Housewife Marjo Selin pumps her iron at the gym; lets off steam there, too. She's a world-class professional bodybuilder from Finland who trains with her husband at the Spartan Total Fitness Center in Merrifield, Virginia. In a full-body leotard, with huge blue eyes and ash-blond hair, she looks like a Nordic goddess, with smooth large shoulders sloping into deep, perfectly tanned, satiny cleavage.

Selin, 22, belies the she-Schwarzenegger stereotype that people imagine when they start thinking about barbelles. Right away, without a second thought, most men wrinkle up their noses and sneer, "Ugh. Why would any woman want muscles?"

"Flex appeal," says Rachel McLish, bodybuilding's reigning queen, Ms. Olympia, a strong-woman superstar who, like Lisa Lyon, is winning international acceptance for cheese-steak. Lyon posed nude for "Playboy" and also for photographer Robert Mapplethorpe's coffee-table and beefcake book "Lady." A sexier set of latisimus dorsai you never saw. And closer to home, Vivian Bacon, heroine of a muscular Cinderella story, adds "None of us are in bodybuilding to look more masculine."

Heads nod all round at a verbal workout at Spartan's gym with Bacon, runnerup in the Ms. D.C. contest; Selin, one of a few pros in the area; and Barbara Nolan, a blond beauty who's still recuperating from her first meet in June. She laments a losing bout with a bag of Cheetos. All three women got into bodybuilding because the men in their life train with weights and because they hated something about themselves: "I hated my waist," says Selin. "I hated my chest," says Nolan. "I hated my legs," says Bacon.

"You've got great legs," the other two chorus. We're not talking gams a la Betty Grable. Although Bacon's legs don't exactly bulge, they certainly do ripple. And they were judged best in her class at the Ms. D.C. competition. Her trainer made her bundle them in warm-ups near contest time to psych the other contestants. The rest of her ain't bad, either. She's sinewy, thin, taut. But the 33-year-old stewardess never dreamed she'd be showing off in a bikini in front of a thousand people when she first started weight- training 18 months ago.

It sounds like a colossal case of narcissism, but it really isn't, she says. "It's not that we worship our bodies. It's that we're proud of them."

Or were proud of them, past tense. "That was me," they say. They're like day lilies, these body bountifuls, blooming for just one day, for one contest. They have seasons, a series of cycles they're called, meant to leave their bodies prime, without an ounce of body fat. Overwise, women's muscles don't show well, nor do the veins that rope around them like grapevines at contest time.

Nolan, founder of several local fitness programs, says you can only peak three or four times a year; weight-training is cyclical. Most amateur builders spend the winter months bulking up, lifting heavier weights fewer times for muscle mass, explains Bacon. The summers, when most contests occur, are spent cutting up -- that is, lifting lighter weights in more repetitions for muscle definition. All the stress forces periods of recovery, says Nolan, followed by maintenance programs.

Nolan trains with her boyfriend, the current Mr. Metro. He's the one who reminds her to slather on the body lotion after a shower. It's part of the skin game. Their portraits are side by side on the bulletin board at Spartan's with other Herculean pin-ups. Bacon runs up with a stack of color Kodaks to supplement the one she's tacked on the board already. "This was me. This was me." She's excited all over again. We admire her poses -- front and rear double bicep, lat spread -- and the flower she had pinned in her upswept hair.

She trains three times a week for 21/2 hours at a time, including her time jumping rope for aerobics -- a must to metabolize fat. Selin trains harder, lifting weights four times a week for a solid hour and a half, just to maintain her shape. But before a contest, many women work out six days a week, enduring hours of intense training and posing work. Unlike a powerlifter, whose aim is to heft ever-heavier loads, a bodybuilder works with light weights (dumbells, barbells, Universal and Nautilus) to sculpt her body, enlarging some muscle groups, slimming others. The exercises include everything from the standard sit-up to more complicated procedures like the barbell reverse-wrist curl.

But, as Selin points out, "Diet is fully 70 percent of competitive bodybuilding. Never eat anything you can't use." Though Selin has never had any problems, Nolan took off so much weight, she stopped menstruating -- sometimes a consequence of the rugged pre-contest diet. Men's hormonal systems aren't quite so fragile, though they do get grumpy right before a contest from the lack of carbohydrates.

