Arnold Schwarzenegger, once a competitive body builder and now an actor, squeezed his muscle-bound frame into a witness chair at U.S. District Court yesterday and did what he could to protect the name "Mr. Olympia," the ultimate in strongman titles that Schwarzenegger held no less than seven times.

The Mr. Olympia contest, sponsored since 1965 by the International Federation of Body Builders, is said to be the Super Bowl of muscle beauty.

But the U.S. Olympic Committee says the use of the word "Olympia" amounts to a trademark infringement. The committee wants Judge Thomas A. Flannery to order Mr. Olympia and, lately, Ms. Olympia, to change their names.

The Olympic committee also wants a former official of the IFBB to stop marketing high protein food for body builders that uses the word "Olympian" in its name.

Schwarzenegger, five times Mr. Universe and once Mr. World, testified yesterday that no one has ever confused him with an Olympic athlete or asked him if he ever competed in the Olympics, which do not include body building in its amateur sports competition.

Known to this summer's moviegoers as "Conan the Barbarian," Schwarzenegger said that the Mr. Olympia contest, which he called "the supreme championship for body builders," is well known as a competition for professional muscle men and women.

During yesterday's court session, Judge Flannery watched the 1979 Mr. Olympia competition on closed-circuit television along with a small group of spectators who had gathered when word spread through the courthouse that Schwarzenegger was on the witness stand.

Schwarzenegger, dressed for court in a conservative blue blazer, pink shirt and patterned tie, had provided the color commentary for the program, part of a CBS network "Sports Spectacular."

Flannery, yawning at times, saw a parade of muscle-bound men, dressed in red briefs, pose and flex for the judges, accompanied by organ music, as Schwarzenegger commented on the tape, "Incredible!" or "Fantastic! Look at the thighs!" or "Look at the definition of his pectoral muscles!"

Flannery is expected to hear closing arguments in the case in late August.