CHICAGO - Ethan Coon was working to develop good posture and strengthen his core muscles, though from all his wiggling and giggling you would hardly know it.

By the time he hit potty break, the 2-year-old and his classmates had bounced on a mini trampoline, rolled on their bellies on scooters and crawled through mazes.

The children were participating in a park district class in Highland Park, Ill., that uses preschool playtime to teach good habits in posture and movement and prevent sports-related injuries later in life.

Many doctors agree they are seeing young children with injuries more commonly seen in adults, from torn knee cartilage to lower back pain. About 2.4 million children ages 5 to 18 were treated for sports-related injuries in U.S. emergency rooms in 2000-2001, according to a study by the Centers for Disease Control and Prevention, which does not have comparable research to show whether the numbers are rising.

"They're way too young to be having these problems," said kinesiotherapist Marianne Vuckovich, who in her private practice once saw an 11-year-old girl who shattered her tibia after rotating her leg too much during soccer.

Vuckovich, 43, evaluates children before and after her 10-week sessions at the Recreation Center of Highland Park, leading them through dozens of activities that target specific muscles and good posture.

She has taught classes to all age groups for 20 years, refining her techniques with the youngest clients over recent years. Unlike the usual playtime activities aimed toward toddlers, Vuckovich brings a therapeutic approach to children who are developing normally, unlike most kinesiotherapists who focus on children with developmental problems. The course, which consists of weekly hourlong classes, costs $155 for Highland Park residents and $170 for nonresidents.

Other medical experts stressed there is no data that examines the effectiveness of working with children so young.

During a recent class, the 2-year-olds were a blur of motion, heading from one activity to the next in a room that looked to be a kids' paradise, filled with foam blocks, balls, balance beams, scooters and balancing devices.

They balanced on one foot and dropped balls into cups, tiptoed along a balance beam or threw balls into hoops while lying on their backs.

They built towers of foam blocks and then, starting from about 10 feet away, attempted to sprint toward the tower so they could knock it down. Vuckovich held each child back with a stretchy elastic band around their tummies, as if loading ammunition in a slingshot, prompting them to pull harder before reaching their goal.

"You can't push them into doing something they don't want to do," she said, joking that her clients have an attention span of about 15 seconds. "We're trying to develop skill levels but, we want them to enjoy themselves."

Vuckovich believes children as young as 2 can learn basic skills such as carrying their weight more evenly and squatting to pick up a toy through the activities, even if they don't know it.

"You have to balance things out," she said.

Ann Coon of Highland Park said she enrolled Ethan in the class after watching how much her daughter enjoyed another Vuckovich class geared for children ages 4-6.

"I just wanted Ethan to be more active," Coon said. "I think she encourages and pushes him a little bit more than he would get in a regular play class. I also appreciate that she's trying to teach him these good mechanical skills."

Mary Weck, a physical therapist at Children's Memorial Hospital in Chicago, said she sees children as young as 3 to 5 with pain in their back, knees and ankles, which she blames on a lack of strength in the core muscles. She believes more children are suffering from the "container syndrome," in which they spend too much time sitting upright in car seats, bouncy seats and chairs.

Instead, newborns should spend more of their waking time on their stomachs, so that they have to work to develop muscles, she said.

As they move into organized sports, children should be allowed to take frequent breaks during training, said Dr. Cynthia LaBella, medical director of the Institute for Sports Medicine at Children's Memorial. She cautioned parents to observe and ask questions to be sure coaches provide age-appropriate training.

"I see a lot of kids trained like adults. That's when the injury rates are high," said LaBella, whose youngest patients are 6 and 7. "Kids need freedom of movement ... let the kid be boss."

Vuckovich shares that philosophy. Letting the child call the shots can lead to entertaining moments, as well.

One Halloween, she recalled, a small client dressed in a lion's costume was working up quite a sweat under his fur while trying to balance on a device.

"He was very frustrated," she said. "He finally pulled off his mane and said, `I think I need a cookie.'"