Seattle ? Lucy Brown somersaults backward down a ramp, leaps over a mat, rolls across a platform, jumps a series of hoops, squats into a yoga pose and hops over a row of cones.

And all with her diaper intact.

The cherubic, grinning 2-year-old was one of eight toddlers stretching and puffing their way through yoga poses and aerobic exercises at a recent toddler fitness class at the Seattle Holistic Center.

As the country’s population of overweight children swells, parents are flocking to baby exercise classes where tots as young as 1 day old can start getting fit.

While mainstream medical experts remain dubious, baby fitness advocates say getting babies and toddlers involved in exercise can set them up for a life of good health and improve motor skills and parent-child bonding.

“I have this little mantra and it goes like this: fit baby equals fit toddler equals fit child equals fit teen equals fit adult,” said Helen Garabedian, author of “Itsy Bitsy Yoga: Poses to Help Your Baby Sleep Longer, Digest Better, and Grow Stronger.” She also teaches an Itsy Bitsy Yoga class in Marlborough, Mass.

Yoga comes naturally to babies, who often learn the positions independently as they develop, said Garabedian, who works with babies as young as 3 weeks old. Babies will commonly move into “downward dog” just before they begin crawling, she said.

But do the children really understand what they’re doing?

Three-year-old Jasper Dean seems to. During the cool-down portion of his class in Seattle, Jasper sits in his mother’s lap, eyes closed and legs crossed while serenely chanting, “Ohm.” He then places his hands together, bows his head and murmurs, “Namaste.”

Merel Adriaanse, 7 months old, is propelled through the water by a Waterbabies instructor, left, and into the arms of her mother Frederieke Adriaanse, right, during an infant swim class in Bellevue, Wash. As the country's population of overweight children swells, parents are flocking to baby exercise classes around the nation.

“He has lots of energy, so it’s a good place for him to run around and learn about his body,” said his mother, Cindy Hazard, 40, of Seattle, who has been practicing yoga for five years.

Even though such classes are fun, medical experts say they do little to make babies physically fit. Very young children aren’t capable of the sustained exercise needed to improve cardiovascular health, strength and flexibility, said Dr. Eric Small, chairman of the American Academy of Pediatrics committee on sports medicine and fitness, and author of the book, “Kids and Sports.”

“Fitness is an adult concept,” Small said.

Nationwide, 30 percent of children and adolescents ages 6 to 19 are considered overweight or obese, according to government figures. Despite that, the academy doesn’t recommend fitness classes for babies, arguing the fragility of infants’ bones can set them up for injury.

But Bonnie Prudden, who wrote the book “How to Keep Your Child Fit from Birth to Six,” argues that babies’ muscles can be strengthened through exercise. Prudden created a YMCA swim program for infants in the 1950s, and her research on childhood fitness helped create the President’s Council on Youth Fitness during the Eisenhower administration.

“Every movement they’re making in the water is exercise,” said Vera Garibaldi, who teaches Waterbabies aquatics in Bellevue, Wash.