HOW DOES MOVEMENT give expression to feeling? "You're doing it now," Daniel Nagrin says, as he talks about his concert appearance tonight at the University of Maryland, as guest soloist and chreographer with the Improvisations Unlimited troupe. "You're doing it," he says, pointing to his visitor's hunched-forward form, "by your choice of posture. I'm doing it with my choice of posture, by slapping my knee as I say this word. All gestures are metaphors. The sign that says 'Rome' is not Rome; it's a sign for Rome.

"The swift turn of a head, and a kind of visual focus, can throw a blinding light on a certain piece of the stage, just by that simple gesture. Just transforming the dynamic of a time step, in tap dancing, can give it a potent metaphoric sense."

Nagrin is 64, but you'd be apt to lose a quick bundle betting on his age. His body is lean, wiry, taut -- the trim product of ceaseless workout and performance, and his face, creased in intensity or stretched with grinning, has the craggy look of a born scrapper. In the world of rugged individualism which was modern dance in the days Nagrin first came to it, he has been rugged individuality personified. In earlier years, he won prizes on Broadway, dancing in shows like "Annie Get Your Gun" and "Plain and Fancy," and his partnership with the charismatic Helen Tamiris (1905-66) constitutes a significant slice of American dance history. But he's probably best known for his biting, wide-ranging dance solos, the backbone of his stage career since the late '40s. The evening-length "The Peloponnesian War" in 1968 was a landmark in the solo form; since then he's made such pieces as "Jacaranda" (1979), with a text specially written for him by Sam Shepard, and the recent (1981) "Poems Off the Wall." The first half of tonight's program will have Nagrin performing excerpts from some of these and other, earlier pieces; the second half will be devoted to a new, improvisatory work he's mounted with the Improvisations Unlimited company, directed by Merian Rosen.

The notion of movement as metaphor is central to Nagrin's dance concept; in an era dominated largely by abstract formalism, he has persisted in making dances that, in some sense, act out life experiences (dramatic acting has also been a life-long interest of his). "Important feelings and events in life shove us around, and imprint themselves on our bodies," he says, "in secret signs -- heaves and twists of the head, the neck, the fingers. As a choreographer, it's up to me to allow a visible gesture -- a suppressed movement of the thighs, for example -- to follow through until the secret begins to sing out loud. If art is not about the unknown and precise feelings that we'd like to make known, I don't know what it's about. A photographer friend of mine has done a series of pictures of the South Bronx -- kids, against a background of incredible rubble, the end of the world. In one there's this girl, 7, maybe 8, and she's leaning forward with such excitement into the camera, it's fabulous.It always charges my batteries to see it, and if that spirit can exist in that filth and destruction, I tell myself, you better wake up and do something good today."

Though he lives in his birthplace, New York City, to which he returns to work on new material, Nagrin, who calls himself a "traveling salesman," spends a good deal of time on the road, performing, teaching and giving workshops. Lately, he's also begun to do some writing -- there are several books he'd like to do, including one on Tamiris (whose story, Nagrin thinks, has never been fully told), one on acting, and one on the teaching of choreography. His approach to the last subject is disinctive, and based on improvisation or its source material.

"I'll tell a class there's something or someone of which you are in awe -- find a movement metaphor to become it.To find the image, they must allow their minds to wander over the literature, myth, newspapers, movies or stories they know, or their own lives. There's only one criterion for what the right image is. You never go for the image that's good,' or that you think might interest the critics, or that seems intelligent or 'innovative.' You wait passively for the one image that demands attention, that upstages all the others, that asserts itself. It may even be an image you don't like, or one that upsets you -- but it'll be the one that's personally moving and meaningful."

This is what modern dance used to be all about, Nagrin feels. "People used to complain about Martha Graham's bare, dirty feet. Did she put on stockings? Tamiris chose to dance about Negro songs of protest. All of them -- Doris Humphrey, Charles Weidman, Anna Sokolow -- they weren't auditioning for success. Or Merce Cunningham -- he's been booed off plenty of stages. It never occurred to him to fix it up so they'd like it. He did what he had to do." And that's Nagrin's philosophy, in essence -- doing what you have to do.