The hair is frizzing and sweat is glistening, and arms are flouncing and body parts bouncing. UP, DOWN, IN, OUT. And in front of the cruel reality of mirrored walls, their savior for an hour is flexing his pectorals, those delicious muscles that surround his chest.

He's rattling a tambourine and looks like a bronze Jolly Green Giant encased in baby-blue spandex.

KABOOM! KABOOM! The beat is bopping and the floor is throbbing and his chest is heaving like a muscular seascape.

"I have more cleavage than most women," says Fairfax Hackley III later, with all the pride of an Arnold Schwarzenegger in his pumping prime. "I am very, very, very well built, and that is an attraction in itself . . . My classes are booked a month in advance.

"Some of my personal clients, they go nuts when I'm not around . . . And I am good. I am very good."

Shake hands with Mr. Hercules, 1981.

Mr. Apollo, 1980.

Mr. Delmarva, 1981.

Mr. Ego, 1985.

"I'd like to think of myself as more confident than egotistical," he says. "I mean, it's important that women understand what I am trying to do. After all, they are handing their bodies over to me."

He's 32 and his arms are 17 1/2. His license plate says BICEPS, and his thigh-sized forearm says "Hack" in tattooed script surrounded by blue roses.

Fairfax Hackley III, or Hack, as his devoted Washington clientele calls him, is what is known as a personal body trainer, someone who by definition has to love your body almost as much as he loves his own. Renting him may sound a bit narcissistic -- even for this hedonistic Me Generation -- but Washington is actually a little late getting into the sauna.

Personal trainers have been used for years in New York and Hollywood, cities where they number in the thousands. Morgan Fairchild, Linda Evans, Ben Vereen, Stephen Sondheim and Sylvester Stallone all have them, some of whom roll drive-up gyms to their clients' homes and charge upward of $200 a hour.

Here, among 20 or so others, we have Hack, the Maserati of musclemen -- an aerobics instructor, nutritional guru and flab counselor available by the hour ($25) or by the class ($9) at such tony spas as Suissa and the Watergate Health Club. This is a man who caught on early to the essence of Washington: To become a celebrity you must do something well. And promote yourself.

For Hack, a Harlem-born preppy who flirted with medicine and stumbled through half-finished degrees in biochemistry, endocrinology and premed, that something turned out to be his body. The rest came easy.

"Do you think I am well built?" he asks over his fettuccine at lunch one day.

His chest is exploding out of his fuchsia Polo shirt, his biceps overwhelming the sleeves. His waist looks girdle-trim. Why wouldn't women turn their bodies over to him for a tone here and tuck there?

Madonna did (when she was here for her last concert): "She is unbelievably strong. She is probably one of the fittest women I have ever encountered. I mean, hard . . . She was so appreciative for what I did for her that she gave me backstage passes and front row seats to her concert here."

Sharon Percy Rockefeller did (for his aerobics class): "If you had seen her before -- she was very, very heavy. She's made marvelous progress."

Gail Hecht, wife of the U.S. senator from Nevada, does: "She goes berserk when she doesn't have me to train her."

Anthony Quinn, too: "We trained when he was here for 'Zorba the Greek.' He's into the Lifecycle more than weights. Very, very private man . . ."

Private Fairfax Hackley is not. Nor is he just another pretty face. He's also well-degreed: The Collegiate School in Manhattan, A.B. Dartmouth, and a degree in nutrition from the Clayton School of Natural Healing in Birmingham.

Although no one could possibly be as good as he thinks he is, his classes are packed, and his clients are, shall we say, taut. He's particularly adept at the Big-Brother approach, sprinkled with some basic sex appeal. In other words, he's a master at telling women what they want to hear. ("I know you can do it. I believe in you." Or "You have very strong legs. Most women would love to have an hourglass body like yours. You just need to work on it a little.")

While some women will only work out with other women -- like those who favor Somebodies in Georgetown with its high premium on female camaraderie -- many prefer the boy-girl sort of attention a male trainer provides. Hack takes to that like Firm Grip to Miss America.

"Hack has got a good bedside, or call it gym-side manner," says Steven A. Michalik, a New York trainer and former Mr. America, Mr. U.S.A. and Mr. Universe, who recently trained actress Bev Francis for the film "Pumping Iron." "Women who turn to personal trainers do not want to be treated like a piece of meat, and he takes an intellectual approach to their bodies."

"He gave me confidence," says Argil Gablis, a real estate broker. "I'm in my mid-forties, and I'm worried about flabby arms and thighs. I'm very concerned about my image . . . He has changed my entire body. Of course, I have a ways to go before, as he says, I'll look 'super.' "

Says another client, Margaret Winters: "A trainer has to know what you are going through at that part of your life in order to motivate you. He has to be close without being nosy. Hack is good at that."

His clients include a financial analyst, a real estate broker, a journalist, a State Department professional, and various celebrities who track him down when they come to town to perform.

