Voigt R. Hodgson

An Assessment of Compressive Neck Loads Under Injury-Producing Conditions

Injury reference curves for axial compressive forces on the neck were derived from impact tests of a spring-loaded tackling block on foottball helmets. Results suggest that helmets, especially those with resilient liners, reduce these forces.

THE ROLE OF IMPACT LOCATION IN REVERSIBLE CEREBRAL CONCUSSION

Mechanical impacts were delivered by an air propelled striker to the front, side, rear and top of rigid protective caps worn by six anesthetized monkeys. These tests were to produce reversible concussion and to determine differences in tolerance to concussion among the four impact sites. Striker force and cap accelerations were measures of the impact severity and animal blood pressure, respiration and ECG changes were measures of the physiological effects. By distributing the blow with a protective cap, allowing free head movement after impact, skull fracture was eliminated and simple reversible concussion could be produced without symptoms of residual neurological deficit. Higher linear and angular accelerations produced longer periods of unconsciousness (more than 3 times) on the side than at any of the other locations. It is hypothesized that the decrease in concussion tolerance accompanied by higher accelerations for side impacts may be the result of lower mechanical impedance due to the oval shape of the animal head. For the covering abstract of the conference see HS-036 716. (Author/TRRL)

Studies on mechanical impedance of the human skull: Preliminary report

The steady state vibration response of the human cadaver head across a frequency range including its first three modes of vibration has been studied. Important first and third modes were found near 300 Hz (antiresonance) and 900 Hz (resonance), respectively. Below 200 Hz the skull moved relatively as a rigid body. In the antiresonant mode, maximum mechanical impedance (force/velocity) was found to occur, with acceleration amplification on the occiput greater than frontal input acceleration by a factor of 3. In the resonance mode minimal mechanical impedance occurs in which only that part of the head adjacent to the driven point is moving under the action of a vibratory force. It is hypothesised that long duration impacts (t > 0·005 sec) produce predominantly rigid body motion because the frequency spectrum of the pulse is too low to excite the lowest natural frequency of the skull. Consequently the driving force produces primarily acceleration of the head. Shorter duration pulses, particularly those with short rise time, have a broader frequency spectrum which can excite skull modes, thereby augmenting or modifying the skull flexure patterns produced by the force. It is not yet understood in what proportions these factors influence head injury, but impact head accelerations have been recorded opposite the blow for short duration impacts (t < 0·004 sec) that do not correlate with rigid body acceleration (a = force/head wt.) sometimes being greater by a factor of 2.

Response of the Seated Human Cadaver to Acceleration and Jerk with and without Seat Cushions

Human cadavers were subjected to seat-to-head accelerations to a maximum acceleration and jerk (rate of change of acceleration) of 18 g and 2600 g/sec, respectively, with six different types of seat cushions. Strain gages were cemented to the vertebral column and accelerometers were attached to bone at several levels of the body. The object of the experiments was to observe the effects of varying magnitude of acceleration, jerk, and seat cushions on the strain and acceleration response of the cadaver. Results indicate that dynamic load factor (ratio of peak to mean response) increases with jerk at low jerk levels to a maximum and thereafter remains relatively independent of jerk; increases with mean or terminal acceleration in the range of these tests; and increases for all types of cushions used in these tests. Strains measured on the body of vertebrae, particularly in the lumbar region, correspond closely to body accelerations, but strains measured on the rear of vertebrae were not related to body accelerations.