2. Occupant lower extremity anatomy and injury biomechanics

The lower extremity is the most frequently injured body region in frontal crashes that do not produce fatal or severe head/chest trauma. In real-world NASS-CDS and CIREN data, knee–thigh–hip (KTH) and foot–ankle injuries account for a large share of AIS 2–3 occupants, and they are the primary reason knee airbags (KABs) exist as a distinct restraint. To design a KAB that actually reduces injury rather than merely shifting it, the developer must first understand the load paths, tolerance limits, and failure mechanisms of the femur, knee joint, tibia, and foot–ankle complex.

Load Path and Skeletal Architecture

In a frontal impact the occupant's lower extremities are loaded through two principal interfaces: the knee–thigh–hip complex (knees contacting the lower instrument panel or knee bolster) and the foot–ankle complex (feet on the toe pan, pedals, or floor). The femur acts as a long column loaded primarily in axial compression and bending. The knee joint transmits this load through the patella, femoral condyles, and tibial plateau. The tibia then carries the load distally toward the ankle, where the talus, calcaneus, and distal tibia/fibula form a complex that is highly sensitive to dorsiflexion, inversion/eversion, and axial compression.

Two coordinate conventions are used throughout this course and must be kept consistent. The SAE J211 vehicle coordinate system defines +X forward, +Y to the occupant's right, +Z upward. Injury reference values are expressed in this frame unless stated otherwise. When a different frame is used (for example, a tibia-fixed local axis), it will be declared explicitly.

Femur: Axial Compression and Bending Tolerance

The femur is the strongest long bone in the body, but it is still vulnerable under combined axial and bending load. The widely cited tolerance for the human femur under axial compression is approximately 10 kN for a 50th-percentile male, with a corresponding bending moment tolerance on the order of 300–400 N·m. These values are used to calibrate the Femur Force criterion in the Hybrid III and THOR dummy legs.

In a frontal crash, the femur is loaded when the knee contacts the bolster. The load is transmitted proximally to the hip joint, where it can produce posterior acetabular fracture or hip dislocation if the knee bolster is too stiff or the load is applied too high on the knee. This is the classic trade-off in KAB design: a stiffer bolster reduces femur bending but increases hip loading; a softer bolster reduces hip load but allows greater knee excursion and potential femur fracture.

Knee Joint: Patellofemoral and Tibiofemoral Loading

The knee is not a simple hinge. Under frontal loading, the patella is compressed against the femoral trochlea, and the tibial plateau receives the transmitted load. The patellofemoral joint tolerates roughly 4–6 kN before chondral or osteochondral fracture. The tibiofemoral joint is more tolerant in pure compression but is vulnerable to shear and rotation, which can produce meniscal tears and cruciate ligament rupture.

For KAB development, the critical observation is that the knee bolster contact area and the KAB deployment pressure determine how the load is distributed across the patella and tibial plateau. A small, hard contact patch concentrates load and can fracture the patella; a large, compliant patch distributes load and reduces peak pressure. This is why KABs are designed with a broad, low-pressure cushion rather than a narrow, high-pressure one.

Tibia: Shaft and Plateau Injury

The tibia is loaded in axial compression and bending. Tibial plateau fractures occur when the femoral condyles drive into the proximal tibia under high axial load. Tibial shaft fractures occur under combined compression and bending, often when the knee is loaded while the foot is constrained by the floor or pedals. The tibia's tolerance is lower than the femur's, making it the more likely fracture site in many frontal crashes.

A practical design implication: if the KAB deploys too low or too stiffly, it can load the tibia directly rather than the femur, converting a potential femur injury into a tibia injury. This is a common failure mode in early KAB designs and is why deployment geometry must be validated against both femur and tibia criteria.

Foot–Ankle Complex: Dorsiflexion and Axial Compression

The foot–ankle complex is the most distal and often the most overlooked load path. In frontal impacts, the foot is typically trapped between the toe pan and the pedals. As the occupant's body moves forward, the ankle is forced into dorsiflexion. The ankle tolerates approximately 45° of dorsiflexion before ligament failure, and axial compression of 5–7 kN can produce pilon or calcaneal fractures.

Ankle injuries are strongly influenced by footwell intrusion and pedal displacement. Even a perfectly tuned KAB cannot prevent ankle injury if the footwell intrudes more than about 150 mm. This is why KAB development must be integrated with body structure design, not treated as a standalone restraint.

Injury Mechanisms and KAB Design Implications

The following SVG summarizes the load path from knee bolster contact through the femur, knee, tibia, and ankle, and maps each segment to its dominant injury mechanism and the KAB design parameter that most directly influences it.

Practical Design and Validation Workflow

Translating these biomechanical principles into a KAB design follows a repeatable workflow. The following example uses a 50th-percentile male occupant in a 56 km/h frontal rigid barrier test as the reference condition; all values are illustrative and must be replaced with program-specific targets.

1. Define the load path target. Set femur axial compression ≤ 10 kN, femur bending moment ≤ 300 N·m, tibia axial compression ≤ 8 kN, and ankle dorsiflexion ≤ 45°. These become the KAB performance requirements.
2. Establish the baseline. Run a sled or full-vehicle test with the KAB disabled. Record knee excursion, femur force, tibia force, and ankle angle. This baseline quantifies the injury risk the KAB must mitigate.
3. Tune cushion pressure and contact area. Start with a low-pressure (typically 0.5–1.5 bar internal) broad cushion. Increase contact area until femur force peaks below target. If tibia force rises instead, the cushion is loading too low—raise the deployment height.
4. Set deployment timing. The KAB must be fully deployed before the knee contacts the bolster. In a 56 km/h barrier test, knee–bolster contact typically occurs 40–60 ms after impact. Set time-to-fire (TTF) so that full deployment is achieved by 30–40 ms.
5. Validate against the full matrix. Test at 0°, ±15° oblique, and with out-of-position (OOP) occupants. OOP is critical: a KAB deploying against a knee already in contact with the bolster can produce high localized forces.

Key Takeaways

The lower extremity is a chain: femur, knee, tibia, and ankle are mechanically coupled, and a KAB that optimizes one segment can worsen another. The femur tolerates roughly 10 kN axial and 300–400 N·m bending; the knee tolerates 4–6 kN patellofemoral; the tibia tolerates 8–10 kN axial; the ankle tolerates 45° dorsiflexion and 5–7 kN axial. Effective KAB design requires low cushion pressure, broad contact area, correct deployment height, and timing that precedes knee–bolster contact. Ankle injury is dominated by footwell intrusion and must be addressed through body structure, not restraint alone. These biomechanical targets and design levers form the foundation for the KAB architecture and CAE correlation work in the chapters that follow.