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A Study of the Design and Use of Automobile Head Restraints MEC420 Human Factors in Engineering Design Human Factors Case History Robert Caig Main Features of Presentation Introduction - The world of ergonomics and head rests Background - A short history of head rests
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MEC420 Human Factors in Engineering Design
Human Factors Case History
An automobile seat must be
built with anthropometry and
human biomechanics in mind.
It must be designed to fit
This presentation will show how head restraints have developed over the years to benefit the health of car passengers involved in rear impact collisions.
The investigation will draw on sources such as journal articles, governmental standards and the internet.
The first head rests were placed in cars
manufactured by Volvo in the mid 1960’s.
It was realised that they could prevent
the snapping-back movement of the
head that occurs in a rear-end collision.
They were a standard feature of
all makes of car by the mid 1970’s.
There now exists laws as to the design
of safe head rests but as will be seen,
there are still some types with their
Rear-end collisions are caused when one car is slowing down or has stopped,
e.g. at a junction or a set of traffic lights, and another car travelling behind
the first does not stop before hitting the back of it.
This may be because the second driver is travelling too
close (tailgating) to the first, is not paying attention to the
road or has faulty brakes.
When this happens, the car in front is pushed forward. The car seats, rigidly
connected to the car frame, push the driver and any passengers’ bodies
forward but the inertia of the head combined with the flexibility of the neck
means the head will snap back.
It is this snapping motion that causes the injury in the neck commonly termed “whiplash”. This has symptoms other than neck pain such as headaches, numbness, weak hand grip and it can also lead to further disease of the spine.
These are the basic motions of the human body during
a rear end crash:
Occupant in vehicle
– head erect.
Car is hit, pushing seat
forward. Head strikes
Occupant rebounds – head
moves beyond body.
At first, researchers assumed injury was due to extension of the
spine beyond its normal range. However, injury also occurred at
low speed crashes with only small extensions of the spinal column.
The cervical spine consists of 7
vertebrae, as shown here. The
complexity of the neck allows it
Each vertebra has what are called facet
joints in the rear portion of the vertebrae.
About 75 milliseconds into the collision, the spine forms an
S-shaped curve before the musculature of the neck has a
chance to react. This S-shaped curve results in sharp
bending in just a few spinal segments.
The joint capsule undergoes excessive stress in those few segments of the spine; so much so that the joint capsules can be torn or the cartilage in the joint itself can be "pinched“, resulting in tissue damage and pain.
Some reports have suggested that even when a car does have
a head restraint, this may not be to the advantage of its occupants.
When the head strikes the restraint, a rebounding effect can occur.
This acts to magnify the forward bending motion.
It has been discovered that when a head rest cannot lock in
position, it can be pushed down by the head when it jerks back
during a rear impact collision.
Also, the jerking movement is again not prevented when
the head rest is placed too far behind the head.
How can these positions be measured?
How far is too far...
A test can be performed using the device pictured above right
combined with an H-Point machine, pictured below left.
The head restraint measuring device has two probes that
project out from the head to measure vertical and horizontal
distance from head to restraint.
The measuring machine represents an average sized male.
Many head restraints were tested and each was
classified into one of four geometric zones, as
shown below right.
Studies have been conducted to study the effects of head restraints
(as well as other safety components) using human volunteers.
It has been said that the tests carried out on modern vehicles equipped with
seat belts, head restraints and bumpers are comparable, in terms of the
accelerations experienced by test subjects, to vintage, World War II vehicles.
There are tests you can perform yourself to make sure your
own car’s head rest is as safe as possible:
If the headrest height is moveable, check that the top of it is
level with the top of your head.
If it tilts, rotate it
to be as close to
the back of your
head as possible.
Perform these checks every
time you drive your car – a
passenger may have altered
its position whilst using the
head rest as an aid to exiting
Lock the head rest in position if this feature is available.
Add On Head Rest
This is a cushion that sits between driver and head rest in order to
provide extra support.
Its makers claim it not only reduces the severity of whiplash injury
but by ‘promoting a relaxed, neutral sitting posture’ it also helps to
reduce fatigue whilst driving.
Safeguard Head Restraint
These head rests are made from a specially developed polymer. They
are soft to the touch under normal conditions and use but under sudden
impact, they firm up and act as an energy absorber.
Legislation concerning head restraints will become more stringent in
the future; this innovation may be an answer to solving the problem
of head and neck injuries in rear-end impacts collisions.
The picture on the left is a Saab active head restraint. It moves forwards and upwards in the event of a rear-end collision.
An independent study performed for a doctoral thesis highly commended it, saying it reduces the risk of
major neck injury during rear-end impact by up to 75%.
Autoliv have produced a self-inflating head rest for rear seat passengers.
When the car is jolted forward, the occupant compresses an air filled cushion in the seat back, filling an airbag in the head restraint.
Autoliv have also developed an anti-whiplash
The three images on the left show the stages
undertaken in a rear impact situation.
The seat is designed to yield in such a collision but
will tilt in a controlled manner (shown bottom right).
This absorbs energy and
reduces the forward
rebound of the occupant.
This technology was introduced
in Volvo cars in 1998.
1. A procedure for evaluating motor vehicle head restraints. Research Council for Automobile Repairs,
January 2001: Issue 1.
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Ono K, Kanno M. Influences of the physical parameters on the risk to neck injuries in low impact speed
rear-end collisions. International IRCOBI Conference on the Biomechanics of Impacts, Eindhoven, Netherlands,
3. Thomson RW, Romilly DP, Navin FPD, Macnabb MJ. Energy attenuation within the vehicle during low speed
collisions. Report to Transport Canada, University of British Columbia, Aug, 1989.
4. West DH, Gough JP, Harper TK. Low speed collision testing using human subjects. Accid Reconstr
J 5(3):22-26, 1993.
5. Szabo TJ, Welcher JB, Anderson RD, et al. Human occupant kinematic response to low speed rear-end impacts.
SAE Tech Paper Series 940532 23-35, 1994.
6. Croft AC. Understanding low speed rear impact collisions: whiplash injuries. Spine Research Institute of San
Diego Press, 1997.
7. Michael Melton. The Complete Guide to Whiplash. Body-Mind Publications.
8. Injury Resources, www.injuryresources.com.
9. Arthur C. Croft & D. Michael Batty. Whiplash: The Epidemic. Medforum/Lifelines, www.medforum.com.
10. Cheryl Jensen. Devices That Can Save Your Neck. NY Times May 29. 1998.