If Tracy Caulkins shocked the swimming world by demolishing the supposedly invincible East Germans in winning five gold medals at the World Swimming Championships in West Berlin last week, her closest friends weren't the least bit surprised.

They cite as an example of the 15-year-old's mental toughness the incident last September when she fractured an ankle, an injury that would dry-dock most swimmers. Caulkins, however, requested her doctor to put on a Fiberglass cast, so she could continue swimming.

With little mobility in her legs, Caulkins increased her arm strength by pulling her 5-foot-8, 110-pound frame through two-a-day practices. As a result the native of Nashville improved her times, putting her in a perfect physical and mental state of the world championships that concluded Monday.

But Caulkin's victories, which included breaking four world records and another, meant more than the emergence of a new American swimming phenomenon (few people outside the swim community knew she already owned 14 U.S. records and one world record).

Her victories, coupled with those of her teammates, seemed to many to signal the return of American women to swimming eminence after the humiliating defeats in the 1976 Olympics in Montreal.

At those Games, East German women won 11 of 13 events while U.S. women returned home with only one gold, in the 400-meter freestyle relay. At the world championships, the American women dominated, taking 19 golds to the East Germans' one.

"I was expecting them (the East Germans) to be tougher," Caulkins said in a telephone interview. "I've swum against most of them before. But this time we were more sure of ourselves and they may have been a little more afraid because the (American) women swam so well at Houston," where the qualifying races were held.

Of her own success, Caulkins said, "I'm really pleased with the way I swam. But right now, I'm relieved that it's over. I'm going to take a break from training and then start up again in a while. I want to make the nationals and the Pan American Games. But my ultimate goal right now is to win a gold medal in Moscow (1980 Olympic Games)."

Caulkins said she is proudest of her record in the 400-meter medley (4 minutes 40.83 seconds), in which she shaved two seconds off the world mark. She was less pleased with her 200-meter butterfly performance that tied the world record (2:09.87), since she said she should have done better.

Her other gold-medal, world record-breaking achievements: the 400-meter freestyle relay (3:43.43) 200-meter medley (2:14.07 and 400-meter medley relay (4:08.21.

Caulkins and her sister Amy, 17 (who was on the women's water polo team that finished third in exhibition matches at the championships, began swimming as children in a summer club with their brother Tim, now 18.

Her mother, Martha, recalled: "Swimming and tennis were the only sports girls could play then and my husband (Tom and I thought there was a value in sports experience."

She is an art teacher at a Nasville school. Her husband is with the school system's testing and evaluation department.

By 11, Tracy was competing at the national level and her self-imposed weekly regimen was established: Up at 5:15 a.m., two hours of practice in the pool before school, another two hours after school, to sleep at 9:30 p.m. weight-training three times a week, exercise sessions twice a week.

"We make sure she gets to the meets," said her mother. "Yes, it's the traveling that's the expensive part. We've never figured the cost out. I don't think I want to know."

Until this week, Paul Bergen was Tracy's coach with the Nashville Aquatic Club, which also produced two other double gold-medalists at the world championships - Joan Pennington and Nick Nevid. Bergen has moved on to become coach of the Longhorn Swimming Club in Austin, Texas, where he is also women's coach for te University of Texas.

Bergen was one of six assistant coaches under head coach George Haines at the championships, and noticed a marked difference among the American women from a 1975 world championship team he coached.

In 1975, he said, "The American boys were talking about breaking world records. Their conversation was about sweeping everything, getting all the medals they could . . .

"The girls were quiet and wondering whether they'd even make it to the finals. But this time, the U.S. women's attitude was fantastic. Rather than (acting like) they had just come along for the trip, they acted as though they had come to compete."

"We were trying to win and I don't think winning is an incident. We worked hard," Bergen said. "What eventually happens depends on how well the kids take the mental stress."

Caulkins, he said, "is very easy to get along with . . . She's extremely coachable. She's able to handle criticism better than any person I've run into in 15 years of coaching. She's able to rebound off disappointments."

Extra practices in 50-meter pools scattered around Berlin during the two weeks of training that preceded the one-week championships helped improve performances, Bergen noted.

But, he added that he doubts the world will see swimming being dominated by one country in future Olympics. "There's an evening-out of good athletes all over the world. We (U.S.) were winning races over there, but by small amounts."

Of the East German women, Bergen said, "I think they're a lot better than what they showed there.If you could think of 100 things that could go wrong, then 99 would happen to them . . .

"There was some sickness, a flu or a fever, that was going around, and some got it. When that happens, I think it makes the others feel mentally down . . .

"They really improved dramatically between 1972 and 1976, when they were very good. But from 1976 to 1978, there hasn't been that great an improvement. There are new faces and new athletes. They don't have people like Kornelia Ender around now . . .

"What happened to the United States women then (Montreal) had been gradually coming on. I don't think American swimmers will allow another downslide."