At 5 months old, Marty Lesner isn't quite ready for aerobics.

But his mom has led him through a daily workout since he was 2 weeks old.

Several times a day, Donna Lesner gives Marty a chance to strengthen the neck, shoulder and arm muscles he'll need to crawl. She props him on the floor with a toy. She burps and soothes him on her lap, tummy down. And instead of carrying him against her shoulder, she cradles him on her arm like a football, allowing Marty to lift his head and gaze out at the world.

The positions are part of "Baby's First Workout," free suggestions developed by the Pathways Awareness Foundation, a Chicago-based nonprofit that promotes early detection of movement delays. The "tummy time" ideas are available at www.pathwaysawareness.org.

Supervised exercises like these have become more important now that most babies are sleeping on their backs, a precaution to prevent sudden infant death syndrome, or SIDS, says Garry Gardner, a pediatrician from Darien, Ill., who leads Pathways' medical round table.

The number of babies who sleep on their backs has grown from 13 percent to 73 percent since 1992, when the American Academy of Pediatrics recommended it. Deaths from SIDS have fallen by more than half since then, according to the National Center for Health Statistics.

SETBACKS FOR BACK SLEEPERS

Experts agree that sleeping on the back is safest for babies. But more and more infants are now on their backs all day as well, spending hours reclining in car seats, bouncers and strollers. That gives them little chance to raise their heads or perform "mini push-ups" to look around, says Gay Girolami, a physical therapist and member of Pathways' medical board.

Gardner says some of his young patients skip over some milestones, such as rolling over from tummy to back. And more babies are developing flat spots on the back or side of the head, a problem called plagiocephaly, from spending too much time on their backs.

Youngsters who sleep on their backs also tend to achieve major milestones — such as rolling, sitting, crawling and pulling themselves up to stand — later than those who sleep on their stomachs, says Rachel Moon, a SIDS expert at Children's National Medical Center in Washington.

In a new survey of 400 physical and occupational therapists conducted for Pathways, two-thirds say they've seen an increase in movement delays in the past six years.

Moon notes that most back sleepers do catch up with other babies. And even though babies are reaching milestones later, most still fall within the normal range, she says.

But doctors are concerned enough that the pediatric academy now encourages infants be placed on their "back to sleep, tummy to play," says Marian Willinger, special assistant for SIDS at the National Institute for Child Health and Human Development.

Yet many parents don't know where to begin. In the new survey, therapists said 70 percent of parents had little or no understanding of how to provide tummy time. Often, parents delay introducing tummy time so long that babies begin to cry after only a few seconds, Gardner says.

"Most moms have never even thought about this," Gardner says. "They don't realize the importance."

START WITH JUST 10 SECONDS

Lesner says she learned about tummy time while working as a receptionist at Pathways. She turns the exercises into a game, kneeling beside a bed or the changing table so that she and Marty are eye-to-eye. She lets Marty look at himself in the mirror and reach for toys on his tummy. She even enlists her 3-year-old son, Frankie, as entertainment.

If Marty starts to fuss, Lesner says, she changes his position or tries again later.

Parents can start gradually, Girolami says. She suggests 10 seconds of supervised tummy time for newborns, building up to an hour a day — broken into several segments — by 3 months. Parents shouldn't leave infants alone on their stomachs.

Can you cuddle too much?

Madi Ditkowsky says she struggled to get her son, Ronan, to play on his stomach.

Because Ronan loved to cuddle, Ditkowsky says she carried him everywhere for three months, rarely placing him on the floor to play.

Eventually, Ronan lost interest in tummy time.

When he was 4 months old, he could barely lift his head and showed no interest in rolling. Ditkowsky enrolled him in physical therapy at Pathways, where therapists worked to strengthen his muscles. Ronan also wore a helmet for four months to round out his head.

Now 11 months, Ronan is out of his helmet and crawling on his tummy, says Ditkowsky, of Highland Park, Ill.

Experts say tummy time doesn't need to be structured, and parents don't need to buy special products. Placing a rolled-up blanket under an infant's chest works just as well as the pillows sold in baby stores, Girolami says.

"If babies are fussy, just get down and try to distract them," Gardner says. "Make some funny faces and some noise, and they forget what they were fussing about."