DETROIT (AP) _ Car seats can be a pain in the neck - and back, leg and posterior - for millions of motorists, and Dr. Timothy Hosea is trying to help cure this ''disease of the automotive age.''

Hosea, an orthpedic surgeon who is a clinical fellow at Harvard Medical School and a consultant to General Motors Corp., has studied the ways Americans are treated by their car seats.

''We found something very interesting, that we're calling the squirm factor,'' he said. ''That is, you're sitting in the car seat and you drive for 30 or 40 minutes and you start squirming around.''

The cause, he said, is muscle fatigue that is relieved, temporarily, by fidgeting.

''There's an increase in (muscle) activity and you squirm and knock it down. There's another increase in activity and you squirm and knock it down,'' he said. ''There is muscle fatigue there.''

Most manufacturers say today's car seats are far better than they used to be, but a lot of work still is needed.

''We're still lacking the optimum seat,'' Hosea said.

Hosea and others say the car seat, especially the driver's, should be considered a workplace.

But unlike office workers, car drivers are constantly reaching with hands and feet for the fuel pedal, brakes, clutch, steering wheel, radio tuner of cigarette lighter. They also cannot walk away for a coffee break.

''The occupants of an automobile are compelled to adopt an almost unchanging body posture for a quite lengthy period,'' Albert Weichenrieder and Hans-Gunther Haldenwanger of the West German carmaker Audi AG said in a recent paper presented to a Society of Automotive Engineers convention in Detroit.

Hosea, who addressed a convention workshop, said carmakers must consider the contours and back supports in a seat and the effects of vibration, acceleration and deceleration of the car, centrifugal force and road conditions and terrain.

In his main study, Hosea's team attached electrodes to the skin of the backs of male volunteers.

But, he said, ''We have been dealing with young, healthy males at this time. We have to look at females and the older population.''

''In order to get a good seat that will help decrease the incidence of back disease in the American driving public, we have to put the function first,'' he added. ''Following that, send the car seat to the designer to dress it up.''

But the manufacturers have to worry about production costs and weight, and that has led to more emphasis on exotic foams and other materials and the use of robots to put seats together.

One method touted by German seatmakers is the pouring of liquid plastic into a seat cover, with robots programmed to pour harder plastic into places where body support will be needed and softer plastics where cushioning is desired.

''You can't think of the back as a whole,'' Hosea said. Adjustability of seats, he said, ''is an essential feature.''

''Back pain affects 75 percent of all Americans at some point in their lifetime,'' Hosea said.