New Sport Rolling Up a Following : In-line skating: Aerobic exercise draws growing numbers of participants from all age groups, skill levels.

Kelly Lind, 14, remembers the dark ages in the evolution of in-line skating. Three years ago, he was the first kid on his block to own a pair of Rollerblades--neon-colored plastic boots with a single row of polyurethane wheels--and wound up power-sliding around his Reseda neighborhood by himself.

“None of my friends would even try them out,” Lind said. “Nobody was interested.”

Since those lonely days, however, in-line skating has become “the most popular new form of bipedal transportation since the skateboard,” Fortune magazine wrote in listing Rollerblades among its “hot products” of 1990.

“All my friends want to try it now,” Lind said. “When you start seeing Rollerblades in Reseda, you know they’re hot.”

According to Rollerblade, Inc.--the dominant company in the field--the number of in-line skaters in this country has increased from 20,000 in 1984 to an estimated 1 million last year. The surge in popularity was no doubt triggered by the company’s marketing strategy (see accompanying story), which expressly targeted Southern California.

On a recent Wednesday evening, a black Rollerblade van with bright fluorescent designs parked by the lighted tennis courts at Balboa Park in Encino to provide a free clinic. A company representative handed out Rollerblades, knee and elbow pads and wrist guards to about 25 skaters, ranging from young kids to graying adults, many preparing to take their first steps in in-line skates.

Eileen Papa, a 29-year-old transplanted New Yorker living in North Hollywood, finished putting on the gear and stood up cautiously. “I’m an athlete, so I should be able to do anything,” she said bravely.

Whoops. She lost her balance and grabbed a metal folding chair.

“I can jump out of airplanes, but I can’t do this?” she asked with disbelief.

Another skater stopped to offer encouragement. Rob Vilarino, a Van Nuys typographer, was given Rollerblades for Christmas and already looked good enough to visit Venice Beach. “I was a klutz the first day too,” he told Papa. “But after a couple of hours, I felt comfortable.”

Lind was at the clinic, but not to learn. A member of the Rollerblade Stunt Team, he gives lessons. Blowing a whistle, he called the class to order.

“Stick with me and I’ll teach you everything you need to know about skating,” he promised. Then he showed them how to stop and how to fall, important skills when you’re flying down a path at speeds up to 30 m.p.h.

“I’m definitely going to fall down,” Papa said as she wobbled down an asphalt path with the class. Whoops. She fell but not on her knee pads as Lind had instructed. She got up, wiping a swatch of dirt off the seat of her stretch pants.

Although in-line skating improves aerobic endurance, it can be a dangerous activity. According to a recent Associated Press story, some doctors are reporting an increase in injuries to in-line skaters.

“People are just not having enough training, or they’re just not apprehensive enough about the risks,” Dr. Jacob D. Rozbruch, chief of orthopedics at Beth Israel Hospital North in New York City, told the AP. “It’s an instability problem related to downhill skating. People don’t seem to know how to stop.”

To educate skaters, Rollerblade and other in-line skate-makers have joined with the National Safety Council to create the Skatesmart program to promote safety. Skaters are advised to wear helmets, wrist guards and knee and elbow pads and learn to control speeds and obey traffic regulations.

In-line skates start at just under $100 and go up to $330 for competition models with five wheels instead of three or four. (Wheels, which must be replaced every 1,500 miles, cost about $50 a set.) Compared to in-line skates, conventional roller skates, with side-by-side wheel configuration, are slower and offer less control. Most world speed records have been set on in-line skates.

While most of the people at the park were trying to learn in-line skating as an aerobic activity, a couple of men were training for roller hockey, a recreational sport usually played on asphalt playgrounds or parking lots. Because Los Angeles has so few ice rinks, roller hockey has boomed in the past couple of years.

“Every day, it seems that somebody’s forming a new league and coming to us for help,” said Phyllis Fleschler, co-owner of Valley Skate & Surf, which sells and rents in-line skates.

Papa, however, was not ready for roller hockey, or even skating in a straight line, but she had determination. “I have to be able to skate with all those girls down at the beach,” she said. “But I need to practice in private.”

(In-line skating lessons are held in the Los Angeles area weekly. Information: 714-827-7655. )