Loss of the Lumbar Curve in the Driving Seat

A Twenty Person Study

 

Reprinted & edited from British Osteopathic Journal Vol XIX pp 19-23, 1996.

 

Abstract; The risk of herniated lumbar intervertebral disc is known to be high in drivers. This may be due to high intradiscal pressure caused by lack of lumbar support. 20-50mm lumbar lordosis has been suggested by different authorities. A method of measuring spines in the erect position and in an adapted car seat was developed. A reliability trial was conducted which showed the equipment to be accurate. A twenty person study was conducted. All subjects in the study lost most of the lumbar lordosis. The lumbar support in the car seat was shown not to coincide with the lumbar lordosis of the subjects.

 

Key Words; Motor car, herniated lumbar intervertebral disc, intradiscal pressure, lumbar support, profile recorder, car seat

 

Introduction

 

Despite much anecdotal evidence of back pain caused by motoring , the first study to show a link between driving and back pain was not published until 1975, some ninety years after the appearance of the motor car [1]. Kelsey and Hardy demonstrated that men who spend half or more of the time at work driving are almost three times as likely to develop an acute herniated lumbar intervertebral disc  as  those who do not hold such jobs. Persons of either sex who drive to or from work are also more likely to develop an acute herniated lumbar intervertebral disc than those who do not drive at all. Subsequent epidemiological studies have confirmed that there is a causative link between driving and the incidence of back pain [2,3,4].

Since its inception, the fundamental purpose of the motor car has been the transportation of people, and almost without exception the occupants have been carried in a seated position.  The choice of this position for driver and passengers is a  logical one [5,6], but it is a direct consequence of this posture which leads to damage to the lumbar spine [7].

The pressure within the intervertebral discs ( the intradiscal pressure ) has been demonstrated to be higher in sitting than in standing.  Of particular interest to this work is the 1974 study by Andersson et al [8], in which volunteers had miniature pressure transducers inserted into the third lumbar intervertebral disc. The subjects were then placed in a modified car seat which was mounted in a car simulator complete with pedals. The modifications to the seat included a lumbar support  which could be varied in height and depth independently, and devices for measuring the inclination of the seat back. Andersson et al demonstrated that the intradiscal pressure in the test subjects could be reduced  by increasing the amount of lumbar support ( to a maximum of  50mm in this study ), and also by inclining the backrest away from the vertical position ( to a maximum of 1200 ) [9].

However, even though a car seat has a back support, this may not be sufficient to maintain the correct attitude in the lumbar spine. Car seats differ from theatre, aeroplane and train seats in that there is a limitation on headroom imposed by the shape of the car [ 6,10]. The solution to this problem adopted by car designers is to place the occupants in a position where the knees are more extended than in other types of seat [11]. Both Keegan and Floyd & Roberts have demonstrated that the extension of the knees causes an increased pull on the hamstring muscles which in turn creates a posterior rotation of the pelvis and a consequent flattening of the lumbar spine [12,13]. Thus in the motor car, there exists a situation where the design of the seat may actively force the lumbar spine into flexion.

Porter & Norris [14]  performed experiments which used a laboratory seat with adjustable lumbar supports and found that  to accommodate a range of male and female subjects, a range of more than 60mm vertical adjustment is required. They also recorded the amount of support that was preferred by their subjects on an objective assessment of comfort. This value turned out to be 20mm which is half of that recommended by Andersson [7].  It is possible that this is more comfortable because although the intradiscal pressure is slightly higher in this situation, the pull of the hamstring muscles is substantial, and a slight decrease in lumbar lordosis will decrease this pull. It is in effect a compromise situation.

 

Measuring the Seated Spine

Given the background of research, it would seem desirable that at least some element of the lumbar lordosis should be maintained  whilst driving to prevent injury to the low back. Few cars have a lumbar support which is adjustable although all seats which the author has examined have a shaped element to correspond to the occupant’s lumbar spine [11].

It is difficult to assess the effectiveness with which a car seat preserves the lumbar curve as the spine is hidden from view. Most current techniques for recording the shape of the spine are not suitable for this problem. Techniques such as those used by Adams et al [15] make use of inclinometers attached directly to the spine. Obviously these devices would prevent the subject sitting properly in a car seat and so their use is precluded. Other researchers have used sophisticated three dimensional computer scanning techniques [16,17,18]. Whilst these methods have been shown to be useful for measuring the erect spine it is impossible to use them in a car seat since the spine is completely hidden. Other techniques which are currently in use include the use of pressure transducers embedded in the surface of the car seat, but these devices record areas of pressure and not the actual shape of the spine [19]. Of course lateral radiographs of the seated subject could be taken, but apart from the cost, there are serious ethical issues surrounding the use of unnecessary  X-rays.

