Car Seats, Back Pain and McKenzie Therapy
by John Gorman
- Associate Member of the McKenzie Institute, UK (in 1998), McTimoney Chiropractor (till 2014), Chartered Engineer
The following article was published in the Journal of McKenzie Institute (UK) and in the Journal of the McKenzie Institute (USA), in Summer 1998. It was a follow up to a similar article in Automotive Interiors International in that year.
Have you ever wondered why McKenzie suggests the McKenzie roll and not the lumbar pads proposed by most other therapists? The reason is that a McKenzie roll placed correctly at about belt level is pelvic support, whereas pads which support the lumbar spine are true lumbar support. The two are not the same, in fact, in their effect on the lowest joints, where the spine connects to the pelvis, they are the exact opposite of one another.
If you think about it - a McKenzie roll which supports the posterior iliac spine and part of the iliac crest pushing forward, is exerting a force to the L5-S joint in exactly the same direction as is applied by extension in lying or extension in standing exercises.
A direct forward pressure on the lumbar spine has exactly the opposite mechanical effect on the L5-S joint. It flexes it by pushing the spine forward while the pelvis rolls backwards.
This incredibly simple mechanical point has been understood by the engineers of the car industry and we are now working with about 2/3 of the world’s car industry on the development of the next generation of car seats. (This was true at the time!) These will be based on pelvic support, not lumbar support. (This has proved a trifle optimistic!)
The following article appears this month in Automotive Interiors International. This does not give the basic mechanical arguments put above because they have been put forward in articles and at conferences in 1996 and 1997 and are now generally accepted inside the car industry. In this article, I am attempting to explain why the design of the car seat and its mechanical deficiencies are so absolutely critical in causing the epidemic of low back pain among car drivers.
Note how well this explanation of what is happening to L4-5 and L5-S fits in with the principles of the McKenzie system of therapy.
Why does it matter so much?
The principle of pelvic support in the Iliac Vehicle Seat has been accepted quickly and enthusiastically in the automotive industry. The mechanical argument is so simple it cannot be ignored. However, it is basic to this principle that the spine is meant to be flexible and that we can sit with a flexed spine - so why does the form of the backrest matter?
The answer to this question is as follows:
For the lowest two joints of the spine, and for these two joints only, there is a very specific geometrical limit to the "safe" range of joint flexion. Beyond this means problems.
This limit is simply that these two joints should only flex through half the range that you would expect. (Note; for non-medics there is a brief note on discs at the end of this article.)
A disc is the flexible cartilage joint between two vertebrae - one would expect the range motion of the joint to be;
From this extended shape--------- to this flexed shape
This is indeed the range for almost all the joints of the spine.
For the lowest two joints, however the range is different.
Very approximately the range is:
From this extended shape ----to this maximum flexion
Because this wedge shape of the disc is always maintained,
the soft jelly like nucleus of the disc always stays near the anterior of the disc.
If for any reason, such as sitting or heavy lifting, one of these two joints is distorted
This normal full flexion to this flexed shape where the direction of the wedge shape is reversed
then the jelly like nucleus will ooze backwards in the disc.
This will be detected by nerves in the back of the disc which will trigger a severe warning pain.
This is the critical unrecognised point of low back pain.
Before looking at why this surprising situation has evolved we should look at the other effects that are involved in low back pain.
There are three other components,
1. Muscle problems. Muscles become tight or go into spasm. Either causes pain and is helped by various massage therapies.
2. The Sacro-Iliac joints at the back of the pelvis become locked causing pain. Much chiropractic and osteopathic effort is
devoted to these joints.
3. Any spinal joint can become locked causing pain. This effect is central to chiropractic and is called a sublesation.
If the shape warning pain described earlier is referred to as (4), an episode of low back pain can include any combination of these four effects and each will tend to cause the others.
The main reasons why driving and car seats result in back pain is the distortion of the shape of the spine resulting in the triggering of the shape warning pain at one of the lowest two joints. It is therefore this effect that needs to be understood as part of car seat design.