"There may be a stronger body builder around," says Hack, "and maybe someone else can do more in aerobics, but I am the best all around . . . I have the right body image, the ability to teach, the best music -- and well, if you put it all together, I don't think there's anyone else in the city who has that winning combination going for them."

Hack approaches body-building like a zealot, sounding at times like a cross between a therapist and a faith healer.

"I decided I wanted to lift 500 pounds. I wanted to do this so badly, I wanted to lift a quarter ton," he says. "I started with 225 pounds and each week I added five pounds, and my legs were so wobbly . . . I would never jump more than 10 pounds, because I didn't want to meet failure. The day that I was to try the 500, I sat down and fear came over me and I didn't do it, and I walked back into the locker room and I kicked over a trash bin. I asked myself what can make me lift this 500 pounds? What can make me do it?

"So I went back out and I started with 480 pounds and with each five pounds I added I imagined that my mother was out on a lake drowning. My mother is fearful of water . . . I thought I was swimming to save my mother, and every pound meant one stroke closer to saving my mother, knowing how fearful she is of death. And I loaded the bar to 505 and I saved her. Ever since then I have never been afraid of a weight."

"Now look at her legs," Hack is saying, like a manager at Safeway talking up his bananas. "You should have seen her body before -- mostly in the hips and legs."

Judie Steinwedel, 33, blond and pudgy-faced, seems totally unfazed by this candid appraisal of her anatomy and its history. It's called divorcing the body from emotion, and Steinwedel and Hack have mastered this in their four-month crash body-building relationship.

"I was a real tubette," she says to a visitor at the gym. "I've gone down four sizes. I wish I had brought my Size 4 pants to show you how I fit into them."

"You see," he explains, "she used to be even more busty, so I worked at strengthening the muscles that support her . . ."

"Well, I am very chesty, and I told Hack I was worried about the bouncing up and down in aerobics," she says. "And so, he put me on a combined aerobics and weights program and helped me build up my pecs."

Part of the whole psychology behind using a personal trainer is to give a client a feeling of being taken care of, of knowing a professional is focusing sympathetically on your anatomical shortcomings for an uninterrupted period of time. It's a lot like having a shrink, only cheaper.

Steinwedel, a political appointee at the State Department, met Hack when she joined the Watergate, and eventually they moved their routine here to the Dupont Athletic Club. "I had been disgusted with my body for a long time, and he just kept pushing and pushing," she says.

Day in and day out, as with all his clients, he meets her among the barbells, in a room where the carpets look like cement and the lights are unforgiving. For two hours each evening, he guides her through the weight machines, forcing her to force her muscles to capacity. They also meet every Sunday at 8:30 a.m.

Tonight, her legs, tightly wrapped in gray shiny tights, look like Ann-Margret's.

She says she has lost 12 pounds. Later he says as an aside, "That's bull. She was a lot heavier than that."

A huff here and wheeze there, she tries to lift 50 pounds on a machine used to strengthen those generous breasts.

"Come on, push, Judie, PUSH," he says.

"Uhhhhhh," she groans.

He holds her hips down and presses his pelvis against her legs. She does it.

There is a lot of touching in this business, hip-grabbing and shoulder-hugging. Hack says he tries to maintain his own standards for professional behavior, in a profession where it's obligatory to ogle body parts.

"I was telling somebody recently, I get so many come-ons, in so many different ways, all the time. Some of them are so overt . . . We do a movement with a pelvic tilt in aerobics. Women dare me to put my eyes in their pelvic areas . . . But I gotta tell ya, I don't bring my honey where I make my money."

He did say, however, that he dates some of his clients.

Like many inner-city youths, Fairfax Hackley discovered in high school that athletics was his ticket out of Harlem. He says he's sure his upbringing has something to do with his drive. He was raised in a project on 125th Street.

"I wanted to get out badly," he says. "I wanted to learn."

His father managed a sporting goods store and his mother was a housewife, and today his parents proudly display all the trophies of their only child in a room at their apartment on Long Island.

In eighth grade, he won a scholarship to Collegiate, one of the country's older, more prestigious prep schools, which numbers among its alumni John F. Kennedy Jr. While there, Hack was the eastern judo champion for three years and All-Prep in football and he played basketball and ran track.

He worked his way through Dartmouth as a janitor, studied sociology and played football, and he was married briefly in his senior year.

Eleven years ago, Hack started lifting weights at Dartmouth.

"I was too skinny," he says. "I just wanted to be bigger."

After his undergraduate work, he stayed on at Dartmouth for an MA in biochemistry, which he never completed, transferred to Georgetown for an MA in endocrinology, also never completed, and went through a year of premed there.

Then he switched full time to the care and feeding of his body.

"In medicine, I could only meet people who are sick," he says. "The potential is so much wider in this business. I can get into books and into videos or maybe even acting. I think I would be very good in the theater. It's just a broader spectrum of opportunity. Know what I mean?"