics opinions are challenged predictably, on five grounds: that the reconstruction assumed the occupant position it needed, that the velocity change came from a single method with no cross-check, that the model was never validated for the loading applied, that a consistency finding was argued as proof of causation, and that the inspection happened after the evidence had been altered.
Injury biomechanics work that documents each assumption, its source and the sensitivity of the conclusion to it withstands all five challenges. A conclusion stated more strongly than the evidence supports does not, and the overstatement is usually confined to a sentence or two.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.