Los Angeles — After jumping into the pool, synchronized swimmers Tracie Ruiz and Candy Costie stay under water until you think they'll burst. Their first breath doesn't come until about 53 seconds later.
By that time they have already completed about one fourth of their freestyle routine in this new Olympic sport, which puts precise arm, leg, and body movements to music. It's an ''Oh, wow'' start designed to grab the audience and impress the judges.
It's also a way of showing off the sport's artistry and athleticism simultaneously. The art comes from the ballet their legs perform while sticking straight out of the water, the athleticism from the endurance and strength required to make it look easy.
That they come up smiling instead of gasping for breath is amazing enough. That they do so after nearly a minute of constant exertion is even more incredible.
''To understand what this means, imagine running as fast as you could for 50 seconds without a breath,'' says Charlotte Davis, the US coach.
And from Ruiz, who has the dark complexion of her Hawaiian father, ''In any sport, there is a certain element of danger. Staying under water for such long times is what makes our sport difficult.''
Because a fair amount of any routine is spent upside down, synchronized swimmers wear nose clips. An extra set may be tucked under a swimmer's suit for an emergency, but even with a noseful of water the show goes on.
And part show it is, for the sport has to some degree grown out of those Esther Williams movies of the '40s and '50s. Williams, an ABC-TV commentator here, was a national freestyle champion before going Hollywood.
Because of its water show image, synchronized swimming struggled to gain recognition as a legitimate sport. ''It's taken 40 years to get into the Olympics,'' says Davis of what might be considered an aquatic cousin of figure skating and gymnastics.
In addition to choreographing routines, swimmers must execute six compulsory figures or rigid positions. The objective is to work in sync with each other and the music, which is audible via underwater speakers.
Costie and Ruiz have spent some time visiting Williams and looking through her scrapbooks with her.
''Comparing us to Esther Williams is like comparing figure skater Sonja Henie with Rosalynn Sumners,'' says Ruiz. ''That's how much our sport has evolved.''
It's not enough to be graceful, competitors must also be superbly conditioned athletes. To prepare for the Olympics, Costie and Ruiz dropped out of the University of Arizona to devote themselves fulltime to training.
Workouts have run 6 to 8 hours six days a week for the past two years. The regimen not only includes dance training, but countless laps of the pool and constant practice on the eggbeater kick which allows the body to ride high in the water. They even lift weights regularly in order to make some of the the more strenuous moves.
Early in their freestyle routine, for example, Candy lifts Tracie totally out of the water. That takes incredible strength, given the rule that prevents swimmers from pushing off from the pool's bottom.
Canada's Sharon Hambrook and Kelly Kryczka, meanwhile, are utilizing three such lifts - two of which come late in the four-minute program, which adds to their degree of difficulty.
These two pairs are the leaders in the sport at present. The Canadians won the most recent world championship in 1982, but Costie and Ruiz won the gold at the 1983 Pan Am Games and scored the sport's first perfect 10 at a pre-Olympic meet in Rome earlier this year.
To prevent scouting before the Olympics, Canada's synchronized swimming trials were closed to the public.
Meanwhile Davis, who has coached Costie and Ruiz at the Seattle Aqua Club for 10 years, had them practicing their routine behind closed doors - this so no one would get a look at a complex new move they invented called ''thread the needle.'' The trick is so involved that neither swimmer could explain exactly what the other does after unveiling it in the prelims on Monday.
The US duet leads entering today's final, with Canada second, followed by a strong Japanese twosome.
A solo competition was added to the Olympics after the Eastern Bloc pullout and Ruiz, the reigning world solo champion, will be favored to win the gold later in the week. Team synchronized swimming, perhaps the most eye-pleasing of all, didn't get accepted into the Games this year, but it could gain entry in the future.
Although synchronized swimming is more athletic than ever before, some of the glitter remains. The women achieve Rockettes-style uniformity by wearing sequined swimsuits, heavy makeup, and small caps. These caps anchor the swimmer's hair, which has been cemented down using unflavored gelatin. The gelatin is washed away with warm water in about 20 minutes, a tremendous improvement from the week it took Esther Williams to get ride of the Vaseline and baby oil she once used.
What comes after the Olympics? Well, would you believe synchronized swimming will go high brow? Costie and Ruiz are scheduled to begin a ''Classical Splash Series,'' in which symphony orchestras will provide accompaniment for the swimmers in Colorado Springs, San Diego, Indianapolis, San Jose, and Palm Beach, Fla.