Greg Blair had barely warmed up during yesterday's regional competition of the Crystal Light National Aerobic Championship when an 18-year-old spectator declared him a winner hands down.

"Oh, this is great," said Michelle Ruhn of North Dakota, as Blair's lean, spandex-clad frame dropped to the hardwood floor in a one-arm push-up. "He's hot. I rate him a 10 and a half. Oh, he's great."

As the music throbbed and the male competitors worked the stage of the crowded mall of Ballston Common, the judges took a more calculated view of Blair's three-minute routine.

The judges checked the competitors' jumping jacks, high leg kicks, push-ups and sit-ups for form and safety. They also studied the men's showmanship, flexibility and transitional moves.

In a sense, so did Kelly Mattei, 25, of Fort Myer. "They look like they're in really good shape."

Yesterday's competiton at Ballston Common was the ninth such event held in shopping malls across the country, according to the competition's creator, Howard Schwartz.

In part, it was an effort to show the public that aerobic exercise is not reserved for the weak-minded and flabby-bodied, but that it is a serious sport for men and women.

"We feel if we can get accepted on a world level, we're headed in the right direction of making it an Olympic event," Schwartz said.

Some have dubbed the Crystal Light competition the Olympics of aerobics. The first-place winners of the regional competitions are propelled into the national finals. The winners of the finals receive a one-year touring contract with Crystal Light, a computerized stationary bike and trips to international health spas, according to the competition's creator.

The action began yesterday with the individual male and female competitions; the mixed pair and team competition will get under way today.

For Charles Little, 24, of Alexandria, aerobics is his life.

Little, who wore white spandex bicycle shorts and a matching white top with blue stars, said he was trained in ballet, jazz and modern dance, and just decided to give aerobics a try.

Yesterday, as Little leaped, twisted and pranced to Aretha Franklin's "Can't Turn You Loose," it was obvious from the claps and hoots that he was a crowd pleaser. Little won first place in yesterday's male competition.

Some spectators, though, expressed different preferences.

"I'd rather watch the women," said Asa McCain, 44, of Oakland, Md., who watched the spectacle from a mall level above.

Perceptions about the exercise have changed dramatically over the years, said the competition's creator.

When the event began five years ago, Schwartz said, it was difficult finding men to compete.

This year, five of the six 1987 national aerobics champions are men.

Professional athletes "probably could not come close to the stamina, strength of any athletes here," Schwartz said.

Robert Kermodle, 33, of Greensboro, N.C., can attest that aerobics is serious business.

"You can never be certain until you do the action whether you've got something," Kermodle said, rehearsing his routine in a quiet corner in the mall.

"When you perform, there's an extra type of energy that infuses you. Sometimes the energy can override your mind. It's performance energy."