The human neck is a remarkable device for its function, flexibility and strength. It supports the head while permitting a wide range of motion and sustains itself under some vigorous head impacts in violent sports and accidents. Nevertheless, the neck has limits both of motion and of the forces it can sustain. In rollovers, the neck is usually loaded through the top or back of the head with the torso providing an inertial reaction mass. Skull fractures, head and brain injuries generally involve higher impact velocities than are necessary to fracture the cervical spine, but which can also load and critically injure the neck.

Accident injury statistics, tests of living and post-mortem human subjects (PMHS), analysis of athletic impacts, tests of anthropometric dummies and computer simulations of human and dummy kinematics, illustrate injury mechanisms and suggest injury criteria measurements for the human neck. Using this data a simple head impact measure as a neck injury criterion was developed to address the problem of neck injury in vehicle rollovers and to help identify appropriate vehicle design considerations for rollover occupant protection.

The analysis defines a head impact speed of 3 m/sec. (7 mph) which produces a neck load of 7,000 N in a 50th percentile male Hybrid III dummy, as the onset of serious neck injury, and that a head impact speed of 4.5 m/sec (10 mph) which produces a dummy neck load of 10,000 N represents the onset of severe to fatal neck injury. NHTSA has already accepted that a head impact velocity of 7 m/sec (16 mph) is the threshold for the onset of serious head and brain injury. These criteria are shown to reasonably represent available human injury accident and experimental statistical distributions.