REMEMBER WHEN you first saw them -- whizzing down the street at bullet speeds, swerving artfully around potholes, dodging traffic like pinballs and spinning circles as tightly as Tonya Harding?

A fad, you scoffed as they zipped by. That'll never catch on.

Well, in-line skating is now among the fastest-growing sports in the United States. More than 12 million Americans strapped on blades last year, making skating more popular than skiing, mountain biking or soccer. In fact, industry leaders predict the sport will overtake tennis in popularity this year, with nearly 15 million people participating.

"This has gone beyond the extremists, the hot-doggers, the daredevils," explains Jack Murphy, president of Power Rollers Inc., one of the area's largest in-line skating schools. "This is a sport for everybody -- fit or fat."

On any sunny weekend in the Washington area, evidence of the sport's broad appeal is everywhere, from the elementary school kids playing roller hockey on the street to the middle-aged gliders taking in the scenery along Rock Creek Park.

Sure, there are still plenty of hotshots whirling 'round in Freedom Plaza or sliding stealth-like along the Mount Vernon trail. Stroking side-to-side, these adventurous athletes look like hyper-cool speed skaters: bodies tucked, arms pumping, a rush of Spandex and motion. But, as a recent visit to a skating clinic showed, the hipper-than-thou set no longer dominates the scene.

Of the 52 people who came to the Ski Chalet in Arlington for their first lesson several Saturdays ago, only a handful belonged to the superior species once associated with the sport. The rest were folks like us, fat folks, old folks, folks who looked as though they hadn't moved from the sofa since Oprah went on her last diet.

"I've taught 2-year-olds and I've taught 60-year-olds," said certified instructor Krista Heubusch, herself a perky twenty-something with a runner's build. "Now, it may take them a little longer to learn, but ultimately they get it just like everyone else."

Jack Murphy of Power Rollers, which runs the clinic every Saturday morning and Thursday evening, said many of these first-timers wouldn't have even considered skating a few years ago.

"Back then, there was no methodology behind the sport. No instruction, no clinics, no one showing you how to spin or how to avoid an injury," he said. Today, thanks to Murphy and other members of the International In-Line Skating Association (IISA), it's entirely different.

Those looking to get a feel for the sport can choose from any number of free skating clinics offered weekly by area ski and skate shops. For the more serious, there are private and group lessons for every skill level. Instructors in all of the schools stress safety and technique, and teach you how to keep your balance, how to control your speed and, most importantly, how to stop.

"It's just like skiing," explained Henry Zuver, executive director of the IISA. "You wouldn't rush down the mountain for the first time without taking a lesson, would you? The same rules apply here. You must learn the skills first."

But proper instruction is only partly responsible for the skating craze. New skating technology, including easier-to-control brakes and improved protective gear, have boosted the sport tremendously.

The original in-line skates, designed with a single row of wheels on each boot in place of the conventional side-by-side wheel alignment, were introduced to the mass market in 1984 by Rollerblade Inc. of Minnetonka, Minn.

The company initially envisioned the skates as training devices for such professional athletes as hockey players, skiers and speed skaters. There was little emphasis on the skates' braking system, since there was little focus on the recreational skater. To stop or slow down, a skater had three choices: drag one foot along the ground, spin around quickly to face the opposite direction or slowly lift the toe of one skate, forcing a rubber heel-stop against the pavement until you slowly ground to a halt.

None of these methods was very efficient for the novice skater who could barely stay balanced, let alone lift one skate into the air.

As in-line skating became more popular, however, manufacturers became more savvy. New braking technologies were introduced, including hand brakes that attach to the blades by cable and work just like drum brakes on a car. The more popular ABT brake by Rollerblade was introduced just a few months ago and is already selling out in local stores. This advanced brake, a modified version of the original heel-stop, allows skaters to apply pressure to the brake simply by moving one foot forward while keeping all wheels firmly on the pavement.

Other manufacturers are expected to introduce easier, safer braking systems in the coming months, according to Zuver. Although the mechanisms behind these systems have not yet been revealed, Zuver said all of them are aimed at making skating as easy and risk-free as possible.

Of course, safety wasn't always such a big consideration. In the early days, there was no such thing as in-line skating protective gear; helmets, wrist guards, knee and elbow pads just weren't part of the gig.

"Only sissies wore that stuff," said John Vanderwolf, president of Washington Area Rollerskaters, a club for in-line enthusiasts. "Today, it's considered smart."

Although protective gear can add as much as $75 to the cost of skates (which run about $200 a pair for the more durable models for adults), it is quickly becoming required equipment. The reason is simple: people, high speeds and concrete don't mix. By wearing helmets, wrist guards and pads, and by taking lessons before hitting the pavement, the majority of skating injuries can be avoided, said Zuver.

"We want people to skate smart and skate safe," he said. "Anyone who is trying the sport for the first time must take a lesson and everyone -- no matter what their skill level -- must wear the gear."

Zuver's point was backed up by the Consumer Product Safety Commission earlier this month when it predicted that 83,000 people would be injured while skating in 1994, more than double the number injured in 1993.