A further method is that developed by Branton [20] and also Porter and Norris  which is based on the former [14]. Both use profile recorders which consist of sliding rods which, when advanced, make contact with the spine and thus reproduce the shape of the spine which can then be recorded. It was decided that this method could be adapted for the purposes of this study. The author constructed two profile recorders; one for measuring erect subjects, and another for measuring subjects in a car seat. A fixed measure was also constructed to allow a reliability trial of the equipment to be performed [21].

 

1 The Erect Profile Recorder.

The erect profile recorder consisted of a large base board on which the subject would stand, to which was attached a vertical post.   Midway up the post the actual profile gauge was attached, which consisted of  46 lengths of 25cm long  doweling which was 12 mm square in section. These dowels were secured between a clamp bar which could be tightened up by wing nuts at the top and bottom thus locking the dowels in position when necessary. A thin layer of compressible vinyl secured to the surface of the clamp bar enabled light even pressure to be kept on all the dowels and yet allow easy movement when required. A vertical scale in millimetres was attached to the upright to allow the height of the subjects to be recorded, and a board on which graph paper could be mounted was provided . ( Figure 1 below)

 

 

 

2. The Seated Profile Recorder.

The seated profile recorder was constructed from a modified car seat. It was decided that the seat should be from modern production car but should not feature an adjustable lumbar support. It should also be a popular model.

An undamaged driver’s seat from a 1994 Ford Fiesta was purchased from a vehicle dismantlers. At the time of writing ( May 1996 ), this seat is still current in the Fiesta Classic model. The author inspected the donor vehicle and confirmed that it had covered 25,000 miles and that the seat was not worn down or damaged.

Close inspection of the floor pan showed that the area to which the seat was attached was actually flat except for a cross member which supported the front seat fixings. This cross member was removed from the floor pan and mounted on a large piece of plywood and the seat bolted to the plywood at the rear and the cross member at the front. This allowed the seat to be in the same attitude as when in the actual car. A dummy pedal board was fixed to the front of the plywood at the same distance as the real pedals would have been.

The profile gauge which  was attached to the car seat differed in construction from the erect device. In this case the probes would have to pass completely through the back of the car seat. It was considered desirable that the fabric of the seat was disrupted as little as possible by the probes and for this reason 9mm round dowels at 10mm intervals were used. A similar clamping arrangement allowed the dowels to be locked in position, and a board for mounting graph paper on was provided in similar fashion to the erect recorder. ( Figure 2 below )

 

 

The choice of a Ford Fiesta seat was fortuitous as it overcame one of the anticipated problems; fixing the profile gauge to the seat. The rear surface of the Fiesta seat is formed by a semi-rigid plastic shell which inspection showed to perform part of the supportive function. However this shell allowed the fixing bolts of the gauge to be attached at the rigid periphery.

 

Reliability trials

A two-stage intra method reliability trial was constructed to test the equipment.[21] Using a plywood profile man with three different spines as a fixed measure, a confidence interval for the comparison of the sitting and erect measures was developed. For a 95% CI, the difference in the measures had spread of 2mm. This could be ignored for most practical purposes and the devices considered as identically accurate against a fixed measure.

Inter- and intra-operator reliability was next tested using repeated measures of five subjects by three operators. Statistical analysis of the resultant graphs showed them to be highly correlated, but it was noticed that there was an axial displacement of the subjects between measures in the car seat. In other words the subjects did not sit in exactly the same position each time. The discrepancy was small but greatly complicated the analysis of the data. For this reason it was decided that a single measure would be used for future studies.

The intra-method reliability trial showed that the equipment was accurate and easy to operate and that different operators could produce graphs  that were highly correlated with each other.


 

 

 

20 Person Study

A twenty person study was conducted using the validated equipment. The subjects, 15 male and 5 female were selected at random from members of a car club, the only criterion being that they were aged 17 or over and were therefor potential drivers.

Each subject was measured for height and weight. Height ranged from 1.56m to 1.82m, mean height 1.732, standard deviation 0.0721. Weight ranged from 55kg to 111kg, mean weight 74.025kg, standard deviation 15.878.

The mean height and weight of the group were close to the mean values for adults in Great Britain [22]. The range and standard deviation of the heights was fairly small, but the weights had a large range and large standard deviation with the heaviest subject having twice the body mass of the lightest.

 

Method

The subjects were first measured sitting. For this study the seat back was set to an angle of  1100. If a lumbar lordosis of 20mm is maintained with the seat back at this angle, the  intradiscal  pressure would be approximately equal to that observed in erect standing [7].

