In the eight years since the opening of the Kennedy Center Opera House, the hall has evolved into a national center for ballet second only to the big New York houses.

At the same time, however, it has earned a reputation for having one of the hardest floors in the business -- floors that are hellish for rehearsals, hazardous and fatiguing for performances.

The foot injury which sidelined Mikhail Baryshnikov from the New York Ballet performances earlier this week has thrown a spotlight on this chronic and objurate problem. Baryshnikov's Achilles tendon condition had its onset in New York, but was unquestionably exacerbated by the Center's unyielding stage.

Hard floors are endemic to the ballet world, but the problems of designing and building a stage able to cushion the punishment to dancers' bodies has usually defeated architects, engineers and ballet masters -- the Kennedy Center included.

At the moment, the New York City Ballet itself is attempting to devise a sort of traveling solution, in the form of a portable, modular, flexible dance flooring to accompany the troupe on its tours.

Baryshnikov has complained often in the past about the Kennedy Center floor, and he's scarcely alone. Dancers from visiting ballet troupes at the Center seem united in two opinions -- they relish the enthusiasm and sophistication of Washington audiences, and the amenities of the city; and they detest the Opera House floor as a constant menace to muscle, bone, art and morale.

The list of dancers who have expressed unhappiness over the floor in varying degrees -- one prominent ballerina was overheard to say she'd never come back because of it -- would include such ballet eminences as Nureyev, Makarova, Dowell, Ulanova, Kirkland, Bruhn, Alonso, McBride and Bujones.

Ironically, every attempt was made to head off just such a problem when the center was designed and constructed. Balanchine and officials of American Ballet Theatre were called in as consultants; prominent dancers were flown in to test sections of the flooring before and during installation. The best laid plans, however -- in this and many another multipurpose opera house -- have frequently gone astray under real performance conditions.

The Center's executive director, Martin Feinstein, who took over the reins three months after the Opera House opening, recalled the situation yesterday.

"The theory was," Feinstein said, "that the stage could be 'tuned' to varying degrees of elasticity."

At regularly spaced intervals along the steel braces under the wood floor of the Opera House are large bolts which, it was presumed, could be tightened or loosened to adjust the resiliency of the floor according to production needs.

"In actual usage of the floor for ballet, however," Feinstein said, "when complaints began and they tried to adjust it was found that the bolts were already at their maximum expansion. It was assumed that somehow the wood had swelled. In any case, no further adjustment was physically possible." Feinstein said that when he subsequently discussed the situation with Balanchine, Balanchine told him to hold off trying to repair matters until his own company's research on a portable flooring made some progress.

Two years ago, before a Stuttgart Ballet visit, Balanchine had another suggestion: Try tightening the bolts to the utmost to compress the wook for a few days before the Stuttgart troupe arrived, and then loosen them. It seemed to give some relief during the Stuttgart engagement, but when the same tactic was applied again for ABT it was no help at all.

"We're determined to do something about this, definitely," Feinstein said, "Whether it's the New York City Ballet solution, or redoing the floor, or some other solution. But because of the costs and because we'd have to close the house, we have to wait until we close it anyway for needed refurbishing of the interior. I'm hopeful this can take place by the end of summer in 1980."

Edward Bigelow, the company manager in charge of the NYCB experiments, says at this point that "we don't have the solution yet, but we're actively looking for it."

Bigelow says the problem has been of long standing because no one paid much attention to ballet until recently. "Opera houses weren't built with dancers' needs in mind -- we were lucky at the New York State Theater to design our own floor and it works beautifully. It was a good step when companies began using linoleum surfaces about 15 years ago, but this only gave us a uniform surface texture wherever we performed -- it didn't help at all with the resiliency problem."

The concept Bigelow is working on, in association with Balanchine, NYCB production stage manager Ronald Bates, and consultants including a professor of industrial design at Cornell University, is that of a portable dance floor.

"It would be like a little wooden sandwich, about three inches thick," Bigelow says, "that could be transported, handled, taken apart in sections, stored, and reassembled. We're also looking at some work done recently at Harvard, where they've developed a flexible indoor track for runners that seems to have improved their speed and reduced fatigue."

To compound the difficulties, many dancers also complain about the hardness of the floors in some of the rehearsal studios at the Kennedy Center.

"Being sensitive to the problem is one thing," Bigelow says, "but knowing how to deal with it effectively is another. The solution isn't simple -- however desirable."

In the meanwhile, the Kennedy Center floor will remain hard.

And the show will remain on.