Page 305 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

282 BIOMECHANICS OF THE HUMAN BODY

formed from several air bladders sewn into the palm, fingers, and thumb of the glove appears most
likely to fulfill these requirements (Reynolds, 2001).

11.4.4 Protection against Mechanical Shocks and Impacts
Protection against potentially injurious shocks and impacts is obtained by distributing the dynamic
forces over as large a surface area of the body as possible and transferring the residual forces
preferably to the pelvic region of the skeleton (though not through the vertebral column). Modifying
the impact-time history to involve smaller peak forces lasting for a longer time is usually beneficial.
Progressive crumpling of the passenger cabin floor and any forward structural members while
absorbing horizontal crash forces, as well as extension of a seat’s rear legs, are all used for this
purpose.

Seat Belts and Harnesses. For seated persons, lap belts, or combined lap and shoulder harnesses,
are used to distribute shock loads, and are routinely installed in automobiles and passenger aircraft.
In addition, the belts hold the body against the seat, which serves to strengthen the restrained areas.
Combined lap and shoulder harnesses are preferable to lap belts alone for forward-facing passengers,
as the latter permit the upper body to flail in the event of a spineward deceleration, such as occurs in
motor vehicle and many aircraft crashes. Harnesses with broader belt materials can be expected to
produce lower pressures on the body and consequently less soft tissue injury. For headward acceler-
ations the static deformation of the seat cushion is important, with the goal being to spread the load
uniformly and comfortably over as large an area of the buttocks and thighs as possible.
A significant factor in human shock tolerance appears to be the acceleration-time history of the
body immediately before the transient event. A dynamic preload imposed immediately before and/or
during the shock, and in the same direction as the impending shock forces (e.g., vehicle braking
before crash), has been found experimentally to reduce body accelerations (Hearon et al., 1982).

Air Bags. Although originally conceived as an alternative to seat belts and harnesses, air bags are
now recognized to provide most benefit when used with passive restraints, which define the position
of the body. The device used in automobiles consists, in principle, of one or more crash sensors
(accelerometers) that detect rapid decelerations, and a controller that processes the data and initiates
a pyrotechnic reaction to generate gas. The gas inflates a porous fabric bag between the decelerating
vehicle structure and the occupant within about 25 to 50 ms, to distribute the shock and impact forces
over a large surface of the body.
An example of the use of the MADYMO model to simulate air bag inflation and occupant
response to the frontal collision of an automobile is shown in Fig. 11.13. In this diagram, the
response of a person wearing a shoulder and lap seat belt has been calculated at 25-ms time intervals
following the initiation of air bag inflation. The forward rotation of the head is clearly visible and is
arrested before it impacts the chest. Also, the flailing of the arms can be seen.
Air bags can cause injury if they impact someone positioned close to the point of inflation, which
may occur in the event of unrestrained, or improperly restrained, vehicle occupants (e.g., small children;
see Fig. 11.9).

Helmets. Impact-reducing helmets surround the head with a rigid shell to distribute the dynamic
forces over as large an area of the skull as possible. The shell is supported by energy absorbing
material formed to the shape of the head, to reduce transmission of the impact to the skull. The shell
of a helmet must be as stiff as possible consistent with weight considerations, and must not deflect
sufficiently on impact for it to contact the head. The supporting webbing and energy absorbing foam
plastic must maintain the separation between the shell and the skull, and not permit shell rotation on
the head, to avoid the edges of the helmet impacting the neck or face.
Most practical helmet designs are a compromise between impact protection and other consider-
ations (e.g., bulk and weight, visibility, comfort, ability to communicate). Their efficacy has been
demonstrated repeatedly by experiment and accident statistics.