Biofidelity of TRL Legform Impactor and Injury Tolerance of the Human Leg in Lateral Impact 2001-22-0023

In nonfatal car-pedestrian accidents, lower extremities are the most commonly injured body parts. The test device used to evaluate the car-front aggressiveness regarding the risk of these injuries is a legform impactor. Injury-related factors causing AIS 2+ injury in the human lower leg when exposed to a lateral impact representing a pedestrian accident should be identified. One of the test devices commonly used to evaluate the risk is the legform impactor developed by the Transport Research Laboratory (TRL). However, information about the biofidelity of this impactor and leg injury tolerance curves is lacking.

Thus, the objectives of this research are: 1) to determine possible injury-related factor(s) causing AIS 2+ injury in the human lower leg when exposed to a lateral impact simulating a pedestrian accident; 2) to evaluate the biofidelity of the current version of the TRL legform impactor; and 3) to propose injury reference value for estimation of the leg and knee joint injury risk by means of a legform impactor.

To determine factors causing leg fracture and knee joint ligament injury, injury-related factors were statistically tested in the present study. In this estimation, we analyzed knee lateral force, knee bending moment, knee shear displacement (i.e., relative displacement between the leg and thigh at the knee joint level in a lateral direction), knee bending angle, leg acceleration, and the impact force as factors that could correlate with the occurrence of the leg injuries. Regarding tibia fracture, both the impact force and leg acceleration were found to be significant factors. For ligament injury, it was found that only the shearing displacement was indeed very likely to be a significant factor.

The TRL legform impactor was evaluated by comparing its responses with the published experimental results obtained using postmortem human subjects (PMHSs). The evaluation was done under two conditions: impact point 84 mm below the knee joint center (shearing tests) and impact point near the ankle area (bending tests). Two impact speeds were used: 5.56 m/s and 11.11 m/s. Impact force, shear displacement, and bending angle were applied to measure the TRL legform impactor biofidelity. In general, the impactor responses were outside the biofidelity corridors determined using PMHSs. The exception was the impact force in the bending setup at a speed of 5.56 m/s.

To estimate the leg injury risk when using the TRL legform impactor, we determined transfer coefficients between the responses of this impactor and the human leg. Using these coefficients, we suggest an impact force of 6.9 kN and a shearing displacement of 7.9 mm as criteria for 50-percentile risk of the leg fracture and ligament injury with the TRL legform impactor.