3. Injury mechanisms and tolerances

Injury biomechanics ultimately reduces to a single question: how much mechanical load can a given tissue sustain before it fails, and how do we translate that failure into a standardized severity score? The answer requires two parallel languages. The first is the language of physical quantities — strain, stress, acceleration, and deflection — measured or estimated at the tissue or body level. The second is the language of clinical coding — the Abbreviated Injury Scale (AIS) and the Injury Severity Score (ISS) — which converts anatomical damage into a numerical severity that engineers, regulators, and insurers can compare across crashes, vehicles, and populations. This chapter connects those two languages by examining the mechanisms that produce injury and the tolerance thresholds that separate reversible from irreversible damage.

From Mechanical Load to Tissue Failure

Every injury begins with deformation. When a crash pulse delivers energy to the body, tissues deform, and the resulting internal forces produce stress (force per unit area) and strain (fractional change in length). For a uniaxial tensile test on cortical bone, strain is defined as ΔL/L₀, where L₀ is the original length. Bone typically fractures at tensile strains of roughly 1.5–3.0%, depending on loading rate, age, and bone mineral density. Soft tissues such as ligaments and tendons tolerate much larger strains — 10–20% before partial failure — but their failure is progressive and rate-dependent.

Stress and strain are related through the tissue's constitutive behavior. At low rates, many biological tissues exhibit viscoelasticity: stiffness increases with loading rate, and the same strain can produce different stresses depending on how fast it is applied. This is why a slow compression of the thorax and a high-speed steering-wheel impact can produce different injury outcomes even at the same peak deflection. The strain rate (dε/dt) therefore becomes a critical variable, not just the peak strain itself.

Acceleration enters the picture as a body-level surrogate. Because direct tissue strain is rarely measurable in a crash, engineers use acceleration of the head, chest, or pelvis as a proxy. The head injury criterion (HIC) integrates resultant head acceleration over a 15 ms window:

HIC = max over t1,t2 of [ (1/(t2-t1)) ∫ a(t) dt ]^2.5 · (t2-t1)
where a(t) is resultant acceleration at the head center of gravity in g,
and t2 - t1 ≤ 15 ms.

A HIC of 1000 corresponds approximately to a 15–20% risk of severe head injury (AIS 3+). This threshold is not a tissue strain limit; it is a statistical correlate derived from cadaver and animal experiments, and it remains the regulatory backbone of FMVSS 208 and Euro NCAP frontal impact assessments.

Deflection and Compression Thresholds

For the thorax, deflection is the preferred metric because chest compression directly relates to rib fracture and pulmonary contusion. The chest deflection criterion uses the maximum sternal deflection relative to the spine, often normalized by chest depth. A common tolerance is 40–50 mm of sternal deflection for a 50th-percentile male, corresponding to roughly 25–30% compression. The Viscous Criterion (VC) combines deflection and velocity:

VC = max [ v(t) · C(t) ]
where v(t) = instantaneous chest deflection velocity (m/s)
and C(t) = instantaneous chest compression ratio (dimensionless).
A VC of 1.0 m/s is associated with a 25% risk of AIS 3+ chest injury.

For the pelvis and lower extremities, deflection thresholds are expressed as force or moment. A femoral axial force of approximately 10 kN is often cited as the fracture threshold for a 50th-percentile male, while the tibia tolerates roughly 8 kN. These values shift with age, sex, and bone quality, which is why injury risk curves are typically presented as probability functions rather than hard cutoffs.

Mapping Physical Metrics to AIS and ISS

AIS is an anatomically based, ordinal scale from 1 (minor) to 6 (currently untreatable). It is not a linear scale: an AIS 3 injury is not three times as severe as an AIS 1. The ISS is calculated from the three most severely injured body regions:

ISS = A1² + A2² + A3²
where A1, A2, A3 are the highest AIS scores in the three most severely
injured ISS body regions (Head/Neck, Face, Chest, Abdomen, Extremity,
External). Only the highest AIS per region is used. If any AIS = 6,
ISS is automatically 75.

Consider a hypothetical occupant in a 56 km/h frontal offset crash. Suppose the injury list includes: a rib fracture with hemothorax (AIS 3, Chest), a femoral shaft fracture (AIS 3, Extremity), and a minor facial laceration (AIS 1, Face). The ISS is 3² + 3² + 1² = 19. If the same occupant instead sustained a small subdural hematoma (AIS 4, Head) plus the rib fracture (AIS 3, Chest), the ISS would be 4² + 3² = 25. The squaring term means that a single severe injury dominates the score, which is clinically realistic but also means ISS is insensitive to multiple moderate injuries in the same region.

The bridge between physical metrics and AIS is the injury risk curve. These curves are derived from paired biomechanical and clinical data — cadaver tests, animal models, and real-world crash databases such as NASS-CDS and CIREN. A typical curve gives the probability of AIS 3+ injury as a function of HIC, chest deflection, or femoral force. For example, a chest deflection of 50 mm might correspond to a 30% risk of AIS 3+ chest injury, while 75 mm might correspond to 60%. These probabilities are population-level, not individual predictions.

Practical Application: Estimating Injury Risk from Crash Data

Suppose an engineer is evaluating a frontal crash test where the Hybrid III 50th-percentile male dummy records the following: resultant head acceleration yielding HIC₁₅ = 850, peak chest deflection = 42 mm, and peak femur axial force = 7.5 kN. Using published risk curves, the engineer might estimate approximately 12% risk of AIS 3+ head injury, 18% risk of AIS 3+ chest injury, and 10% risk of AIS 3+ lower-extremity injury. These are not additive; they represent independent body-region risks. The overall ISS cannot be predicted directly from these probabilities because ISS depends on the actual anatomical injuries sustained, not on risk percentages.

To connect the two, the engineer would use the risk curves to identify which body region is most likely to exceed AIS 3, then examine the restraint system's performance in that region. If chest deflection is the dominant risk, the countermeasure might be a load-limiting seat belt or a more compliant steering column. If head HIC is dominant, the focus shifts to airbag deployment timing and windshield/pillar padding.

Limitations and Cautions

Injury risk curves are population-level statistical constructs. They do not predict individual outcomes, and they are derived disproportionately from 50th-percentile male cadavers and dummies. Female, elderly, and pediatric occupants have different tolerances — often lower for bone fracture and higher for soft-tissue injury. The age adjustment for AIS risk is a recognized need in modern biomechanics, and databases such as the Global Human Body Models Consortium (GHBMC) are working to provide more representative finite element models.

Additionally, AIS and ISS were designed for epidemiological comparison, not for engineering design optimization. They are ordinal and nonlinear, which makes them statistically awkward. Many researchers now use injury risk functions directly as design objectives, bypassing AIS entirely. But for regulatory compliance and cross-study comparison, AIS and ISS remain the standard currency. Understanding their relationship to strain, stress, acceleration, and deflection is therefore essential for anyone analyzing crash safety performance.

The key takeaway is that physical metrics and clinical coding are two views of the same event. Strain and stress describe what happens at the tissue level; acceleration and deflection are measurable surrogates; AIS and ISS translate the outcome into a standardized severity score. A competent safety analyst must be fluent in all three layers and must know when each is appropriate.