2. Injury biomechanics fundamentals

Injury criteria only make sense once you understand what physically fails inside the body and why. A HIC value, a Nij value, or a chest deflection limit is a proxy for a specific tissue-level failure mode. If you cannot name the mechanism, you cannot judge whether a given criterion is appropriate for the loading direction, the occupant population, or the restraint concept under evaluation. This chapter builds that mechanistic foundation body region by body region, and closes with the loading-path logic that connects mechanisms to restraint design targets.

Head: Contact vs. Inertial Loading

Head injury splits into two mechanically distinct families. Contact loading produces focal effects: skull fracture from a localized impact, and epidural hematoma from middle meningeal artery rupture. Inertial loading — angular acceleration of the head without direct impact — produces diffuse effects: diffuse axonal injury (DAI), subdural hematoma from bridging vein rupture, and concussion. The distinction matters because the same HIC value can correspond to very different injury risks depending on whether the pulse is dominated by translation or rotation.

Linear acceleration primarily drives focal contusion and skull fracture; the brain's tolerance to pure translation is relatively high. Rotational acceleration, by contrast, generates shear strain in the brain tissue because the skull and brain have different densities and are only loosely coupled. Strain concentrated at the gray–white matter interface and in the corpus callosum and brainstem produces DAI. This is why modern criteria increasingly supplement HIC with rotational metrics such as BrIC (Brain Injury Criterion) based on angular velocity components. For a restraint engineer, the practical implication is that a head path that avoids hard contact but permits large rotation — for example, an airbag that pushes the head laterally while the torso is held — can still generate injurious strain.

Skull fracture tolerance is on the order of 5–6 kN for frontal impact in adult specimens, with substantial variation by age and sex. Pediatric skulls are more compliant but the sutures are unfused, so the same load produces different fracture patterns. When setting targets for a child occupant, do not simply scale adult force limits by mass.

Neck: Compression, Tension, and the Flexion–Extension Envelope

The neck is a slender column of seven vertebrae, discs, ligaments, and muscles that must carry the head's mass (roughly 4.5–5 kg) while permitting large range of motion. Its injury mechanisms are organized by load direction:

  • Compression — axial loading through the cervical spine produces burst fractures and, in severe cases, spinal cord injury. This is the dominant mechanism in rollover and in head-first contact with a roof or A-pillar.
  • Tension — distraction loads injure ligaments and can produce facet separation or, at extreme levels, atlanto-occipital dislocation.
  • Extension — rearward bending beyond physiological range injures the anterior longitudinal ligament and, in the classic rear-impact scenario, produces whiplash-associated disorders. The facet joints and capsular ligaments are the primary load-bearing structures in this mode.
  • Flexion — forward bending loads the posterior ligament complex; combined with compression it produces the most severe fracture–dislocation patterns.

The Nij (normalized neck injury criterion) captures the combined compression–extension and tension–flexion envelope by normalizing the instantaneous axial force and bending moment against their respective tolerances. The normalization is the key idea: a neck can tolerate more moment when the axial load is small, and the interaction surface defines the boundary. When you evaluate a restraint system, plot the (Fz, My) trajectory against that surface rather than checking each component independently.

Chest: Deflection, Rate, and the AIS 3 Threshold

Thoracic injury in crashes is dominated by compression of the rib cage. Rib fractures are the sentinel injury; three or more rib fractures, or any rib fracture with associated hemothorax or pneumothorax, typically reaches AIS 3. The widely used chest deflection criterion (often expressed as a maximum deflection limit around 63 mm for the 50th percentile male Hybrid III, with age-dependent scaling) reflects the strong correlation between sternal deflection and rib fracture risk.

Two refinements matter in practice. First, deflection rate influences injury: the same peak deflection applied faster produces more fractures because viscoelastic tissue stiffens and energy is absorbed over a shorter stroke. Second, loading location matters: a concentrated load from a shoulder belt or a steering-wheel hub produces local rib fracture at lower total deflection than a distributed load from a broad airbag. This is why chest criteria are increasingly paired with belt-force or pressure-distribution measurements rather than deflection alone.

