2. Thoracic anthropometry and biomechanics

Anthropomorphic Test Devices (ATDs) are only as valid as the human anatomy they are intended to represent. For thoracic injury assessment, the critical design question is not simply "how much does the chest compress?" but "where does the load enter, how does it propagate through the rib cage, and at what deflection does structural failure become likely?" Answering that question requires a working knowledge of thoracic anthropometry — the geometry of the rib cage, sternum, and costal cartilages — and of the biomechanical pathways that carry belt and airbag loads into the thorax. This chapter establishes that foundation for both the THOR Hybrid III and WorldSID families, because the two devices embody different anthropometric assumptions and therefore respond differently to identical loading.

Rib Cage Geometry and Its Design Consequences

The human thorax is not a symmetric barrel. It is a truncated cone that is wider at the base, with ribs that sweep downward and forward from the spine, and with a sternum that sits anteriorly and slightly superiorly relative to the lower rib cage. Ribs 1–7 are true ribs, attaching directly to the sternum via costal cartilage; ribs 8–10 are false ribs, joining the costal margin; ribs 11–12 are floating. This hierarchy matters for ATD design because load applied to the upper sternum is distributed across a stiffer, more constrained structure than load applied to the lower costal margin, which is more compliant and more prone to localized deflection.

In the THOR Hybrid III, the rib cage is represented by a set of curved steel or aluminum ribs with defined stiffness, mounted to a rigid spine box and a deformable sternum. The WorldSID takes a different approach: its rib cage is a more anatomically contoured structure with separate left and right rib modules and a floating sternal plate, intended to capture asymmetric loading. The practical consequence is that the same 40 mm chest deflection measured on a Hybrid III and on a WorldSID does not represent the same injury risk, because the underlying geometry, mass distribution, and rib stiffness differ. Any tolerance criterion must therefore be tied to a specific device, not applied generically.

Sternal and Costal Load Paths

Load enters the thorax through two principal pathways. The sternal path carries anterior loads — seat belt webbing, airbag reaction — through the sternum into the costal cartilages and then into the rib arcs. The costal path carries lateral and oblique loads directly into the rib shafts, where bending and shear dominate. In a frontal impact with a three-point belt, the shoulder belt loads the upper sternum and the outboard costal margin, producing a combined anterior-lateral deflection pattern. In a side impact, the door trim and side airbag load the lateral rib cage almost purely through the costal path.

This distinction drives sensor placement. Sternal deflection sensors capture the anterior path well but can under-report lateral costal deflection. Rib-mounted or multi-point deflection measurement is necessary when the loading is oblique or lateral. For the WorldSID, the floating sternal plate and independent rib modules are designed precisely to preserve these separate load paths so that the measured deflection reflects the actual loading geometry rather than a rigid-body average.

Deflection Tolerance Criteria and Their Device Dependence

Deflection tolerance is the maximum chest compression — typically measured in millimeters at the sternum or at a specified rib location — beyond which the probability of skeletal injury rises steeply. For the Hybrid III, the widely cited reference threshold is on the order of 63 mm for a 50th-percentile male, derived from cadaveric and volunteer data and embedded in regulatory and consumer test protocols. For the WorldSID, the corresponding reference is lower, roughly 45 mm, reflecting both its more compliant rib design and its calibration against a different injury-risk curve. These are not interchangeable numbers, and treating them as such is one of the most common errors in ATD-based thoracic assessment.

The tolerance is also load-path dependent. A purely anterior sternal load produces a different deflection-to-injury relationship than a lateral costal load, because the rib cage's structural resistance is anisotropic. In practice, this means that a single scalar deflection limit is a simplification; defensible assessment uses the deflection value together with the loading direction, the measurement location, and the device family. Where multi-point deflection is available, the peak local deflection — not the average — should drive the injury-risk judgment.

Practical Calibration Check

Before trusting any deflection reading, verify the sensor chain against a known input. A reproducible bench check is to apply a controlled quasi-static displacement to the sternal plate with a calibrated linear actuator and compare the sensor output to the commanded displacement. For example, command a 20 mm step at 5 mm/s, hold for 2 s, and confirm the sensor reports 20 ± 1 mm with no hysteresis greater than 0.5 mm on return. A reading that drifts, saturates early, or fails to return to zero indicates a sensor or mounting fault — not a biomechanical finding. Record the device family, sensor location, and loading direction alongside every deflection value so that the tolerance criterion applied is unambiguous.

In summary, thoracic assessment in ATD design rests on three linked ideas: the rib cage's asymmetric geometry determines where load enters and how it distributes; the sternal and costal paths carry load differently and therefore require different sensor placement; and deflection tolerance is a device-specific, load-path-specific criterion rather than a universal constant. Keeping those three ideas coupled — geometry, path, and calibrated tolerance — is what makes a deflection number meaningful for injury assessment.