Seatback strength and injury potential in moderate to high-speed rear-end collisions were investigated in a series of 12 HYGE sled tests. The test methodology included the use of instrumented Hybrid-III anthropomorphic test devices (ATDs). Four tests employed a 95th percentile male ATD ballasted to a total weight of 300 lbs and subjected to approximate 15 mph Delta-V impacts. The remaining tests employed an unmodified 50th percentile male ATD with impacts of approximately 25 mph Delta-V, and three ATD positions, including two "out of position" postures corresponding to leaning forward ("forward" position), and leaning forward and inboard ("radio" position). Seats from three different vehicles were tested, representing a range of strength values. Upper neck values for N were less than 1.0 in all cases. Lower neck N values sometimes exceeded 1.0 with the 50th percentile male ATD out of position, and these values did not trend with seatback strength. With the 300 lb ballasted 95th percentile male ATD, tested "in position," all values of N in the upper and lower neck were less than 1.0. Thoracic spine loading did not demonstrate a relation with seatback strength. Lumbar spine compressive loads in the 50th percentile male ATD increased with seatback stiffness but were all within the range of loading associated with activities of daily living. Within the range of seatback strength tested and within the configurations observed in the present study, increasing seatback strength did not decrease injury risk.