The steadiest, most consistent performer during Thursday night's U.S. Olympic Gymnastics Trials was not Phoebe Mills, Kelly Garrison-Steves or even, heaven forbid, Bela Karolyi. Sorry, folks. That recognition goes to . . . Carol Stabisevski - the piano player.

Think of it: 22 times Stabisevski played the same song during the compulsory floor exercise; over and over and over again, ad nauseum, he played the same nameless song and, even with a fairly high difficulty factor, never once made a mistake - a 10.0 on the piano, if you will. Only Joyce Wilborn was able to manage such a score (on the vault) and she had to do it only once.Even Karolyi, the busybody coach who seems to find fault everywhere, couldn't resist admiring the performance of his long-time pal and countryman. Between performances, Karolyi walked over to the piano and gave Stabisevski a hug, not unlike the one Stabisevski received from Mary Lou Retton on worldwide TV during the 1984 Olympic Games.

Actually, playing the same song 22 times in a single meet is a mere sprint for Stabisevski. A marathon would be, oh, say, 140 times in one day, which he did once last year. That doesn't even include warmups.

This is no small endurance feat. Stabisevski's fingertips get numb, his back gets tight (which is why he used to tape a sign above the keyboard: RELAX), his forearms get sore (from the finger movement), and "my mind gets tired because I have to follow the gymnasts so closely," he says. "Each gymnast has a different style, a different rhythm, a different speed. And they all like different accents in different places."

Stabisevski must watch the gymnasts on the floor almost constantly while playing the piano, trying to match, among other things, the accents of the music with the precise time the gymnasts strike a pose - with a flip of the hand or a tilt of the head - at several points throughout their routines.

"I watch as much as I can, but when I must transfer from one side of the keyboard to the other I have to look, so I miss a little of the performance," says Stabisevski, who practices with the gymnasts only two days before each meet. "I've known most of the kids two or three years anyway," he says.

Stabisevski knows something of both gymnastics and music. Growing up in his native Rumania, he "was supposed to become a concert pianist," but at age 13 he gave up music for sports. He took a degree in physical education, coached gymnastics and eventually wound up playing the piano for the Rumania's national gymnastics team.

In 1976, Stabisevski played for the Rumanian team in the Munich Olympic Games. He would play again in the Olympics, but for a different country. In 1977, he defected to the U.S. while on a tour with the Rumanian national team (Karolyi defected four years later). The United States Gymnastics Federation quickly asked him to become its official pianist.

"It's a hobby," says Stabisevski, who operates his own gymnastics club in Valencia, Calif.

Come September in Seoul, Stabisevski will play in his third Olympics (he would have played in '80 if not for the boycott), and, he hopes, his third Olympic all-around champ (as he did for Nadia Comaneci and Retton). But it will mark the first time he will play music in the Olympics that he did not write. The USGF, which selects one piece of music to be played during all compulsory floor routines from one Olympics to the next, chose a score that originated in Russia.

Stabisevski knows it well by now. He has been played it several thousand times during the last four years. And yet, for all the tedium, he rarely makes mistakes, although during the final routine at nationals last month - on his 60th rendition of the song - he lost his concentration momentarily. "I really messed up," he confesses. "My finger slipped off the key and I couldn't find it again. I kept playing for several seconds until I finally found the right notes. But I didn't make any mistakes tonight."

Alas, this will be Stabisevski's final year of playing for the national team. The new compulsories will be orchestrated (more than one instrument) next year. "Some (gymnasts) feel more comfortable with recorded music because they practice with it all the time and with me just two days before the meet," he says. "But I try to put a bit of flavor in their routines."