How to find the right driving position for your back

 


 

I would refer you to this study carried out by Brian McIllwraith a friend colleague and Osteopath of over 30 years experience
His web page is http://www.car-seat-data.co.uk

An Analysis of the Driving Position in the Modern Motor Car.
BRYAN MCILWRAITH Bsc (Hons) Ost. Med. DO.
 
Reprinted and edited from British Osteopathic Journal Vol XI pp27-34 1993.
 

Most people are probably aware that driving is detrimental to the general health of the spine, and many authorities believe it is an aetiological factor in many conditions. The most obvious reasons are the loss of the lumbar curve during prolonged driving and the vibration and jolting to which the spine is subjected. Whilst these factors are of great importance, there are other less obvious ones at work all of which are due to the design of the modern motor car. The object of this article is to highlight the design faults to which the hapless motorist is subjected on a daily basis
 
Pedal offset is a result of the juggling act, which is forced on the small car designer who has to accomodate passengers and components in a limited spac. However,there are mitigating factors for its inclusion. Examination of the steering wheel in many cars, both large and small, will show, that it too is offset when there is absolutely no justification at all.
 
Up to the end of the 'forties' most cars featured a solid steering column connected to a steering box. This layout was superceded by rack and pinion steering which was more accurate and, due to the revision in the geometry of the steering gear, it became necessary to include universal joints in the steering column. With this new layout, it was possible to site the steering wheel almost anywhere on the dashboard. Unfortunately the designers did not take advantage of this and, until very recently, the greater percentage of cars produced featured offset steering wheels. These are the main shortcomings in the driving position but let us examine just how the driver is affected by them, in reverse order:

 1) Wheel Offset: When this occurs the driver may be forced to hold his arms at a slight angle to the body.

  2) Steering Wheel Angle: Some cars have the wheel mounted at an angle to the dashboard, i.e. with the left hand edge nearer the driver than the right so that the driver sits with left elbow more flexed than the right.
In this case, or with wheel offset, the net result is that most drivers tend to rotate the dorsal spine to the left so that they face the steering wheel.

 3) Pedal Offset: When sitting in a car seat the thighs naturally tend to abduct. The left foot is then usually close to the clutch pedal. The right foot will not be close to the accelerator due to the offset and so the driver will have to adopt one of several stategies to bring the foot in contact with the pedal.
Whilst it may be that some drivers will sit with their pelvis at an angle to the seat back or possibly they may adduct the right thigh, observation shows that most drivers leave the right thigh abducted but externally rotate it. Due to the flexion of the knee this brings the right foot back to the midline and into the desired position.
 4) The Accelerator. It is necessary to select a seat position which allows the driver to fully depress the clutch. Although the accelerator may need to be fully depressed at times, it is usually held somewhere between one third or half of its range. Even though many designers place the accelerator slightly below the other two pedals, this still tends to lead to a situation where the right thigh may be unsupported by the seat squab. If this happens, the weight of the thigh is transmitted down through the calf to the ankle, reaching the foot which is supported only on the point of the heel.
  
 5) The Bucket Seat:  Lowering the roofline of cars reduced the space available for the passengers and required a modification of the seating position.  To accommodate the driver in this reduced space, the seat is lowered and tipped backwards. This will have two basic effects. Firstly the knees will start to extend, straightening the leg. This will create a pull on the hamstrings which in turn pull on the ischial tuberosities and tend to roll the pelvis backwards, forcing the lumbar spine into flexion (forward bending). In this position the intravertebral discs of the lumbar spine are subjected to increased pressure and are susceptible to damage. Secondly, with the straightening of the legs, there is more mechanical strain on the low back
 
The seat back may be  inclined at an angle of twenty degrees (minimum),and this allows a person of average height to sit reasonably comfortably. However, the restriction on headroom means that taller people will have to tilt the seat further back to give sufficient headroom which will result in an even greater flexion of the cervical or cervicodorsal spine.

A worrying trend in new cars recently is the introduction of full sliding sunroofs, by many manufacturers as standard. Sometimes this has been done without restyling the shell which results in a loss of between 2 and 4 cm of potential headroom. All too often, this is exacerbated by the fact that the sunroof has a thick rim or edge which sits directly above the drivers head. Paradoxically the sunroof is only standard in the British market where company car sales are still of great importance to the dealers. 
 
Drivers who are more than 1.83m (6ft) tall simply do not have sufficient headroom in many cars. Despite this the author has found that very few of the many drivers questioned seem to be aware of the problem.
How then do tall drivers cope with fitting a quart into a pint bottle? Once again observation provides the answer. In addition to tilting the seat back the tall driver tends to move the hips forward on the seat squab and allow the lumbar spine to roll backwards into flexion. In other words they deliberately slide down the seat into a slouched position. Apart from the obvious detrimental effect on the lumbar spine, this position may also lead to an increased flexion of the knees.  This leads to the distal part of the thigh being unsupported.

To recap so far; the driver is likely to be sitting with the cervicodorsal spine flexed and rotated, the lumbar spine flexed and the right thigh abducted and externally rotated, and the right foot inverted. Bearing in mind that this position may be held for hours at a stretch this begins to look like a recipe for disaster and certainly nothing like the "ergonomic design" so frequently quoted in advertising material.

The faults described so far are by no means a complete list. They are general to many cars, but some models have their own particular ones. In rounding off the corners of the passenger box to achieve a low drag factor, some stylists have brought the door tops very close to the side of the drivers head. Although they are in no danger of coming into contact with the door, many drivers instinctively sidebend their necks away from the door towards the centre of the car. Once again you may observe this for yourself when following some vehicles.

