What they are and what they do
Head restraints are a crash protection feature. They are connected to the top of each seat with the purpose of protecting the neck, head and spine during a crash.
How they work
A head restraint should be adjusted to be at least as high as the passenger’s ears and directly behind their head for it to work effectively and provide support to the neck and spine. (See diagram on right).
The main purpose of a head restraint is to prevent whiplash. Whiplash is caused when the head extends forwards and then accelerates as it moves backwards and away from the torso. This movement is usually due to a sudden jolt, such as a rear impact crash. Whiplash relates to Newton’s second law, which states: “When an unbalanced force is applied to an object, it will cause the object to accelerate”. The unbalanced force in this scenario would be the impact of the crash, which causes the head to accelerate as it moves backwards. Head restraints are in place to stop the head from suddenly moving backwards, helping to prevent whiplash.
Development
Prior to the 1950s, all head restraints were non-adjustable. However, due to the high injury rate, vehicle manufacturers and safety researchers, with the assistance of medical advisories, spent the next decade designing head restraints that effectively extended the seatback.
In 1969, it became compulsory for all cars in the US to have head restraints and, after this, many other countries enacted similar laws. Head restraints can now be found in all cars made and sold around the world.