crash impact Along with the increasing speeds of transport and fighting machines in the twentieth century have come the study and implementation of methods of limiting human damage in the event of a crash. Body restraint has been a crucial consideration in aircraft since the beginning of aviation, and the high incidence of head injuries occurring in accidents in military jet aircraft led in the 1950s to the introduction of protective helmets for military aircrew. The use of seat belts in the front seats of cars has been required by law in the UK since 1983, and motor cyclists have been required to wear helmets since 1974. Since the late 1980s many pedal cyclists wear helmets by common consent or institutional requirement. All these measures, enforced by law or adopted by choice, reduce the risk of serious injury from crash impact.

An abrupt acceleration, imposed upon the human body by a collision between a person in motion and a stationary object, or by an object striking a stationary individual, can cause injury which may be fatal. The sudden change of velocity which occurs, whether it be produced by a fall from height onto solid ground or by the vehicle in which a person is travelling hitting a stationary object, is the cause of the injury. The degree of injury is related to the magnitude, duration, and direction of application of the accelerative forces. The inertial force which results from the application of an acceleration and which causes displacement of organs within the body is equal and opposite to the applied acceleration. The effects of short duration acceleration (less than 0.5 sec) are related principally to the structural strength of the part of the body upon which they act, the peak acceleration (measured in G, i.e. multiples of the acceleration due to gravity) (see G and G-suits), and the duration of the acceleration.

The direction in which the accelerative force is applied to the body is a major factor in determining the body's tolerance to an impact and the risk of injury. The other major factors are the support to and restraint of the body, and the possibility of a flailing head or limb impacting with a solid structure. The levels of acceleration which occur on the crash of a road vehicle or an aircraft can vary from a few G to a peak acceleration of 150–200 G, acting for 0.2 to 0.4 sec.

Human tolerance

The effects of short duration accelerations are usually categorized as tolerable, injurious, or fatal. Tolerable forces may produce bruises and abrasions but do not incapacitate. Injurious forces result in moderate to severe trauma, including fractures and injuries to internal organs such as liver, spleen, and brain, which may or may not incapacitate. The limits of human tolerance — for the seated adult subject, well supported by a seat, and effectively restrained by a harness which prevents impact with other structures — have been estimated for different directions of acceleration, from experiments using whole or parts of human cadavers, non-human primates, anthropomorphic dummies, and human subjects. The lowest tolerance (around 11–12 G for 0.1 sec) is for sideways accelerations and the highest (greater than 45 G for 0.1 sec) is for forwards acceleration. The limit of tolerance for headwards acceleration (25 G for 0.1 sec) is an important factor in defining the performance of ejection seats in high-performance military aircraft and of attenuating devices designed to reduce the risk of injury on a vertical impact with the ground. The limit of tolerance of backwards acceleration is important in relation to provision of crash protection in motor vehicles and aircraft with conventional forward-facing seats.

Restraint systems

The more effectively the occupant is restrained in a seat, the higher is their tolerance of an impact acceleration. The seat in turn must be adequately secured to the structure of the vehicle. There has been, for example, a progressive increase in the strengths of aircraft seats and their attachments to the aircraft floor, which in the past were woefully inadequate. The familiar restraint harness comprises one or more straps which are secured across the trunk of the seated individual by means of a quick-release buckle. The simplest type is the lap strap, which, whilst easy to use, does not restrain the upper trunk, leaving it free to jack-knife on impact with backwards or sideways accelerations, which may well result in serious injury, especially if the head hits the vehicle structure. The addition of a diagonal strap running from the origin of one of the lap straps, up over the front of the trunk and the top of the opposite shoulder, to be secured to the seat or vehicle structure — the ‘three point’ harness as fitted to the front seats of cars — provides very good restraint provided that it is well designed and adjusted. Most modern fighter aircraft are fitted with a ‘five point’ harness, which comprises two lap and two shoulder straps, and a strap from the seat structure between the legs to the quick-release box. This harness gives excellent restraint both during aerobatic manoeuvres and on crash impact and ejection.

Another method of enhancing tolerance of impact accelerations in use is the air bag, which is located in front of the seat occupant and inflates automatically in response to deceleration to prevent the occupant moving forward and striking the vehicle structure. An air bag typically inflates in less than 0.1 sec and then deflates slowly over the next few seconds.

Head injury and protection

Head injury is common in all forms of crash impact. One third to a half of car crash fatalities are due to injury to the head. Blows to the head may cause fracture of the bones of the skull, tearing of the membranes lining the inside of the skull — causing bleeding which may raise the pressure within the skull — and local damage to the brain itself where it underlies a fracture. But the brain is often damaged within the intact skull by the linear and angular acceleration of the head induced by the blow; the brain moves within the skull, and layers of its substance move relative to other layers, causing distortion or at worst disruption of connections between neurons. A relatively minor blow to the head can give rise to concussion — a transient paralysis of cerebral function, with loss of consciousness, which is followed by complete recovery. The concussed individual may, however, be unable to escape other consequences of the crash such as fire. Whilst the factors responsible for the concussion are complex, it is generally accepted that the human brain can withstand crash impact forces of 300–400 G without either concussion or skull fracture, provided that there is no local deformation of the skull to inflict direct injury.

Prevention of injury to the heads of the occupants of a vehicle on crash impact depends upon adequate restraint of the trunk, the presence of an adequate space free of rigid structures around the head, and making sure that any surface with which the head may come in contact is made of a material which deforms when hit, allowing a measure of energy absorption. Frequently, however, especially where the risk of head impact is increased, the most satisfactory solution is to provide personal head protection. A well-designed protective helmet comprises an outer semi-rigid shell and a suspension system either of a strap harness or a layer of crushable foam beneath the shell. The shell distributes the impact over a larger area and provides some attenuation by partial disintegration, whilst the gap (18–25 mm) between the shell and the head reduces the average acceleration applied to the head to 1/6–1/8 of that applied to the surface of the helmet. Such protective helmets have done much to reduce the toll of fatalities and serious head injuries in motor cyclists and in military aircrew.

John Ernsting

Bibliography

Ernsting, J. and and King, P. (1988). Aviation medicine, (2nd edn). Butterworth-Heinemann, Oxford.
Glaister, D. H. (1978). Human tolerance to impact acceleration. Injury, 9, 191–8.

See also concussion; flying; G and G-suit; injury.