Muscles are in.

In an article on what women "really think about men's bodies" in this month's issue of Esquire, executive editor Priscilla Flood writes:

"Women love coming up against the hard reality of male muscles. Shoulder muscles, back muscles, biceps, pectorals, get special mention as both admirable in themselves and desirable to touch."

Says Arnold Schwarzenegger, who has them all -- in super sizes:

"It's funny how things have changed in the last 10 years." In the '60s and early '70s, an emaciated "hippy" look dominated. But now, "It's become the 'in' thing to look muscular -- to have wide shoulders and a small waist."

The boom is bicep-building -- along with the countrywide craze for physical fitness and weight loss -- is reflected, he says, in the number of new gymnanasiums (from about 4,000 in 1975 to 12,000 now) and the huge sales of body-beautiful books and barbells.

Once the stereotyped hobby of blond beach boys, body-building has taken on new respectability -- in large part through the promotion of seven time "Mr. Olympia" champion Schwarzenegger. Corporations such as Bank of America have installed office fitness centers, and barbell-training has picked up growing numbers of female fans who appreciate new firmness in their muscle tone.

At least one corporate officer, on his trips out of town, is known to tote 50 pounds of weights to keep in shape. (Schwarzenegger himself, just back from 6 months in Spain filming "Conan" -- an ancient-world "comic-book" character known as "The Barbarian" -- insisted on a $50,000 gym on the set. "I had it written in my contract.")

Despite its new populatity, a "large amount of misconception," he says, still surrounds bodybuilding:

It's not a "balanced" exercise program. Not true nowadays, writes Schwarzenegger in a new book (with Bill Dobbins), Arnold's Bodybuilding for Men: How to Develop and Maintain a Superbly Fit, Uscular Body for Life (Simon and Schuster, 240 pages, $14.95).

When done properly, training with weights provides what Schwarzenegger sees as the three components of total fitness: "aerobic conditioning, flexibility and muscular conditioning."

It takes hours of "torture" to accomplish anythiny. False again. You can benefit, he says, from as little as a few hours a week.

If you don't watch out, you're liable to become musclebound. The truth is, "Getting really big" -- his size -- "is tremendously difficult, if not impossible for most people." You must put in 8 to 12 years of "mind-boggling" work, "and that's only if you have the right genetic potential in the first place." Only a relative few bodybuilders go beyound fitness training to competition.

(If you're building brawn to attract women, beware. Warns the Esquire article: To some, the super-muscled look can be "grotesque.")

There's not strength in those bulging biceps. It's "real," he says.

You need a lot of expensive equipment or access to a gym. Actually, you can train in your home with weights and an exercise bench starting at about $150.

It's narcissism that keeps you flexing in front of the mirror. That mirror "is a valuable tool." It aids "concentration" and "technique" and "it provides feedback."

Austrain-born Schwarzenegger, 33, is amiable and articulate, talking earnestly about historic sites in Spain and the economic Opportunities in the United States that have allowed him to "build and build and build" on his investiments. (He has bachelor degrees in business administration and international economics, mostly by correspondence, from the University of Wisconsin in Superior.)

Because he is tall, he carries his bulk well. When he removes a sports jackets, however, to demonstrate exercises, his chest and arms stretch an "extra-large" Izod pullover to its limit. Knits are the only shirts he can buy from the shelf. The rest have to be specially made: "It's one of the disadvantages."

He retired" from competition 5 years ago after capturing six Mr. Olympia championships, open only to Mr. Universe winners. But last year, seeing new faces on the circuit, he decided to test himself again. It ws, he says, "a risk -- if you lose, they just remember you lost. But I love taking risks."

He trained hard for 8 weeks, and won again. Because of his physical conditioning, he says, he's in better shape than he was at 16 or 18. "I have much more energy. I need much less sleep. Nobody can keep up with me."

At the moment, his life is moving at a "hectic" pace, with movies, TV, book promotion tours, lectures and real-estate deals, so he spends only about half his time at his Santa Monica, Calif., home. That life on the road prompted him to develop an "improvisational" fitness program for businessmen and women who travel frequently.

Back in the early 70s, Schwarzenegger says he didn't understand when business executives complained "they didn't have enough time to exercise because they were always going from one hotel to another. I felt everbody had two to three hours a day.

"But as I became more involved in business and working 13 and 14 hours a day," his own training time suffered. Travel, he also found, takes "the edge off your mental faculties" when you need them the most."Now I can relate to businessmen." His approach incorporates aerobic workouts with bodybuilding (without the weights).

"Look for opportunities to run and swim," he advises, and "use stairs when you can." In New York City, for example, he'll run up and down "40 flights to stairs" in his hotel if it's too late to job. To bodybuild without the barbells, he does pushups from the height of two hotel-room chairs, one for each arm.

A competitive swimmer and soccer player before he became a bodybuilder, Schwarzenegger combines swimming and running with "high-intensity" repetitions of barbell exercises to maintain his heart and lung capacity. The flexibility, he says, comes from stretching exercises to begin the workout. "The more stretching you do, the less chance of injury" if you fall, for example, while skiing."

But muscle growth, the says comes most efficiently from "progressive resistance training," the use of weights.

As for good looks, "An inch or two extra around the chest coupled with a loss of a couple of inches around the middle will completely transform how you look. All you have to do is put your mind to it."

A "very limited number of us," he says, "are born with wide shoulders. The rest of us have to do something about it."