"My name is Sue, foot circles, one-and-two-and. . ." the introduction and pre-warm up command begin in one breath.

Obediently, eight women bend and bounce to the beat of "This Must be Magic," a moderately energizing tune. Sue, substitute teacher for tonight's class at St. Mary's Episcopal Church, has enough energy for nine.

Down on the floor now, it's sit-ups on cold lineoleum. Gym shorts, tight, warm-up pants, T-shirts and jogging shoes highlight the fact that some are thin, some not-so-trim. Upbeat R&B music gets them kicking, reaching and snapping their fingers in a battery of exercises learned earlier in the 15-week course. This is aerobic dance, a cross between jogging and dancing, programed to strengthen the heart and trim the fat.

Step turns, leg swings and jogging -- always limply jogging in place -- now to slower music. "The good Lord loves you," a tinny tape blares.

Aerobics, meaning "with oxygen," are geared to make the body work strenuously and to demand increased oxygen over an extended period, 45 to 60 minutes two or three times a week. Not all aerobics classes are alike: Some are heavy on jumping jacks and disco bumps, others lean toward jogs and bends. But all aim to build endurance and body awareness.

"Touch and smile," the teacher exhorts, "two-three-four."

After jogging, jumping, kicking and twisting run-through, they finish the combination in a "pose." Then, concentrating seriously, each exer-dancer takes her heartrate count. Hands on neck, they walk in slow circles and monitor their pumping hearts' progress.

This session, 2,200 students have plunked down $55 to learn the standardized Aerobic Dance Inc. routines. The California-based firm is on its way to becoming a nationally known exercise syndicate, with 35 trained instructors in the District, Maryland and northern Virginia. Jacki Sorensen, the muscle behind this faction of the contagious aerobics craze, has created a series of cues and six aerobic dances, plus a range of warm-up and cool-down exercises, all aranged for maximum circulatory and cardiac benefits, muscle tone and general self-improvement.

It's all very scientific.

Besides the gospel-style tunes, the women step, dip, bend and clap to Donna Summers' disco smash, "Enough Is Enough." And by the end of the hour, they're pouring off sweat and racing to the water fountain as if they're truly reached their limit. Aerobics look safer than jogging, but there is the occasional accident. After twisting her ankle, one woman retired to a chair in the corner to alternately cry in pain and laugh at herself in tears. In a departure from the script, instructor Sue suggested that the casualty might want to put ice on the ankle to avoid swelling.

Just when the members seem to be running out of energy, they are spurred to move to a disco version of "Singin' in the Rain," in a routine complete with imaginary hat, puddle and lamp post. Soon, loosening up, they're looking less like an exercise class than a beginners' chorus line. Are they dancing for the mirrors leaning against the wall, giving distorted feedback, or for a grand, imaginary audience showering silent applause?

"Tom Jones, Tom Jones. Pom-pom, pom-pom, jog, walk, walk." Each flexing movement has its own copyrighted cue, never letting up throughout the hour, except for pulse readings.

Over at Joy of Motion, Michelle Ava's version of Body Dynamics/Aerobics draws 22 pupils (five of them male) to a weeknight class. The Saturday-morning workout similarly stresses double-time shakes, vigorous boogies and disco-style athletic funk. Altogether, 300 people are working out in the Joy of Motion body dynamics programs (including jazz, stretch and conditioning classes), looking to tone up, slim down and release all kinds of frustrations. This co-ed crowd is hip but crowded in the upstairs Connecticut Avenue studio. Mirrored head-to-jogging shoes while running en masse in a colorful circle -- body stockings, warm-up socks and pastel leotards -- the huffing pack is coached by Michelle to "have confidence in it and shake out!" To wind down, the group hangs from the waist, head near the floor, then slowly lifts each inch of sweating torso and reaches for the ceiling in time to the show tune "What I Did for Love."

What they do for firm thighs is another matter.