Joe Namath's decision to retire after his 13th season was a medical one, although he has not mentioned it as a reason why he lost out to Port Haden as the Los Angeles Rams' starting quarterback.

Dr. Robert Kerlan, Rams' physician and specialist in treating athletes' injuries, said yesterday that Namath was unable physically to qualify as the number two quarterback for the regular-season finale in Washington and for the playoffs.

There have been unconfirmed reports that Namath suffered a hairline fracture of or a deviated sternum (chest bone) when speared by a helmet against Chicago in a Monday night game that caused him to leave the contest.

Kerlan said that to the best of his memory, without consulting his notes, Namath suffered a contusion of the sternum in the fourth game of the season, but it was a deterioration of his right knee in the last part of the season that incapacitated him.

Kerlan said of the sternum injury, "Maybe he couldn't throw for a few days after that, but I think he practiced regularly.

We never declared him on our lists of injuries as being unavailable, although in the last part of the season he was not as able as Vince Ferragamo (who had been third-string quarterback behind Haden and Namath).

"For instance, before the game against the Redskins I did not believe him fit to be number two quawrterback. I talked to the coach (Chuck Knox) and told him that because of an increased problem with his right knee he would not be available as number two quarterback, or for a few weeks, extending into the playoffs."

Ferragamo was substituted for Haden in the 17-14 loss to the Redskins. Namath never played a down after the injury against the Bears.

There was wonder why the Rams did not use him in the 14-7 loss to the Minnesota in the rain at Los Angeles as Haden completed 14 of 32 passes and was intercepted three times.

Kerlan noted that Namath came to the Rams with a chest injury suffered in the quarterback's footaball camp for youngsters and it delayed him from throwing in training camp.

The physician said Namath being sacked by several Bears, "He let the ball go. Somebody creamed him . . . put his helmet into Joe's chest. He practiced the next week. It was mostly . . . strictly a contusion, as I remember. It was a direct blow and was pretty painful. We did not treat it the rest of the season as something that caused him not to participate.

"He was able to throw. He did not have any problem throwing because of his arm, chest or upper body. I'm sure his ongoing problem was his knees and they required considerable medical attention.

"He underwent physical therapy and swimming to be able to play and practice. In the last month of the season his right knee made it very difficult for him to participate.

"He did not complain. He is a fine gentleman, one of the best I've ever met. He was very cooperative.

"He did a superhuman job in keeping herself in there (practicing and playing) and ready. He rose above and beyond the average individual's response."

Namath, 34, underwent a regeneration of his cardiovascular system by swimming, to offset the fact that he could not run to get in shape with his knees in braces. His legs were locked by a plastic device so they could not be bent while in the water.

That was done to prevent irritation to the knees by kicking or jerking them in the water. But he was able to build up his wind and strengthen his arms and upper torso.

He is made it clear in Los Angeles after the playoff game that it was his last. After his retirement announcements in New York on Tuesday and yesterday in Phoenix, where he will play in a celebrity golf tournament.