Men obsess about "washboard abs." Women worry about shapely breasts and thighs. And now that both sexes engage in the equal opportunity pastime of bun watching, everyone--it seems--wants a better bottom line.

Working your butt off has been a top trend in exercise videos and equipment sold through "infomercials" over the past decade, according to the American Council on Exercise (ACE), a nonprofit "workout watchdog" organization based in San Diego.

"Depending on the changing tides of fashion, one or another body part becomes the object of intense focus, creating instant demand for just the right regimen, product or exercise to reduce, tone or otherwise alter its shape and appearance," notes a recent issue of the council's magazine, FitnessMatters.

At a time of epidemic obesity--when Americans' broadening behinds are prompting designers to widen seating everywhere from cinemas to church pews--tush-trimming workouts are hot. "Today's clothes show off our bodies, and the gluteal [buttock] muscles are something people can work on to show off," says Richard Cotton, ACE's chief exercise physiologist.

And you don't need any special gadgets to tone and shape the butt muscles, according to a recent survey of more than 36,000 ACE-certified fitness professionals. When asked to name the best exercise for achieving a shapely rear end, most respondents singled out squats--with or without weights. Lunges came in a close second. In fact, none of the top five exercises named--walking up hills, jogging or running and stair climbing were the others--require equipment.

While these exercises can help strengthen and tone the muscles in the lower body--which include the gluteals, hamstrings, quadriceps and calves--"you will only see a change in shape if you are doing fat-burning aerobic exercise and watching your diet as well," Cotton notes. "Otherwise, you will just firm the muscles under the fat."

The "best bet" for busy people is to do an activity that builds strength and burns calories at the same time, says Cotton, who recommends walking uphill. "It's the best combination of aerobic activity to burn fat and anaerobic activity to shape muscles," he says.

Stair climbing is also a good workout for strengthening and fat burning. However, people who spend an excessive amount of time on stair-climbing machines may get "too muscular of a rear end," notes Cotton.

Workouts may help improve your rear view, "but not necessarily to the degree of a Victoria's Secret model," he cautions. "Something none of us can control, genetics, is the number one factor in body shape. So it's important to be at peace with your body and to recognize that these exercises also contribute to your health, strength and longevity."

Despite this reality, people who feel desperate about an inferior posterior are still all too willing to spend money on gadgets that promise to melt inches from their buns. These so-called "spot reducers" are all shams, say exercise experts, who note that it's impossible to target weight loss for any specific area of the body.

"If spot reducers worked, people who chew gum would all have small jaws," notes Peter Francis, director of the biomechanics laboratory at San Diego State University. "The appearance of any body part depends on a combination of factors" including diet, exercise and heredity.

The muscle that gives shape to the cheek of the buttocks is the gluteus maximus, which Francis calls "the biggest chunk of meat in the human body."

Surprisingly, he says, "it isn't a very well understood muscle," and there is little objective data as to which activities strengthen it best. In an effort to determine the most effective exercise for the "glutes," Francis recently completed a study of 31 women, ages 19 to 41, who performed 10 of the most commonly prescribed butt-bettering exercises while wired to a machine that measured muscle activity. The study, dubbed "the Butt Project," was conceived jointly by Francis and the editors of Women's Sports & Fitness magazine.

The most effective exercise, Francis found, was the one-footed squat (see below). "All your body weight is on one foot, so the muscles of just one side are lifting your entire body," notes Francis, who says this is also an excellent balance and strengthening exercise for older adults. (Seniors may want to hold lightly onto a secure object for balance while performing it.)

Men and women who would like to become callipygian--a word meaning "having shapely buttocks" can try these exercises to work their gluteus to the maximus:

One-foot squat. Stand on one foot and extend the opposite leg straight out in front, arms out to the side for balance. Bend your standing leg and lower as far as you can without losing your balance. Try two sets of eight, or as many as you can do.

Lunge. Stand with your feet together and take a long step forward with your right foot as you bend your left knee and come up on the ball of your left foot. Sit straight down, then push back to a standing position. Alternate legs and repeat 8 to 12 times. You may want to begin with a small step and gradually increase to a longer step as you become stronger. Make sure that the knee of the front leg doesn't travel forward past the toes.