By Dr. Gary Farr
Effect of G Force
“G force” is defined as the acceleration imposed by earth’s gravity, or 32.2 feet per second, squared. A force of 5G, for example, means that a body is accelerating at five times the force of gravity. One researcher, I. Macnab, has described the effect of acceleration to injuries of the neck:
“If, as a result of an accident, the head accelerates in relationship to the trunk -backward, forward, or sideways -injury to the neck may result. Because lesions produced in this way differ from those resulting from forced passive movements of the head, it seems worthwhile to differentiate them by the term acceleration injuries of the neck. In acceleration injuries, the force applied to the neck is roughly equivalent to the weight of the head multiplied by the speed that the head is moving”.
The human head has an average weight of 10 pounds. Consequently, a 5G force results in a potential loading of approximately 50 pounds to the head.
West, Gough, and Harper found that at 11.6 km/h (7.25 mph), vehicle peak acceleration was 3. 1 G, while the occupant’s head accelerated in the X direction at a force of 8.3G. This is equivalent to an 83-pound force acting on the head. Extension or X direction acceleration of the cervical spine will result in multiple-plane loading components, creating a shearing force, tensile force, and a compressive or axial force.
Working on cadavers, Przybylski et al. performed ligament uniaxial tension testing on C2-C7. They found that the anterior longitudinal ligament had a mean ultimate load of 107 ± 63 Newtons. Furthermore, according to the Society of Automotive Engineers, the limit on shearing force in the cervical spine is 231 Newtons, or approximately 52 pounds, and the limit on axial force for the upper cervical spine is 249 Newtons, or approximately 56 pounds. One Newton is equal to 0.225 pounds of force.
Watts, Atkinson, and Hennessy performed theoretical mathematical calculations, assuming a delta V of the torso at I I mph. Their calculations revealed that a 60-degree extension would occur in 0.086 seconds, which is not enough time for muscles to be enervated and thereby act as a protective mechanism for the head and neck. Based on prior studies that had detected rupture of the neck ligaments at 178 pounds and cervical disc maximum loading of 230 pounds, their calculations revealed possible disc and/or ligament damage for a delta V of I I mph. Their calculations also demonstrated that cervical injury could be incurred, reaching loadings of 190 pounds of torque, at speeds as low as 7.5 mph in an untensed neck of a normal, healthy individual.
Similarly, Barzelay and Lacy observed that the risk of injury appears to be greatest in collisions with impact velocities between eight and 20 mph. They also concluded that the force to the head can reach 100 pounds in collisions where speed at impact is no more than 15 mph.
Watts and his colleagues observed that tolerance levels may be even more restricted in individuals with prior health problems:
“Consider a person with a prior neck problem such that his or her natural neck extension is limited to only 20 degrees and damage level is only 70 lbs, due to scar tissue. With only a small initial tensing of the neck muscles, this damage level will be reached at a push speed of 2.5-3.0 mph! Note that the lower push speed range is similar to or even lower than the speeds typical for causing vehicle damage. Thus, human body damage can occur with either 0 or very small amounts of vehicle damage and it is not necessary to have copious vehicle damage in order to hurt the human body”.
The injuries sustained in accidents can have long-term sequelae. Hohl, a physician researcher, studied 146 patients, with no pre-existing cervical degenerative changes, who had sustained soft-tissue injuries resulting from automobile accidents. He concluded, after a five-year study period, that 39 percent of the patients showed degenerative changes.