Only a brief introduction can be given here.
The very surprising shape of these two joints comes from the angle of the sacrum in the pelvis.
The sacral end plate, which is effectively the foundation stone of the spine, is not horizontal as would be expected but nearer to vertical! The average angle from the horizontal is 50°. The first three components of the spine (two discs and one vertebra) are all wedge shaped to compensate for this and to bring the spine back to the vertical.
The need for a warning pain comes from the line of action of the most important muscle structure. (The Erector Spinae Apponeurosis)
This is the main tension component whenever we lift.
The mechanical advantage is obviously reduced when the spine is flexed - particularly if the flexion is in the lowest joints. If in lifting a weight the spine bends too much the mechanical advantage can be reduced to almost nothing leading to sudden and excessive flexion of the spine and serious damage.
It is to avoid this occurring that the severe warning pain has evolved. For various detailed reasons, evolution has chosen to limit the range of disc flexion to half the possible range and to trigger the warning pain whenever flexion occurs beyond the half way or parallel sided shape of the disc.
The evolution of the warning pain has of course nothing to do with modern seating. It is just an unfortunate coincidence that the main effect of a backrest is to distort the spine in exactly that way that will tend to cause the warning pain to trigger.
Furthermore, the relatively recent introduction of lumbar support doesn't help. It is intended to give a general lordotic or hollow shape to the spine - which it does - but this actually increases the bend at the lowest spinal joints. This is shown in these two pictures with increased lumbar support flexing the lowest joint into the forbidden range.
It is also obvious that lumbar support distorts the spine, increasing the lordosis but in the wrong place, too high up in the upper lumbar joints or even the lower thoracics. (This was clear in the tabulations in the seminal paper advocating lumbar support in 1975. Click for explanation.) This also increases the thoracic kyphosis unnaturally, as is clear in these pictures.)
With the now inevitable change to pelvic support in vehicle seating considerable effort will be required by medicine - both conventional and alternative - to understand and take advantage of the possibilities. (This was written in 1998. Despite the interest from seat designers and engineers, absolutely nothing happened!)
The responsibility of the seat designer will be to design the seat such that the flexion in the spine is spread equally between all joints and is not concentrated at the lowest joints.
Natural Joint Mobility is not just a design and seal of approval for seats and chairs. It is more.
It is almost a lifestyle - but an easy one to keep to. For instance, slouching on a settee is ok. Never try to sit up straight when sitting on the ground - just slump. Never sit with support to the lumbar or lower thoracic spine but, most important, never sit in a car without pelvic support!
John Gorman. 1998 with a few changes for this website in 2017.
Explanations for non -medics
Construction of the Disc.
The disc is not a disc of hard cartilage that is sandwiched between two vertibra and liable to "slip" or pop out of place.
The disc is best pictured as follows: The vertebrae are cylinders of bone. Adjacent cylinders are linked by a cylindrical wall of ligaments made of layers of fibres rather like the wall of a radial ply tyre. Inside the wall there is a jelly like fluid known as the nucleus.
It is the flexibility of the walls together with the hydrostatic weight bearing capacity of the nucleus, that makes the intervertebral disc the ideal single joint for the spine.
There are a multitude of therapies treating low back pain. Among these there is one that tries to correct the shape of the lowest two discs directly.
This is the McKenzie system, which exists as a separate therapy inside the physiotherapy profession.
This is a self-treatment system by means of exercises. The main exercise is the "floppy press-up". Lying face down the arms push the head and chest up but the pelvis is left on the floor. This forces the lowest joints of the spine back to their correct shape.
Physiotherapists who are members of the McKenzie Institute would be particularly helpful in understanding the system.
Low Back Pain
Only the briefest outline can be given here of this one aspect of the whole low back pain syndrome.
More detailed information can be found in the book "The Evolution of Low Back Pain" and the technical paper "The Obstetric Reason for Lourdosis and the implications for lifting and low back pain" (at http://www.lower-back-pain.info/obstetric.html ) - both by John Gorman.