IN 1919, John Philip Sousa fell off a horse and injured his left hand. That's why Keith Brion's left arm will be relatively inactive tonight at Wolf Trap, never rising up to shoulder level and sometimes simply resting by his side. When he plays Sousa, Brion goes all-out for authenticity.

Brion will be "playing" Sousa in more than one sense tonight when he debuts with the National Symphony Orchestra. His program will be a recreation of the kind of program Sousa conducted with his own band in the 1920s, and the conductor will be a recreation of Sousa himself -- "a little old man who looks something like Arthur Fiedler," in Brion's own description. He will be wearing gray hair and a gray moustache, which he does not wear offstage, and the uniform of a U.S. Navy lieutenant commander with white trousers. His baton will be longer than normal.

A few years ago, Brion was the director of the band program at Yale and had no idea that he would be earning his living as a free-lance conductor doing Sousa imitations. Besides music education, his chief interest, for about 20 years, had been contemporary music. That means he was not accustomed to large, enthusiastic audiences. Then, more or less by accident, John Philip Sousa struck him like lightning. He explained it a few days ago at the Marine Corps Museum, where he had gone to do research on Sousa, who was the leader of the Marine Band before retiring and becoming a superstar composer and conductor with his own band.

Reading a biography a few years ago, he discovered that Sousa, suffering from malaria, had been carried to the hospital "from a New Haven stage." Which stage, he wondered. He began checking and found that it was Woolsey Hall at Yale, where Brion conducted his own band concerts. Then the idea was born.

"I wondered whether I could make a theater piece out of doing a Sousa concert as it had originally looked and sounded. I remembered reading Leonard Bernstein's statement that the symphony orchestra has become a kind of museum, and I wondered: If the orchestra is a museum, is it a good museum? Some museums now are trying to have their visitors live the experience: If you are looking at a desert scene, they blow a hot breeze at you. I thought of taking the latest trends in museum-going and applying them to a concert situation: making the look of the concert, the size of the podium and baton, the conducting style and gestures, the sound of the orchestra, all a reproduction of the way it originally was."

He tried it in Woolsey Hall with the Yale Band. "The audience reaction astounded me," he recalls. "And the New Haven music audience is very sophisticated."

That reaction was one he wanted to see again, and he began doing occasional guest appearances in the Sousa mode -- first with members of the Boston Pops in 1979. There have now been 22 such performances, and practically all of them have been sold out. Brion resigned his position at Yale last year and is already booked with a dozen orchestras for next year.

"I didn't conduct my first Sousa concert until I was 45 years old," he says with amazement in his voice. "At first, I thought the playacting part of it was cheap and theatrical, but it ends up being a stronger musical experience; I was very surprised at that. For one thing, as soon as you give that downbeat, you are a conductor and nothing else, no matter what uniform you are wearing. But there is also something psycho-acoustical about an experience like this: Is it what you see that's important or what you hear? And can you separate what you see from what you hear?"

After a concert in Rochester, he recalls, he was approached by a man who told him that he had played under Sousa -- in Atlantic City in 1932, the last year of Sousa's life. "We talked for a while," Brion says, "and finally I couldn't resist asking him, 'How am I doing?' He told me, 'It's eerie,' and I took that as a compliment."

Keith Brion's introduction to Sousa came when he was quite young, and in greater than normal doses. "When I was 2 or 3 years old," he recalls, "my father had a business renting public address systems for political rallies and county fairs. My parents were not particularly musical, and the only records we had were the march records that he played on his sound systems at these public events. He used to let me play these records, and apparently they had an effect." Brion began playing the piccolo in bands, then studied music education at Westchester State College with the aim of becoming a high school band leader, which he did. Along the way, he picked up an interest in 20th-century music and also began playing the flute. He plays it still.

Although the Sousa boom shows no sign of letting up, Brion is already thinking of other ways his "music as theater" idea can be put to work. He may find another traditional composer to impersonate, but he has considered and decided against at least one. "I thought about Johann Strauss Jr.," he says, "but after watching some of Willi Boskovsky's interpretations I concluded that to impersonate him you have to be a violinist." If he is looking for someone whose instrument he can play, he might consider Frederick the Great, who was a composer, the proprietor or an orchestra and a flute-player. A Prussian emperor might be considered a step up even from an officer in the U.S. armed forces.