Adam Jaffe climbed atop a cube at My Gym, a children's fitness center in Beverly Hills, and wrapped his hands around the plastic handles attached to an overhead wire. With "Ghostbusters" as background music, the 4-year-old shoved off from the cube, zoomed through the air under the watchful eye of instructors and landed on an exercise mat about 50 feet away.

The "Space Flight" activity was the finale of "Whiz kids," Adam's hour-long Saturday morning exercise class, during which he and six other preschoolers had also warmed up, stretched, climbed on pint-sized gym equipment, kicked a soccer ball and jumped in gunny sacks.

"Its very good for coordination," says Adam's mother, Barbara Jaffe, who has enrolled all three of her young sons in exercise classes. "And it helps build self-esteem and confidence." Adam seemed more enthused, however, about the immediate benefits. Asked after class his favorite part, he answers quickly, "Climbing."

A decade ago, preschoolers got their exercise by running around the house, the backyard or the neighborhood park. That's not always so today, when organized exercise programs -- some designed to begin the day the baby arrives home from the hospital -- are proliferating.

Aside from the kind of program offered at My Gym and similar establishments, preschoolers today can enroll in ballet and other dance classes, gymnastics or "movement experiences," usually at a cost of $7 or $8 a session.

Children and parents who can't make it to scheduled classes can sweat it out at home, thanks to do-it-yourself programs like BabyCise, a kit that sells in toy stores for about $80 and includes an instruction video, an exercise mat, baby barbells, weights, sweatbands, and exercise logbooks.

Exactly how many youngsters are involved in organized exercise, either at home or out, is difficult to estimate, but Joan Barnes, founder and president of the Burlingame, Calif.-based Gymboree preschool exercise program, estimates that "20 to 25 percent of preschoolers are enrolled in some kind of movement experience." About 45,000 kids a week attend Gymboree classes here and abroad, Barnes estimates. Soon, other industry observers predict, half the nation's preschoolers will be involved in organized exercise programs.

But while many parents and physicians are enthusiastic about the movement, some doctors and childhood development experts are not as gung-ho.

They're not against exercise, of course, and most experts consider the risk of injury from such programs negligible if the instructor is experienced. But they question whether such organized programs for preschoolers are necessary and speculate that the benefits of such programs -- for both parents and offspring -- may be more psychological and social rather than physical.

By the year's end the American Academy of Pediatrics expects to publish a statement about infant exercise programs in its journal, Pediatrics, says Dr. Paul Dyment, chief of pediatrics at the Maine Medical Center in Portland, Maine, and chairman of AAP's sports medicine committee. According to Dyment, the statement will read, "Passive exercise {those in which the parent moves the limbs of the child} are unnecessary and not recommended for a normal infant."

"Kids will get their own exercise," says Dr. Gayle Leidner, associate director of pediatrics at Orthopaedic Hospital and a clinical instructor at the University of Southern California School of Medicine in Los Angeles. "I don't think parents need to worry about it much."

"Spontaneous, self-generated play will meet the fitness needs of children if they are given the opportunity," agrees Dr. Nathan J. Smith, a professor of pediatrics and sports medicine at the University of Washington, Seattle, also a member of the AAP committee. "But," he concedes, "that's sometimes easier said than done."

Most preschoolers, unless grossly obese, are active enough on their own to maintain cardiovascular fitness, points out Dr. Irving Tessler, a pediatric cardiologist at St. Vincent Medical Center, Los Angeles, and an associate clinical professor of pediatrics at the USC School of Medicine. "They don't need an organized exercise program {at that age}."

The assumption that preschool children will exercise enough on their own isn't always so, counters Tiffany Field, a professor of pediatrics and psychology at the University of Miami Medical School and one of a few researchers who has studied preschool exercise. With Bene Alpert, a master's degree candidate in exercise physiology, Field recently studied 13 preschoolers, ages 3 1/2 to 5 years, children of medical school staff and faculty who attended the university preschool, which Field directs.

"Most preschools assume kids are getting enough exercise," says Field. But, in her study, "Some of the subjects tended to be pudgy. Some tended to be uncoordinated."

Field and Alpert divided the children into two groups. Seven participated in aerobic exercise for 30 minutes three times a week for eight weeks. Another six served as controls and did not participate in the organized exercise program.

After the eight-week program, Field and Alpert compared the fitness levels of the two groups by testing them on pediatric exercise bicycles, asking the toddlers to pedal at an easy pace, a moderate and a difficult pace. They found no difference between the two groups at the easy and moderate pedaling paces, but found that the exercising group did better than the control group during the difficult pedaling pace.

The increased endurance wasn't the only benefit, though, says Field. "The kids {who exercised} were looking more coordinated at the end," she says. "I was as impressed with the gain in coordination as with the physical gains." Field plans to continue her study and eventually to disseminate the results to local preschools and possibly to publish the results.

Dr. Douglas McKeag, coordinator of sports medicine at Michigan State University, East Lansing, disagrees with Field's findings. "There is little or no significant training effect from doing any type of formal exercise in a child who has not yet undergone maturation or puberty," he asserts. McKeag bases his belief on his two-year study of 115 children, ages 5 to 16, reported in a recent issue of the Journal of Adolescent Health Care. In the study, 90 children participated in treadmill exercise while 25 matched control children did not undergo formal treadmill exercise. McKeag found "no measurable training effects" resulting from the treadmill exercise in children who had not yet reached puberty.

Proponents of organized preschool exercise, however, are undaunted by skeptics like McKeag. They point out that physical fitness is not the only goal of such programs.

"Enrollees learn their physical limitations and how to overcome them," says Lloyd Wilfong, program administrator at Olympica USA, a Van Nuys gymnastics program. In addition, other program directors say the experience of organized preschool exercise can build children's self-confidence and self-esteem.

Organized preschool exercise programs in which the parents and children both participate offer another plus, say proponents: a chance to spend "quality time" with their offspring. Even experts who consider organized exercise physically unnecessary for preschoolers have difficulty arguing with the "quality time" premise. "It's hard to fault time spent with a child," notes pediatrician Dyment. "You can't say it is a valueless experience, because bonding goes on."

Enrolling children in exercise programs at an early age may yield some long-term benefits as well, claim proponents. Although there is no hard data to prove it, proponents of early organized exercise claim it may help ward off childhood obesity, which is considered a significant problem by medical experts. Dyment estimates that 10 to 20 percent of children are obese (20 percent above their ideal weight) by adolescence.

Classes may also help instill exercise as a lifelong habit, say proponents. McKeag agrees with that claim, although the key is exposure to exercise, which he says can occur without an organized program. "There is a positive correlation, based on studies, between children exposed to exercise and good exercise habits at a young age and their subsequent habits as adults," says McKeag.

For parents who decide to expose their kids to organized preschool exercise programs -- at home or in the community -- experts offer these suggestions:

Check out the credentials of the instructors.

Don't expose your child to a competitive program.

Keep the program fun for your child.

Don't push too hard or expect too much.

Pediatrician Leidner also advises parents to keep exercise programs in perspective. "Exercise has to fit comfortably into a child's schedule," she says. "I have a hard time with parents who schedule every moment of a child's existence. It gets tremendously stressful for kids. Concentrate on exercise that's fun for kids, and let them relax the rest of the time."

Kathleen Doheny is a free-lance writer based in California.