In this study,rear seat restraint design optimizations were conducted for older children,adults and infants in rear-facing child restraint systems(CRS)using a set of MADYMO models,an automated simulation framework,and occupant belt-fit and posture prediction models.Sled tests were also used to validate the computational models.Results showed that the optimal belt anchorage locations and the seat cushion length for older children,adults,and rear-facing CRS-seated infants conflict with each other.In particular,more-forward lap belt anchorage locations that prevent submarining for older children would reduce the protection to both adults and CRS-seated infants,although the protection is still acceptable based on regulated injury criteria.Shorter seat cushion could provide optimal protection to older children and adults,but would significantly increase the CRS rotation.The findings of this study suggested that adaptive/adjustable restraint systems are necessary to simultaneously improve the rear seat occupant protection for all age groups.