Hyperflexion injuries of the human cervical spine commonly occur in vehicular crashes, contact sports, diving, and falls. Flexion related injuries constitute a high percentage of all cervical spine injuries. The objective of this study was to determine the mechanisms and tolerance of the human cervical spine under hyperflexion loading conditions through in vitro biomechanical experimentation. The impact load was delivered by an electrohydraulic piston to the cranium of a cadaver head-neck complex in a preflexed configuration. Measured biomechanical quantities included head impact forces, neck forces and moments, kinematics, spinal cord pressures, and pre-test alignment parameters. A total of thirteen specimens were tested. Minor injuries included mainly disruption of the lower cervical spine posterior ligament complex at one level. Major injuries included extensive ligamentous injury usually with vertebral fractures and/or complete dislocations. The anterior eccentricity of the occipital condyle with respect to Tl body was the most critical variable which influenced the loading condition and injury outcome. The spinal cord pressures were consistent with the severity of joint disruption and load magnitude. The average moment magnitudes for minor injuries were 52 Nm, and 97 Nm for specimens with major trauma. Using logistic regression techniques, the 25 % probability of major neck injury was determined to occur at 1850 N of axial neck force and 62 Nm of injury bending moment.