The objective of this study was to assess the effect of modifying the seatback angle of a Child Restraint System (CRS) in the dummy readings and in the associated injury criteria in frontal impacts, performed according to the specifications of the regulation UN R129. A multibody model of a CRS was developed and validated using the experimental results obtained in a frontal impact of a commercial rear-facing infant seat (45- 86 cm) using the Q1.5 dummy. Then, the seatback angle of the CRS was virtually modified in the multibody model of the CRS between +10 degrees (more vertical) and -10 degrees (more horizontal) at 2.5 degrees intervals. The average CORA rating resulted on a fair score (0.611 average rating) of the dummy readings. The modification of the seatback angle showed that there is a trade-off between the forces and moments acting on the cervical and the lumbar areas of the spine. Although a more vertical configuration resulted in an improvement of the criteria specified by the regulations (average 15% reduction using +10 degrees), spine loads resulted on values three times higher than the reference configuration. Further research is needed to assess if the lumbar spine loads may reach injurious levels for these reclined postures.