Due to the lack of biofidelity seen in GHBMC M50-O in rear-facing impact simulations involving interaction with the seat back in an OEM seat, it is important to explore how the boundary conditions might be affecting the biofidelity and potentially formulate methods to improve biofidelity of different occupant models in the future while also maintaining seat validity. This study investigated the influence of one such boundary condition, which is the seat back foam material properties, on the thorax and pelvis kinematics and injury outcomes of the GHBMC 50th M50-O model in a high-speed rear-facing frontal impact scenario, which involves severe occupant loading of the seat back. Two different seat back foam materials were used – a stiff foam with high densification and a soft foam with low densification. The peak magnitudes of the T-spine resultant accelerations of the GHBMC M50-O increased with the use of soft foam as compared to stiff foam. However, the change in the average biofidelity of T-spine kinematics, as quantified through both BRS and CORA, was not significant. With an increased rearward excursion of the thorax in the case of the simulation with the soft foam, posterior rib fractures that matched PMHS rib fracture locations were predicted in the GHBMC M50-O, unlike the simulation with stiff foam. Pelvis kinematics of the GHBMC M50-O trended towards PMHS kinematics using soft foam, which was supported by a significant improvement in the average biofidelity as quantified through both BRS and CORA. However, pubic rami fractures were predicted in the GHBMC M50-O pelvis with the use of soft foam, unlike the PMHS. This study found that the peak magnitudes, shape of GHBMC M50-O kinematics and injuries are sensitive to foam material. However, a significant improvement in biofidelity of the kinematics and injury prediction of the GHBMC M50-O would require testing of foam materials at a compression rate that can be obtained from the PMHS tests, to accurately represent the seat foam in the rear-facing simulations, as well as age- and anthropometry-specific modifications to the GHBMC M50-O to capture PMHS characteristics more closely.