Prior to the subject being placed in the seat, a dowel approximately one third of the way up the seat back had a soft wax marking compound applied to the tip. This dowel, which was to be used for aligning the seated and erect graphs was actually 40mm above the apex of the lumbar support in the seat. The subject was instructed to sit well back in the seat and look straight ahead whereupon the dowels were advanced to make contact with the spine. To avoid the problem of relying on the subjects to state if the dowels were touching the spine, the operator firmly tapped the dowels with fingertips to ensure good contact. This would slightly compress the superficial tissues but was felt preferable to relying on the subject’s judgement.

Sheets of graph paper with a pre-drawn horizontal line were used to record a graph of the dowel positions, carefully aligning the marked dowel with this line. The marked dowel was then retracted before the subject was allowed to vacate the seat to prevent any erroneous marking of the spine. ( Line C, figure 3 )

The wax mark transferred successfully to the subject’s spines and the contact points of the other dowels were visible on the skin as small dimples confirming good contact. The subjects did not report any discomfort caused by the firm contact.

The subjects were then placed on the erect profile gauge. The graph board was adjusted vertically until the horizontal line and the wax mark on the subject’s spine coincided whereupon a further graph was recorded. This procedure allowed the same 40mm area of spine to be measured  in both the seated and erect positions irrespective of the actual length of that subject’s lumbar lordosis.


 

 

 

Results

The graphs are displayed in figure 3 (below).  The erect profile is on the left and the sitting on the right.

Loss of Lumbar Curve:

All subjects showed a marked loss of lumbar lordosis. In virtually every subject the lumbar spine had moved into a flexion attitude. None of the subjects could be described as having 20mm of lumbar lordosis maintained as suggested by Porter & Norris, far less 50mm as suggested by Andersson.

Height of Lumbar Support:

The seat under discussion features a shaped area of foam which is presumably meant to perform the function of a support for the lumbar spine. The position of the apex of this support (line A) was known relative to the centralising marking dowel (line C). The position of this support did not appear to coincide with the apex of the lumbar curve in any of the subjects.

 

 

Typical Graph. Erect measure on left, Seated on right

 


 

 

 

 

Discussion

 There was a marked loss of lumbar extension curve in all the subjects, but the amount of lordosis lost varied between subjects. The erect graph of subject 3 shows a relatively shallow extension curve whereas that of subject 6 is very marked. Both these spines moved into a flexion curve when seated but the latter lost more than 50mm of extension. If the loss of extension curve were similar between subjects it might have indicated that the seat was controlling the spine to some degree. However the almost universal loss of lumbar lordosis and the varied amounts observed indicates that the seat contributed little or nothing to the support of the lumbar spine.

There also appeared to be no correlation between the loss of lordosis and the weight of the subjects. It might be expected that lighter subjects were less able to deform the seat and would therefor  be better supported but the two lightest subjects, 8 and 9, were amongst those that not only lost the lumbar lordosis but actually moved into a marked lumbar flexion curve.

The height of the lumbar support in this seat appeared to be at variance with the observed height of the lumbar curve in all the subjects. In all the subjects the apex of the extension curve was much further up the car seat than the apex of the shaped lumbar support (line A on graphs). Once again it is tempting to assume that the seat might be better suited to a certain size of person in general terms. Since the seat came from a small hatchback it might be more suited to women who are on average smaller and lighter [22]  but examination of subject 19, one of the smallest and lightest, shows that  the lumbar support is nowhere near the lumbar lordosis. In fact in this subject the lumbar support would appear to be in line with the middle of the subject’s sacrum. Heavier subjects would be expected to compress the seat squab to a greater extent and thus  be seated lower with relation to the seat back. However, as weight increases so too does height, and even in the heaviest subjects 1,16,17, the lumbar support does not coincide with the lumbar lordosis.

The inescapable conclusion from this small  study is that the lumbar support in this seat is both ineffective and wrongly positioned. Of course it may be unwise to extrapolate this result to the population in general due to the small sample size, but as the mean weight and height of the sample is coincident with the population of Great Britain [22], this may be a reliable indicator that this is the case.

Contemporary Ford literature states that  this  seat was designed using computerised aids [ 23 ]. These included a dummy with pressure sensitive probes which allowed “scientific assessment of the support needed and provided on long journeys”. The results of this survey indicate that the design may be less successful than the literature suggests. However it should be pointed out that these results should not be used to make comparisons with other car seats which have not been tested. This seat may be no better or worse than other seats in this class of vehicle.

