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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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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
  • Accidents - The results of rear-end collisions with and
  • without a head rest
  • Anatomy - A description of the physiology of the affected
  • spinal region
  • Do Restraints Really Help? - Reports on detrimental effects
  • Evaluating Performance - How to identify a good head
  • restraint
  • Best Practice - The optimum procedure using restraints
  • Latest Technology - New developments in the field
  • References


An automobile seat must be

built with anthropometry and

human biomechanics in mind.

It must be designed to fit

body posture.

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:

Phase 1

Phase 2

Phase 3

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

great mobility.

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.


Do Restraints Really Help?

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...


Evaluating Performance

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.


Evaluating Performance

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 problems associated with this type of test:
  • An test subject aware of an impending crash will
  • react differently to a real world crash victim by
  • bracing themselves
  • A turned head on impact has been said to increase
  • the potential for injury yet this situation has not been
  • examined
  • All test subjects were male but statistics reveal women
  • are nearly twice as likely to be involved in a rear-end
  • collision

Best Practice

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

the car.

Lock the head rest in position if this feature is available.


Latest Technology

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.


Latest Technology

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.


Latest Technology

Autoliv have also developed an anti-whiplash

front seat.

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.

2. Pennie B, Agambar L. Patterns of injury and recovery in whiplash. Injury Brit J Accid Surg22(1):57-59, 1991.

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,

201-212, 1993.

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,

9. Arthur C. Croft & D. Michael Batty. Whiplash: The Epidemic. Medforum/Lifelines,

10. Cheryl Jensen. Devices That Can Save Your Neck. NY Times May 29. 1998.