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Motoring: Back where you belong: Are you sitting comfortably? If not, you have the wrong car, says Phil Llewellin

Driving is detrimental to the health of the spine, says Bryan McIlwraith, an osteopath based in Inverness whose interest in cars extends to being a keen amateur mechanic and chairman of the Highland Classic Motor Club. He says that 'dreadful driving positions' are factors that his otherwise beloved 1957 Wolseley 15/50 and 1972 Triumph Stag have in common.

Mr McIlwraith's paper on this important aspect of the car-driver relationship appeared in the British Osteopathic Journal about the time that I was visiting Edmund Ward at the pioneering Institute for Consumer Ergonomics (ICE) in Loughborough, Leicestershire. I wanted to find out more about the seats that support my 53-year-old bones for 50,000 miles a year.

Many of the world's biggest car manufacturers have consulted ICE in its 24 years of operation. And Mr Ward and his colleagues have published a free, 14-point checklist that should help to reduce the cost of back pain in Britain (81 million working days lost in 1992). Not all of this can be put down to driving, of course, but ICE's research shows that people who remain seated for long periods suffer almost as much as those whose work involves a lot of lifting.

Understandably, but unfortunately, seat design is based largely on statistics called 'percentiles', which are used to characterise the average driver. The problem is that people's proportions vary. Here I speak with considerable authority, being a 79 percentile in terms of overall height - meaning that I am taller than 79 per cent of the British adult male population - but a 96 percentile from bum to scalp. Legs that get a mere 45 percentile rating are attached to one of the longest bodies in the land. The inevitable headroom problem is frequently exacerbated by the intrusive surround for a sliding sunroof, so I often adopt tactics highlighted in Mr McIlwraith's report.

'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,' he says. 'In other words, they deliberately slide into a slouched position.'

The design of some car seats is suspect, too. 'To give the illusion of a roomy interior, designers shorten the seat squab by three or four centimetres, which, although it may not sound a lot, 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 . . .'

Plenty of adjustment is one of the keys to a satisfactory seat, Mr Ward says. This enables such elements as height, reach and rake to be tailored to bring the driver closer to the ideal position. But that's not all: 'Avoiding sitting still for a long time is one of the best ways to prevent the lower back becoming painful. However, it is difficult to imagine drivers taking an exercise break every 45 minutes, so the alternative is to change your posture while retaining the driving position's essential elements. The ability to do that - and the ease with which the seat can be adjusted while on the move - are factors that should be considered when choosing a car.'

A made-to-measure seat would be very expensive, but the next best thing is offered by Recaro, which has specialised in car seats since 1963. It uses a modular technique to assemble seats that cater for different heights, weights, thigh lengths, and so forth. Prices range from pounds 665 to pounds 840.

The personal nature of the relationship between driver and seat makes it difficult to deliver balanced verdicts on individual makes and models: what could have been tailor-made for hefty Mr Jones may be purgatory on wheels for petite Miss Smith. This was confirmed as Mr Ward ran through his checklist while I sat in 10 different cars.

Points covered by the checklist include seat height ('Can you get a clenched fist between the top of your head and the roof?'), the height of the head restraint, the backrest's height and width, the extent and shape of any lumbar support, and the relationship between seat, steering wheel and pedals. What the experts call 'pedal offset' is encountered in many small cars where space-saving design constraints bring the offside wheel arch into the space where the driver's right foot should be. Twisting the foot to reach the accelerator initiates a series of distortions that extend all the way to the spine.

A car's size and status are not trustworthy guides. A Rolls-Royce Silver Spur offered much less headroom than the sporty Peugeot 106 Rallye which would almost have fitted in its boot. ICE's caveat about materials that may cause the body to slip and slump argued against the leather upholstery in the Rolls and in a Ferrari 348 Spider. The tiny Fiat Cinquecento's seatback mechanism was more convenient to locate and operate than its counterparts in the bigger and much more expensive Ford Mondeo Ghia and Mercedes C180. A lever reached with the left hand made fore-and-aft adjustment in a Vauxhall Cavalier LS easier than in the Rover 623iS, whose central bar involves more of a stretch. The lofty Nissan Terrano SLX's lumbar support suited my back better than most. The Toyota Carina XLi scored points for having an adjustable lumbar support - but lost them because it was awkward to operate. And so forth.

My advice is to use ICE's checklist when shopping for a car. Thus, unsuitable contenders can be dismissed in the showroom. If you go to the next stage, a road test, think about the seat during the proverbial run round the block, the importance of which is underlined by Mr McIlwraith. 'Any car salesman,' he says, 'will tell you many customers choose a car without test driving it. The most important factor for many people is the relative trade-in value of their old car. This is a ludicrous approach that seems to rest on the assumption that all cars are more or less equal, which is clearly not the case.'

ICE Ergonomics, Swingbridge Road, Loughborough, Leicestershire LE11 0JB (0509 236161). Recaro Ltd, Unit K, Riverside Industrial Estate, Tamworth, Staffordshire B78 3RW (0827 261997).

(Photograph omitted)