The design of the seats in some cars is suspect. To give the illusion of a roomy interior, some designers shorten the seat squab by three or four centimetres. Although this may not sound a lot, it is the difference between  support or lack of it for the thighs.
Some manufacturers proudly proclaim that their seats have a lumbar support. Unfortunately, the seat comes in one size but humans do not and even those of the same size may have vastly different lumbar spines where the lordosis may be short or long, absent or pronounced which means that the chances of the support fitting more than a small percentage of drivers is remote. In the course of measuring vehicles, the author found some seats where the squab was not aligned with the seat back. Some had a slight lateral shift, but were still perpendicular to the seat back whilst some were set at an angle, usually to clear a large gearbox housing.

So far only the use of the accelerator pedal has been discussed. Many back sufferers will complain that using the clutch causes pain, which demonstrates that the clutch transmits a certain amount of force to the low back. Whether or not that force is capable of producing low back conditions is difficult to assess, but it seems likely that prolonged city driving with multiple gear changes will transmit an asymetric force to the pelvis which may well be of aetiological significance. It was felt however, that discussion of this and also the position of the clutch foot when not in use, was beyond the scope of this paper.

The vertical inclination of  the steering wheel should also be considered. A figure of between 20 and 30 degrees is acceptable for a saloon car, where the wheel is mounted roughly at mid chest height, as this allows control of the wheel with a minimum of movement of the upper extremity. A figure of over 30 degrees is really  only suitable for a van or lorry where the wheel is mounted much lower. In a car, it will force the driver to abduct the upper arm and require flexion and extension movements of the elbow and wrist and possibly pro-supination of the forearm when turning the wheel. A figure of below 20 degrees is only suitable for sports cars, where the driving position is quite different to that which is at present under discussion. Suffice to say that it should not be encountered in saloon cars. Fortunately, most cars fall into this range, but a few have a figure of more than 30deg.
 
The  accelerator pedal should  also be inspected. It should be set below the level of the other pedals, as this is generally required for driver comfort; ocassionally  the pedal is nearer the driver than the other two. Fortunately, this is rarely found, as it is exceedingly uncomfortable for anything but the shortest trip as it results in the leg being completely unsupported.

Pedal width over the three control pedals should be checked. Pedal widths of 25cm or less represent a very tight spacing between the pedals and may cause difficulty for larger drivers. Such close spacing is frequently, though not always, caused by the intrusion of the wheel arch into the footwell.. 

 Finally, but very importantly, the kerb height (KH) of the vehicle should be assesed. The lack of overall height in modern cars can cause problems for drivers in a way that is not associated with the seat, but is caused by the low kerb height. In a very low vehicle, the height of the seat squab when measured from the ground as distinct from the floorpan may be such that when the driver attempts to exit the vehicle, his knee joint will be higher than his hip joint. With the lower kerb height, the driver will be forced to increase the flexion of the lumbar spine as he tries to move his centre of gravity over the foot which is outside the car. This will be accompanied by a measure of sidebending and rotation. 
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 By contrast, in the higher vehicle the hip is above the level of the knee. There is already a move towards extension of the hip.The driver can continue to extend the hip and low back as he exits. Try it for yourself if you find it difficult to visualize. Although the tables do show up obvious faults like wheel offset, they do not obviate the fact that drivers come in all shapes and sizes, It was felt it would be too difficult to interpret them in a meaningful way for the average car purchaser.
 
 For this purpose the author has devised five simple tests, which can be carried out in any car without the need for tools or instruments as follows;

1) The Praying Test. Sitting in the driver's seat, place the hands together, fingertips and palms touching, pointing outwards from the chest as if praying. The wrists should actually be touching the chest. In this position, the hands will form a fairly accurate perpendicular to the body. It should be possible to see if they are pointing at the centre of the steering wheel. If they are not then the wheel may be offset.
2) The Fist Test. With the seat in the normal driving position ( i.e. a position where the clutch can be fully depressed without stretching and the hips well back into the seat) make a fist with the left hand keeping the thumb to the side of the index finger. The depth of such a fist will measure approximately 50mm. It should be possible to place the fist on the crown of the head. If it is only possible to insert the flat of the hand between the roof and head, then there is insufficient headroom.

3) The Look Down Test. With both hands placed evenly on the steering wheel look down at the legs. It should be possible to see equal amounts of both legs between the arms. Frequently, the left leg will be visible, but the right leg will be obscured by the right ar. This may indicate that the shoulder girdle is rotated to the left in relation to the pelvis.

4) The Right Leg Test. This test should be performed after driving the car for a short while. Once again, look down and examine the position of the right leg. Is it elevated above the level of the left, or has it fallen out towards the edge of the seat? Is the right foot roughly in line with the thigh as it should be, or has it had to come across towards the centre of the car?

5) The Kerb Height Test. Swing the right leg out of the car as though getting out, and place the right foot on the ground. Try and ensure the lower leg (shin & calf ) is in a vertical position. Now look at the surface of the right thigh. It should be sloping down towards the knee. If it is sloping upwards (ie if the knee is higher than the hip) you will have difficulty when exiting this vehicle.

 If the car can pass these four simple tests, there is a good chance that it is suitable for that particular driver. By utilising the tests, a prospective purchaser should be able to produce a short list of suitable vehicles, from which they can then make a choice.
 

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