Spinal injuries are a great cost to society and the afflicted individuals. It is well known that most spinal injuries are not bony fractures but rather soft tissue lesions falling in the 'subfailure' region. For the clinical diagnosis of spinal injuries, abnormal motion patterns under physiological loads are considered an important factor. The purpose of the present study was to determine the onset and progression of spinal injury, and compare the sensitivity of three motion parameters: neutral zone (NZ), elastic zone (EZ), and range of motion (ROM). Spinal injury was defined as a significant increase in any of the three motion parameters. A repeatable high-speed flexion-compression load vector was applied individually to six porcine cervical spine specimens. Several impacts of increasing severity were applied to each specimen. After each impact, flexion-extension motion was measured. Neutral zone was the residual deformation from the neutral position to the position under zero load at the start of the final load cycle. Elastic zone was the displacement from zero load to the maximum load on the final load cycle. Range of motion was the sum of the neutral and elastic zones. The first significant increase in motion was determined by the neutral zone parameter with few observable anatomic lesions on the specimens. This was the onset of spinal injury. The next significant motion increase was also determined by the neutral zone parameter. After this motion increase, termed the progression of injury, ligament ruptures were observed in some specimens. It was concluded that the neutral zone was the most sensitive motion parameter in defining the onset and progression of spinal injury.(ABSTRACT TRUNCATED AT 250 WORDS)