The workout left Rebecca Bishow speechless. After 45 minutes of calisthenics, trampolining and exercise on varied gym equipment, all she could do was smile.

But then, at age 4 months, Rebecca is a woman of few words. She expressed her opinion of Gymboree--a new fitness program for the diaper set--with pink-gummed grins and one brief squall.

"Except for those sit-ups she loved it," translated Joanie Bishow, a 35-year-old school counselor and one of about 150 parents toting toddlers to Monday night's open house at the new Gymboree center at Temple Beth El, Alexandria.

"Doing the baby boogie with Rebecca and rolling her around on the air log was fun for us, too," added Rick Bishow, a 34-year-old physician's assistant. "It's nice for all of us to meet other babies and their parents in such a playful setting."

Gymboree marks the latest twist in America's obsession with infant improvement--the Better Baby Movement gone physical.

First came the intellectual push for preschoolers--spawned by books with titles like Kindergarten is Too Late and How to Raise a Brighter Child and "toys" like number-concept blocks and word-association flash cards.

Now comes the physical counterpart, Gymboree, a booming nationwide franchise of infant exercise centers that debuted in the Washington area this summer in Gaithersburg and this week in Alexandria, Va., Columbia, Md., and Towson, Md. Centers are scheduled to open in Fairfax County in October and in Rockville in January.

Begun in 1976 by 36-year-old Joan Barnes, a former dance instructor and mother of two from Burlingame, Calif., Gymboree is now a $1.1 million company with 125 centers in 14 states.

"We are growing," says vice president Karen Anderson, "at a very rapid rate." About 120,000 pre-schoolers are Gymboree grads, with about 10,000 to 15,000 tots currently enrolled.

Infant exercise is not a new concept--"water babies" programs are popular at Ys around the country and postnatal "mom and baby" workouts have cropped up with increasing frequency in recent years. But Gymboree's comprehensive training, packaging and marketing strategy on a nationwide level make it unique. Billed as "the most successful parent-child program in the country," it could be described as a cross between Jacki Sorensen and Ronald McDonald.

"There was nothing like this when my son was a baby," says Wilma Hazen, 36, who quit her job as director of the American Home Economic Association's Center for the Family in June to open the Alexandria Gymboree center.

"I read about Gymboree in a professional journal about a year ago and thought it was a great idea. I wanted to spend more time with my son, who is 2 1/2, and running a center seemed ideal."

She and her husband, Charles Linderman, a 36-year-old fossil fuels manager and ex-camp counselor, flew to California for a week of training this summer. Only about one of every two dozen prospective franchises--most of whom are women or couples--are accepted. The minimum franchise fee is about $20,000 for two sites, each of which must be equipped with about $8,000 worth of regulation Gymboree equipment.

The "developmental playground" includes tunnels, slides, mats, mini-trampolines, huge inflatable balls and logs, hoops, bubbles, rockers--even a giant multicolored parachute the parents waft over and under the children.

"Part of the purpose is to have special time between parent and child," says Hazen. "This is particularly important with working parents since they don't always have the chance to set up a network of friends with babies in their own geographic area."

Gymboree is divided into three sessions, according to the child's age:

BabyGym: For infants 3 months to 1 year. Parents sing familiar ditties ("The wheels on the bus go round and round") and help their kiddies do "baby boogies"--choreographed exercises such as bicycling legs or stretching arms.

Gymboree: For tots 1 to 2 1/2 years. Most of the class is devoted to "free exploration," with some structured time for "songs, fingerplays and creative movements."

GymGrad: For preschoolers 2 1/2 to 4 years. "Gymbercises, a combination of stretches, body awareness, aerobics and cool-downs" follow a different theme each week.

"It's important for parents to realize," says Hazen, "that children before the age of 1 learn more than they do at any other time in their lives. And before the age of kindergarten they learn more than they will in the next 12 years.

"How they learn is by manipulating their environment, using all their senses and learning coordination. We call this sensory integration."

Eighteen-month-old Matthew Dickert calls it "Weee."

"We're not into the whole better-baby business," asserts his mother, Sherry Shapiro, a 32-year-old congressional researcher who also brought her 3-month-old twins Emily and Lauren to the Alexandria open house.

"We want them to be happy, well-rounded kids. But we like to do stimulating, fun things with them.

"And this," she said as Matthew explored a fabric tunnel, "sure is fun."