3. Occupant kinematics and injury criteria for the lower extremities

In a frontal impact, the knee bolster is the first structure the lower extremity interacts with after the occupant has been restrained by the seat belt and begun to move forward. The bolster is not a passive trim panel: it is a load path that shapes how force is transmitted from the knee, through the distal femur, into the hip joint and pelvis. Understanding the interaction requires a clear picture of the occupant's lower-extremity kinematics and the four injury metrics that regulators and OEMs use to judge whether the bolster design is acceptable: knee-thigh-hip (KTH) loading, femur force, knee slider displacement, and the tibia index.

Kinematic sequence during a frontal impact

Establish the reference frame first, because every sign convention in this chapter depends on it. Use the vehicle coordinate system: +X forward (toward the front of the vehicle), +Y to the occupant's left, +Z upward. Occupant motion during a frontal impact is therefore predominantly in the −X direction (forward relative to the vehicle interior), and femur axial force is compressive, reported as a positive magnitude in most test protocols.

As the vehicle decelerates, the occupant continues forward. The pelvis is restrained by the lap belt, but the upper body pitches forward and the knees swing toward the bolster. The knee contacts the bolster surface, and the femur begins to load axially. Because the hip is a ball-and-socket joint, the femur can rotate about the hip while the knee is constrained by the bolster, producing a combination of axial compression, bending, and shear at the knee-thigh-hip region. If the bolster is too stiff or too close, the knee is stopped abruptly and the femur absorbs the full load; if it is too soft or too far away, the knee may slide along the bolster, changing the load path and potentially allowing the tibia to load against the lower edge of the panel.

The four injury criteria in context

Knee-thigh-hip (KTH) loading. The KTH criterion treats the knee, distal femur, and hip as a load-bearing chain. It is evaluated from the axial force transmitted through the femur and the associated bending moment at the hip. In the widely used KTH model, the injury risk is a function of the combined axial force and bending moment at the femoral head, with the femur acting as a beam loaded at the knee. The bolster's stiffness profile directly sets the peak of this combined loading: a stiff bolster produces a short, high-amplitude force pulse, while a compliant bolster spreads the same momentum change over a longer interval and lowers the peak.

Femur force. Femur axial force is measured with a load cell in the femur of the Hybrid III or THOR dummy (or a human body model in simulation). The commonly cited acceptance value for the 50th-percentile male is on the order of 10 kN compressive force, with the exact limit depending on the protocol (FMVSS 208, Euro NCAP, or the OEM's internal target). Femur force is the most direct indicator of how hard the bolster is pushing back on the occupant, and it is the metric most sensitive to bolster stand-off distance and foam stiffness.

Knee slider displacement. The knee slider is the mechanism that allows the dummy's knee to translate relative to the femur under load. Its displacement is a proxy for knee-joint shear and for how much the knee slides across the bolster face. Excessive slider displacement indicates that the knee is being driven sideways or that the bolster geometry is deflecting the leg rather than arresting it cleanly. In practice, a large slider displacement often correlates with a bolster surface that is angled or too narrow, causing the knee to skate along the panel.

Tibia index (TI). The tibia index combines axial force and bending moment at the tibia:

TI = (F_z / F_c) + (M / M_c)

where:
  F_z  = axial compressive force at the tibia (N)
  F_c  = critical axial force, typically 35.9 kN (50th male)
  M    = resultant bending moment at the tibia (N·m)
  M_c  = critical bending moment, typically 225 N·m (50th male)
  TI   = 1.0 is the injury threshold

The tibia index matters for bolster interaction because the lower edge of the bolster can contact the tibia directly, especially if the panel is too low or the occupant is small. A TI above 1.0 signals that the tibia is being loaded beyond its tolerance, which is a common failure mode when the bolster's lower lip is not chamfered or when the panel is positioned too close to the shin.

How bolster design maps onto the criteria

The four criteria are not independent; they respond to different aspects of the bolster–panel interaction, and a design change that improves one can worsen another. The table below summarizes the dominant sensitivities, using typical engineering direction rather than exact numbers.

Working through a design check

Consider a mid-size sedan with a 50th-percentile male Hybrid III occupant in the driver seat, frontal impact at 56 km/h. The bolster is a PP foam pad over a steel retainer, with a nominal stand-off of 420 mm from the hip point to the panel face. Suppose the baseline foam has a crush strength of 0.35 MPa and the femur force trace peaks at 9.2 kN at about 55 ms, with a knee slider displacement of 8 mm and a tibia index of 0.72. All values are within limits, but the femur force margin is thin.

To improve the margin, the engineer can lower the foam crush strength to 0.28 MPa and increase pad thickness by 15 mm. The expected effect is a longer, flatter force pulse: peak femur force drops, but the knee now travels further before being arrested, which can increase knee slider displacement if the panel face is angled. The check is therefore iterative: re-run the sled or simulation, confirm femur force and KTH both fall, and verify that slider displacement and TI have not crossed their limits. A useful rule of thumb is to keep the femur force peak below roughly 85% of the acceptance value so that build variation and occupant size variation do not erode the margin.

Reading the results

When reviewing test or simulation output, check the four criteria together and in the correct order. First confirm the contact timing: the knee should meet the bolster before the femur loads, and the force trace should rise smoothly. A sharp initial spike usually means the pad has bottomed out or the stand-off is too small. Second, check the femur force peak against the protocol limit and against the KTH combined loading; if femur force is acceptable but KTH is not, the bending moment at the hip is the driver, which points to panel geometry rather than pad stiffness. Third, inspect knee slider displacement for asymmetry or excessive travel, which indicates the knee is sliding rather than being captured. Finally, compute the tibia index from the measured axial force and bending moment; a TI near or above 1.0 with a low femur force is a strong signal that the lower edge of the bolster is contacting the shin, and the fix is usually geometric (raise or chamfer the lower lip) rather than a change in pad stiffness.

These four metrics form the quantitative bridge between the bolster design and the occupant's lower-extremity injury risk. The next step in the analysis is to connect them to the specific panel geometry and material properties that produce them, which is where the interaction analysis becomes a design tool rather than a pass/fail check.