The flexible pedestrian legform impactor (FlexPLI) is not applicable to high-bumper vehicles because of the lack of upper body representation. Therefore, we have been developing an advanced pedestrian legform impactor (aPLI) with an enhanced biofidelity for such vehicles. First, we developed a baseline aPLI by adding an upper body mass to the FlexPLI through the hip joint. Next, we identified optimization parameters by analyzing their sensitivity to its biofidelity, and optimized their levels by means of a response surface methodology. However, as a potential regulatory tool, the structure of aPLI needs to be as simple as possible to ensure its high repeatability. The goal of this study was to clarify the influence of simplifying the aPLI structure on its biofidelity. First, we identified candidate simplification parameters from the optimization parameters. Second, we analyzed the sensitivity of those parameters to the biofidelity for optimized aPLI specifications. Finally, we determined the simplest structure by accumulating simplification from the parameters in ascending order of their sensitivity without violating the biofidelity requirements. The sensitivity analysis showed that the sensitivity of a parameter of the optimized aPLI specifications may or may not be significantly decreased from the baseline specifications. The former includes the hip translational and ankle rotational stiffness, and the latter includes the hip rotational stiffness, resulting in elimination of corresponding freedom of motion at these joints. The mechanism behind such results will be further discussed in the paper. This study successfully identified simplified specifications of aPLI without sacrificing its biofidelity by clarifying the influence of the simplification on its biofidelity. Additionally, the procedure of simplifying specifications from the optimized ones was revealed to be useful as it utilizes the change of the sensitivity of parameters due to the interaction effect between them.