Objective: This study analyzed available rear impact sled tests with Starcraft-type seats that use a diagonal belt behind the seatback. The study focused on neck responses for out-of-position (OOP) and in-position seated dummies. Methods: Thirteen rear sled tests were identified with out-of-position and in-position 5 th , 50 th and 95 th Hybrid III dummies in up to 47.6 mph rear delta Vs involving Starcraft-type seats. The tests were conducted at Ford, Exponent and CSE. Seven KARCO rear sled tests were found with in-position 5 th and 50 th Hybrid III dummies in 21.1-29.5 mph rear delta Vs involving Starcraft-type seats. In all of the in-position and one of the out-of-position series, comparable tests were run with production seats. Biomechanical responses of the dummies and test videos were analyzed. Results: When the head or upper body was unsupported in a severe rear impact with an upright Starcraft-type seat, neck extension moments were as high as 4.4-times IARV (Injury Assessment Reference Value). When the head, neck and torso was fully supported during the test, the responses were well below IARVs.Conclusions: The design of seats must consider situations where the head or upper body may not be supported as the vehicle is accelerated forward in a rear impact. This can lead to extension forces and moments on the spine and a risk for spinal fracture-dislocation and disabling cord injury with upright seats.