The regulations specified bumper performance; they did not prescribe any particular bumper design. Nevertheless, many cars for the U.S. market were equipped with bulky, massive, protruding bumpers to comply with the 5-mile-per-hour bumper standard in effect from 1973 to 1982. This often meant additional overall vehicle length, as well as new front and rear designs to incorporate the stronger energy absorbing bumpers, adding weight to the extremities of the vehicle. Passenger cars featured gap-concealing flexible filler panels between the bumpers and the car's bodywork causing them to have a "massive, blockish look". However, other bumper designs also met the requirements. The 1973 AMC Matador coupe had free-standing bumpers with rubber gaiters alone to conceal the retractable shock absorbers. "Endura" bumpers, compliant with the regulations yet tightly integrated into the front bodywork, were used on models such as the Pontiac Grand Am starting in 1973, and the Chevrolet Monte Carlo starting in 1978, with significantly lower mass than heavy chromed-steel bumpers with separate impact energy absorbers.