The loss of lumbar lordosis may also be a phenomenon that is not peculiar to this seat. It may not be possible for the driver to maintain an extension curve for other reasons. It has been shown that extension at the knee joints during sitting produces a flattening of the lumbar curve as the pelvis is forced to rotate posteriorly [ 12,13 ]. This effect, which is observed in most modern cars, can be partly counteracted by tilting the seat back away from the vertical [ 11 ].  The driver’s torso is thus inclined backwards which means that in order to see the road, a degree of flexion may have to be produced in the upper dorsal or cervical spine [ 10 ].  However, examination of the subject graphs shows that in many cases the whole spine has moved into a long flexion curve. This suggests that rather than actively flex the cervical spine to aid forward vision, the subjects have adopted a position where the thorax is “balanced” on the abdomen. This sort of posture can be observed at any function where people have to sit for any length of time. Although this will reduce muscle activity in the spine it may well be at the expense of  increased lumbar intradiscal pressure. Further studies with the seat back at  varying angles in this and other seats are required to investigate if this is indeed what the drivers are attempting to do.


 

 

 

References

 

 

1.      Kelsey J L .  Hardy R J. Driving of Motor Vehicles as a Risk Factor for Acute Herniated Lumbar Intervertebral Disc. American J. Epidemiology 102 (1) pp63-73 1975

2.      Pietri F et al. Low-back Pain in Commercial Travellers. Scand J Work & Environmental Health. Vol 18 pp52-58 1992

3.      Porter J M, Porter C S, Lee V J A. A Survey of Driver Discomfort. Contemporary Ergonomics.pp262-267.  Pub Taylor & Francis 1992

4.      Walsh K, Cruddas M, Coggon D. Interaction of Height and Mechanical Loading of the Spine in the Development of Low-back Pain. Scand J. Work & Environmental Health. Vol 17 pp420-424 1991

5.      Rebiffe R. General Reflections on the Postural Comfort of the Driver and Passengers; Consequences on Seat Design. Human Factors in Transport Research. Academic Press 1980

6.      Grandjean E. Sitting Posture of Car Drivers From the Point of View of Ergonomics. Human Factors in Transport Research. Academic Press 1980.

7.      Andersson G B J. The Load on The Lumbar Spine in Sitting Postures.Human Factors in Transport Research Vol 2. Academic Press 1980

8.      Andersson B J G, Ortengren R, Nachemson A, Elfstrom G. Lumbar Disc Pressure and Myoelectric Back Muscle Activity During Sitting. Scand J. Rehab Med 6: pp128-133 1974

9.      Andersson G B J, Murpy R W, Ortengren R, Nachemson A L. The Influence of Backrest Inclination and Lumbar Support on Lumbar Lordosis. Spine Vol 4(1) pp 52-58. 1979.

10.  McIlwraith B. Seating and Associated Back Conditions.  Automotive Interiors International - Seating Review 1994. ISSN 0967-0386  pub Turret Group plc.

11.  McIlwraith B. An Analysis of the Driving Position in the Modern Motor Car. British Osteopathic Journal Vol XI pp27-34 1993.

12.  12.  12.  Keegan J J. Alterations of the Lumbar Curve Related to Posture and Seating. J Bone & Joint Surgery Vol 35A No3 July pp589-603  1953

13.  Floyd W F, Roberts D F. Anatomical and Physiological Principles in Chair and Table Design. Ergonomics Vol 2 (1) pp1-16. 1958.

14.  Porter J M. Norris B J. The Effects of Posture and Seat Design on Lumbar Lordosis. Human Factors in Transport Design. Contemporary Ergonomics pp191-196 pub Taylor & Francis 1987.

15.  AdamsM A, Dolan P, Marx C, Hutton W C. An Electronic Inclinometer Technique for Measuring Lumbar Curvature. Clin. Biomechanics Vol 1 (3) pp 130-134 1986.

16.  Frobin W and  Hierholzer E. Analysis of Human Back Shape using Surface Curvatures. J. Biomechanics Vol 15 No 5 pp379-390 1982

17.  Drerup B, Hierholzer E. Back Shape Measurement using Video Rasterstereography and Three-dimensional Reconstruction of Spinal Shape. Clin. Biomechanics Vol 9 No1 pp28-36 1994

18.  Smidt G L, Van Meter S E, et al. Spine Configuration and Range of Motion in Normals and Scoliotics. Clin. Biomechanics Vol 9 No5 1994

19.  Diebschlag W, Muller-Limroth, Physiological Requirements on Car Seats. Human Factors in Transport Research. Academic Press 1980

20.  Branton P. Backshapes of Seated Persons - How Close can the Interface be Designed? Applied Ergonomics Vol 15  No2.  pp105-107 1984

21.  McIlwraith B. Loss of the Lumbar Curve in the Driving Seat- An Intra-method Reliability Trial. Bsc (Hons) Dissertation. BCNO Library, 6 Netherhall Gdns London

22.  Knight I  .The heights and weights of adults in Great Britain HMSO  London.ISBN 0 11 691114 X  1984.

23.  Seating: The Undercover Story. Talkback Summer 1989. Pub Fleet Marketing, Ford Motor Co Ltd. Talkback, St Mary’s Green, Chelmsford, Essex, CM1 3TU.