For women, thin is in, along with a more feminine physique. Bulky professional Laura Combes, for instance, was recently criticized for being overdeveloped, say these women. And currently, contest judges are ruling out the more masculine body types to attract more women to the sport.

For that matter, you might as well be on the runway at Atlantic City. You've got to keep smiling till your lips ache. And don't forget the lip gloss, either. Contestants are judged on make-up, skin and hair condition, as well as body symmetry and muscle size. Unless you're dark- skinned, you'll need not only the tone but also the tan to show the definition. Selin expresses some mild concern about skin cancer before wandering off to work her biceps. Perhaps a few flyes to prime her pump.

The gym looks intimidating, with its chrome-and-leather torture devices and gargantuan guys in heavy leather belts, grunting and banging the weights. One man helps a competitor get the barbell on her back for her leg work: squats, 10 to 15 reps in four sets at a weight of, say, 125 pounds. Everybody has a different schedule, usually working different sets of muscles on alternate days, with curls, presses, pulldowns and whatnot.

The Weider Book of Bodybuilding for Women, by Betty and Joe Weider, is a good resource if you'd like to set up your own program, from beginner to advanced. And the trainers at most Nautilus clubs and local gyms will help, too. Even working out for three hours a week can really make a difference in your body, say trainers.

But turning cheesecake into beefcake isn't easy. It takes pain to gain: That's the sport's predominant maxim. Adds Selin, "It's the most demanding sport there is. The most demanding sort of lifestyle." And her peers nod in agreement. Still, it's a growing business, with local body shops noticing a rise in female memberships in the past two years.

Heavy gyms, as their devotees call them, usually offer Nautilus, Universal and free weights for all-round training. And most competitive bodybuilders use both types of weights. "Hardcore gyms can be a little intimidating for most women for the first time," says Craig Mattingly of Spartan's. "But every day, I look out there," says the deliriously happy instructor "and I can't believe what I'm seeing -- dancers, models, gymnasts." They're all ages, too: the grandmother of bodybuilders, Doris Barrilleaux, is 53.

''I love the gym," says Nolan.

"You meet the most wonderful people through body building," says Selin, "You feel tired, let down, like you don't have any muscles at all. And somebody says how great you look." BEAUTIFUL-BODY BUILDERS DYNAMO GYM -- College Park. 474-6380. Open 7 a.m. to 11 p.m., seven days a week. Rates are $275 for a year, $180 for six months and $120 for three months. Nautilus and free weights, initial instruction, showers, lockers and a monthly newsletter. GOLD'S GYM -- Wheaton. 949-4653. Open 7 a.m. to 11 p.m. seven days a week. Rates are $295 for 12 months, $195 for six months, and $125 for three months. Includes weight room, locker, showers and training. OLYMPIA GYM -- Tysons Corner. 356-5058. Open 8 a.m. to 10 p.m. on Monday, Wednesday, Friday; 10 to 10. on Tuesday & Thursday; 9 to 6, Saturday; noon to 6 , Sunday. A year's membership goes for $279, which includes weight room, personalized instruction (as much as you need), showers and lockers. SPARTAN GYM -- Merrifield, Virginia, and Temple Hill, Maryland. 560-6955 in Virginia; 423-1992 in Maryland. Open 7 a.m. to 10 p.m., Monday to Friday; 7 to 4, Saturday; 9 to 1, Sunday. Rates are $275 for 12 months, $180 for six months and $115 for three months. Combination weight room, instruction, showers and lockers. Membership entitles you to use either gym. WORLD-CLASS GYM -- In Clinton. 856-1441. Open 10 to 9, Monday to Friday, 10 to 4, Saturday, closed Sunday. Rates for all facilities are $250 for 12 months, $150 for six months and $100 for three months. MR. & MS. NATION'S CAPITAL -- The contest will be held August 27 under the auspices of the AAU (Amateur Athletic Union). Phone Pete Miller for more information on this and other AAU competitions at 703/821-1834.