Washington ? Children riding in the rear seat of compact extended cab pickups are almost five times as likely to be injured in a crash as children riding in the back seats of other vehicles, a study says.

The disparity is due to lack of legroom and shoulder belts in the back of the trucks, according to a study by the Children’s Hospital of Philadelphia that appears today in the Journal of the American Medical Assn.

“This is not the optimal vehicle for transporting children,” said Dr. Flaura Winston, lead author of the report. “There is not much room for the child to move in the event of a crash.”

Owner’s manuals warn against having children ride in the backs of the compact trucks. The auto industry considers the trucks “work vehicles, and children should not be riding in them,” said Eron Shosteck, spokesman for the Alliance of Automobile Manufacturers.

Extended cabs are growing in popularity. Last year, 83.2 percent of small pickups were sold with extended cabs, compared with 57.2 percent in 1998, according to Ward’s Automotive Reports.

Compact extended cab pickups usually have two doors and small rear passenger compartments with side-facing, fold-down jump seats. Full-sized extended cab pickups typically have four doors and larger rear compartments, with standard bench seats.

Jump seats in the smaller pickups have only lap belts, which allow more upper-body movement in a crash. The government considers the jump seats auxiliary equipment and exempts them from safety standards. The report recommends changing that policy.