MEET MR. METROPOLITAN AAU's Mr. Metropolitan contest will be held at 7 Saturday evening, at the Bauer Recreation Center, Bauer and Norbeck Roads, Rockville. Admission is $3. For details call Pete Miller at 790-0143, after 6.

Seventeen-year-old Frank Wilkinson is constricting his left bicep brachium, gorging blood into hundreds of young capillaries. Like tiny pioneers, these blood channels have only recently settled themselves throughout the tender new fiber around the bone and tendon extending from his massive deltoid shoulder muscle.

In other words, Frank Wilkinson is flexing his arm.

And when, on cue, this pumped bicep explodes into a 16-inch pose, the crowded auditorium roars its gratitude and approval.

That was at a novice competition three weeks ago, a preliminary for this Saturday's "Mr. Metropolitan Bodybuilding Championship."

As the teenage event Frank was in began, the crowd was noisily speculating about who -- and, apparently equally important, which gym -- would win the trophies. But as the young men moved into their carefully thought-out routines, chatter turned into shouts: "Step out front, Jimmy, let the judges see your double bicep shot."

Many of the fans, it seems, are friends of contestants, and unofficial cheerleaders. "Don't stop... give us some more back... flex it, honey... that's it... now hold it, hold it."

As each youth goes through his one-minute individual posing routine, the uncommitted spectators start clapping, and when the top three contestants are brought back for a final "posedown," the crowd is on its feet. Applause rolls through the gym.

The next class, the open division, stirs up the entire audience. Twelve older, "fully grown" men walk onto the stage, and the crowd -- especially the women -- erupts into cheers, whoops and remarkably blunt comments. "Jesus," shrieks one redhead who looks like a top Junior League prospect, "catch the lats on No. 5."

Sandy, from Rosslyn, says this is her first bodybuilding show. "I'm here because I know one of the entries, Jimmy. The tall one with the blond hair." She points. But she's not totally unfamiliar with the game. When the announcer evokes the name of Mike Mentzer, Washington's biggest contribution to bodybuilding's elite, Sandy purrs, "Hummm, Mike Mentzer, nice legs."

When Sandy's candidate climbs onto the platform for his moment of individual posing, she yanks a Kodak out of her purse. The announcer reads that Jim is 28, has been entering contests for 2 1/2 years and likes to play tennis. I ask Sandy if Jim actually does hang around the racquet clubs. She shrugs her shoulders. "I haven't been out with him for a couple of years; I suppose he could have started playing tennis since then." But his washboard-looking stomach muscles and tell-tale immense shoulders hint that Jim spends little time relaxing on the courts.

The posing styles of the contestants seem to fall into three categories: The brutal -- those with a deliberately roughened quality in their routines and a Marine recruit's grimace on their faces (in fact, two of the contestants are Marines and a third "hopes to enlist"); the "somewhat shy," who have no chance whatsoever of winning; and the graceful, who use a flowing variety of sweeping arm gestures that seem to invite the audience to appreciate, perhaps adore, each muscle group.

What do the judges like? I'm told the answer to that question depends on whether a particular judge is also a weight-and powerlifting fan. If so, the macho/Marine look could score points.

But the so-called "natural" posers were definitely crowd favorites, and that night winners in both the teenage (Frank Wilkinson of Olympic Health Club) and the open divisions (Charles Bentley of Fun and Fitness Health Club) tended toward that category. This was a "novice" contest, for men who have yet to win a physique trophy.

Pete Miller, AAU's representative in the Potomac area, says that the caliber and number of men competing in these grassroots-level shows "has skyrocketed" compared to past years. But the AAU takes only part of the credit for this boom.

The phenomenal growth of the sport was signaled by the book and movie "Pumping Iron," and Miller says that bodybuilding superstar Arnold Schwarzenegger is largely responsible for inspiring the entrants and drawing the large audiences. Of course, Schwarzenegger will be at Saturday's contest only in spirit. This is, after all, the bush league.

Who treks out to a neighborhood recreation center for this kind of show? Well, unlike the local trampoline finals, senior citizen swim meets and judo classes where one supposedly learns "how to fall correctly," families aren't the big or even primary audience for bodybuilding. The crowds, mainly in their 20s and 30s, look like people who wouldn't have dreamed two years ago that they'd be paying money to see a "Mr. Metropolitan" crowned. Prosperous and healthy, they're about evenly split between women and men. Mainly, it's the women's attendance, evident appreciation and vocal support that distinguishes this from other masculine-dominated spectator games.

Local health-club managers report that interest in bodybuilding far exceeds the few youngsters who someday might be able to compete. Today, most of the 30-ish men and women who join spas demand more than the traditional goal of "keeping in shape" -- they want to improve their shapes. This group is another source for audiences for bodybuilding shows. And the health-and nutrition-minded of all ages seem to be natural allies with the bodybuilding cult.

But there were few obvious parents -- or other middle-aged people -- in attendance at the novice show. One contestant said his folks were home "hiding their heads." "My dad," the well-proportioned athlete explained, "still wonders if jogging can legitimately be called a sport... You can imagine what he thinks of bodybuilding."