Effect of Hybrid III Leg Geometry on Upper Tibia Bending Moments

The knee and ankle joint pivots of the Hybrid III dummy's leg are positioned in approximately the same orientation as the knee and ankle joint rotation centers of a human in a normal driving posture. However, the dummy's leg assembly is not simply a straight member between these two pivots. It is a zigzag-shaped solid link composed of one long straight section in the middle and short angled sections at either end, which form the pivots. The upper and lower tibia load cells are mounted on the straight middle section, making the upper tibia load cell location anterior to the line between the ankle and knee pivots and the lower tibia load cell location slightly posterior to the line between the pivots. Hence, an approximately vertical force on the foot can act along the line behind the upper tibia load cell and in front of the lower tibia load cell, creating bending moments. The upper tibia moment would bow the leg forward (negative Y moment by SAE J211 convention), and the lower tibia moment generated would bow the leg rearward (positive Y moment). Such spurious bending moments, particularly at the upper tibia, can be large enough to suggest a significant risk of proximal tibia fracture that likely would not exist in humans subjected to the same forces. Several instances of forward bending of the proximal tibia coincident with axial loading on the lower tibia have been identified among the data recorded in 64 km/h (40 mi/h) frontal offset crash tests conducted by the Insurance Institute for Highway Safety. The ratio of upper tibia bending moment to measured tibia axial force was approximately equal to the distance between the center of the upper tibia load cell and the line connecting the knee and ankle pivots. In addition, other measures recorded on the same leg indicated the proximal tibia bending was due predominantly to an upward force on the foot. This paper describes how the potentially erroneous indications of proximal tibia fracture risk were identified and suggests a method for estimating more realistic moments at the upper and lower load cell locations.