For the abdomen, the mechanism shifts to organ compression and shear. Solid organs (liver, spleen, kidneys) rupture under direct compression or deceleration-induced shear at their attachment points; hollow organs (intestine, bladder) are more tolerant of compression but vulnerable to burst from internal pressure. Submarining — the occupant sliding under the lap belt — is the classic restraint-induced abdominal loading path, concentrating force on the lower abdomen and producing both soft-tissue injury and lumbar spine loading.

Extremities: Bending, Axial Load, and the Long-Bone Failure Modes

Extremity injuries are rarely life-threatening but carry high disability cost and are a frequent differentiator between restraint concepts. The femur fails in bending under knee–bolster or knee–airbag loading, with a tolerance on the order of 10 kN axial force for the 50th percentile male. The tibia fails under combined axial and bending load in footwell intrusion. The ankle and hindfoot are vulnerable to axial compression from a loaded pedal or floor pan, producing calcaneus and pilon fractures.

The upper extremity follows a similar logic: the humerus and forearm fail in bending when the arm is trapped or when a side airbag deploys against an out-of-position limb. Because extremity criteria are often expressed as force or moment limits rather than acceleration, they are evaluated with load-cell-equipped dummy limbs rather than accelerometers.

When a restraint concept changes, ask which body region's loading path it alters before selecting criteria. A change that reduces head excursion but increases neck tension is not automatically an improvement; the criteria must be evaluated as a set.

Reading the Loading Path: From Mechanism to Criterion

The figure below maps each body region to its dominant injury mechanism, the physical quantity that best represents that mechanism, and the corresponding criterion family. Use it as a checklist when you define a target set: every criterion you adopt should trace back to a mechanism you can name, and every mechanism you care about should have a criterion that captures it.

Body-region injury mechanisms mapped to physical quantities and criteria A four-column diagram linking head, neck, chest, abdomen, and extremity regions to their dominant injury mechanisms, the representative physical quantity, and the criterion family used in restraint target setting. Mechanism → Quantity → Criterion Body Region Dominant Mechanism Representative Quantity Criterion Family Head contact + inertial Skull fracture, contusion (contact) DAI, SDH, concussion (rotation) a_res, angular velocity HIC, BrIC HIC15 / HIC36 BrIC, rotational limits Neck axial + bending Compression fracture, facet injury Ligament distraction, whiplash Fz, My at occipital condyle Nij Nkm (rear impact) Chest compression Rib fracture, hemothorax Rate-dependent stiffening Sternal deflection Viscous response Chest deflection limit VC, TTIm Abdomen compression + shear Solid organ rupture Submarining, belt intrusion Belt force, pressure Penetration depth Abdominal pressure Belt force limit Extremity bending + axial Femur / tibia fracture Ankle axial compression Axial force, moment at load cell Femur force limit Tibia index

Applying the Mechanism Map to Target Setting

Consider a concrete example: a frontal sled test at 56 km/h with a belted 50th percentile male Hybrid III, evaluating a new force-limiting retractor. The retractor pays out belt at a threshold force, reducing peak chest deflection but allowing greater forward excursion. Working through the mechanism map:

  1. Chest — deflection drops, so rib fracture risk falls. Verify that the deflection–time trace does not show a late secondary peak as the occupant loads the airbag after belt payout.
  2. Head and neck — greater forward excursion may increase head rotation and neck tension. Check Nij against the interaction surface, not just the peak moment.
  3. Abdomen — a lower belt force reduces submarining risk, but confirm that the lap belt geometry still engages the pelvis rather than the abdomen as the torso pitches forward.
  4. Extremity — knee–bolster loading may change if the occupant's kinematics shift; check femur force if the knee path is altered.

This is the discipline the mechanism map enforces: a restraint change is never evaluated on a single criterion. The target set is a vector, and the design is acceptable only when every component of that vector stays within its mechanism-derived limit under the specified loading conditions.

Do not transfer a criterion across body regions or loading directions without checking the underlying mechanism. A chest deflection limit derived from frontal sternal loading is not valid for lateral loading, where rib tolerance and deflection patterns differ substantially. Similarly, Nij tolerances are direction-specific and must not be applied to lateral bending without recalibration.

With these mechanisms established, the next step is to quantify them: how criteria are computed from measured signals, how tolerances are derived from cadaver and volunteer data, and how uncertainty in those tolerances propagates into the target you set. The mechanism map above is the reference you will return to whenever a criterion's applicability is in question.

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