The real barrier to postnatal exercise, any new mother will tell you, is not weak abdominals or an aching back. It's a tiny human being with a compelling smile and even more compelling cry who makes it difficult -- if not impossible -- for her to grab 55 minutes for a workout.

This is the main flaw in the American College of Obstetricians and Gynecologists' new postnatal exercise videotape. With the exception of "a few minutes for mother and baby" at the end of the workout, during which the five new moms cuddle their kids while sappy lyrics proclaim, "There used to be the one of us, now there's the two of us," the routine ignores the fact that the exerciser has a new baby to mind.

A more practical program would either be shorter -- say, two different 20-minute workouts -- or would integrate the infants into the longer program. It is possible to perch a wee one on your abdomen while doing sit-ups and lie a baby on the mat beside you while you do push-ups. In selected exercises -- such as raising your arms overhead -- a 4-month-old makes a good 15-pound weight. This turns the workout into a fun game for mom and baby together -- particularly helpful for working mothers who have limited time with their children.

But these doctors are a cautious bunch and present a conservative workout at a relatively easy level. This is done intentionally, cautions Dr. Art Ulene -- the "Today" show's videodoc who introduces the workout -- because the changes caused by pregnancy take time to reverse themselves.

"These changes affect the way the body reacts to exercise," notes the pamphlet accompanying the tape. "Your cardiovascular system must work harder. Your connective tissues are more lax and your joints less stable. Your center of gravity is altered, and it is more difficult to maintain your balance. Your weight increases, your shape changes, and your emotions may vary more easily."

A depressing picture, you might say. The program hammers home the new mother's frailty even further by pointing out six times in 18 pages to consult your physician if you have any questions or problems. Certainly that is an important caveat, but the overemphasis on potential problems smacks of malpracticephobia.

The workout is run by Kimberly Dashiell, the no-longer-pregnant teacher in the companion pregnancy workout. It runs through 12 segments beginning with a stretch called "tune-up time." Then comes a warm-up, an aerobics section, calisthenics for the waist, arms, legs and abdominal muscles, and finally a cool-down. It's a pleasant program, with some particularly good exercises for the abdomen and thighs -- important to firm up after nine months as a human pear.

It's too bad that some of the useful information in the pamphlet is not included in the videotape. The pamphlet says, for example, that "it is very important that you learn to lift your baby correctly, bending your knees and using your legs to do the lifting. Never bend from the waist with your legs straight." Since schelpping a squirming infant may be the most frequent exercise most new moms get, showing the right and wrong way to do it on the videotape would likely save many aching backs.