Commission Chairman Ann Brown said two deaths and 28,000 injuries were reported in the first five months of the year. There were three deaths and 37,000 injuries in 1993.

The commission said that if all skaters wore helmets and wrist guards, the number of injuries would drop dramatically.

A separate study released last week and conducted by the National Center for Injury Prevention and Control, also recommended that consumers wear protective gear. However, this study found that in-line skating actually results in fewer injuries than conventional skating and skateboarding.

"More research needs to be done," said Richard A. Schieber, author of the study, "but at this point, it appears that in-line skating is no more dangerous than other small-wheel recreational activities."

And it may have far more benefits.

According to two recent fitness studies, in-line skating is just as good at burning calories and increasing aerobic capacity as running or stair-stepping.

Researchers at the University of Massachusetts, who compared 20 recreational skaters who skated, ran and stair-stepped at a moderate pace, found that they burned 14.1 calories per minute while skating, 14.9 while running, and only 10.8 while stepping. The researchers said the slight difference between running and skating was insignificant, and that skating might actually be better exercise since it builds just as much aerobic capacity without all the huffing and puffing.

"That's because you overcome resistance to get started, and momentum keeps you going," said Steve Keteyian, who helped conduct the second study at the Henry Ford Hospital in Detroit. That study found that there is no real difference between skating upright and tucked, in terms of burning calories.

That was good news to those enrolled in Power Rollers' skating school, dozens of whom said they planned to skate to get in shape or to stay fit.

"I decided to take this up because I wasn't using my gym," said Alice Martin, 34, of Falls Church. "I think it's great exercise for your legs and rear, and it's a lot more fun than aerobics."

Leo Cabrales, a Fairfax County police officer and self-described weekend jock, said he sees the sport as just another way to stay in shape.

"This is like swimming for the first time," said Cabrales. "You find yourself using muscles you've never used before -- especially when you're trying so hard to keep from falling down."

Vanderwolf of Washington Area Rollerbladers said he is living proof of just how effective the sport can be. One of the first Washingtonians to take up skating back in 1986, he said he lost 20 pounds in the first six months he put on the blades.

"I didn't realize how many calories I was burning until I started losing the weight," he said. "I can't recommend a better exercise for people who aren't looking for the hard burn you get from running but still want something physical outdoors."

"Besides," he added, "is there a better way to see everything this city has to offer? I don't think so."

WHERE TO BLADE

Because of the Washington area's many bike paths, hiking trails and wide tree-lined streets, it is considered one of the best locations for skating in the nation, according to the International In-Line Skating Association. When looking for a place to try your blades, keep in mind the levels of pedestrian and automobile traffic and look for flat, smooth pavement with as much shade as possible.

Here's a list of skating sites recommended by area skate shops and instructors:

THE DISTRICT

ROCK CREEK PARK -- Beach Drive is closed at the District line on Saturdays and Sundays and has become a haven for skaters. The gentle rolling hills, lush greenery and wide pavement make the perfect skating sanctuary for all skill levels. If beginners find the pavement too slick, stick to the flatter section north of Military Road. (Note: Cars are allowed on this stretch so be extra careful.)

CAPITAL CRESCENT TRAIL -- This newly opened section of the C&O Canal trail is perfect for skaters of all skills. The 1.5-mile path, which runs from Georgetown to Fletcher's Boat House, is freshly paved and extremely smooth and clean. Although bikers and joggers are quickly discovering the trail, it is wide enough to comfortably accommodate skaters, as well.

THE MALL -- While it is arguably the most picturesque site for skating in the nation, the Mall is not recommended for novices. The rough, gravelly pavement and heavy pedestrian traffic make it too difficult for anyone who has not fully mastered the art of braking. For intermediates and advanced, however, it is a perfect place to cruise. (Note: Skating behind the Capitol is illegal and strictly enforced. If you have any questions about where skating is prohibited, call the U.S. Park Police.)

MARYLAND

THE INDUSTRIAL PARK on Research Boulevard in Rockville -- This cluster of professional buildings is virtually dead on the weekends, leaving acres of smooth asphalt-covered parking lots for skaters to enjoy. Here is a totally open area where you can practice your turns, play street hockey or just get some exercise. It's a great place for those who are just learning the moves.

MARTIN LUTHER KING JR. -- Recreational Park, Jackson Road off New Hampshire Avenue, Silver Spring. A good place for beginners, the park has lots of paths for skaters and cyclists. Skating around the pond offers a nice view and a good place to practice crossovers.

LAKE NEEDWOOD -- Muncaster Mill Road and Avery Lane, Rockville. If hills don't bother you, this is the place. Asphalt paths surround the lake, offering a peaceful setting for skating. There are few obstacles and not many cyclists, but the terrain can get mighty steep. This is not a place for novices.

NORTH LAKE -- Wisteria Road off Great Seneca Highway, Germantown. The 1.5-mile circuit around the man-made pond is flat, smooth and clean. Although the circle gets crowded after work, it's a good place for first-timers and anyone else who doesn't mind skating in circles.

SENECA CREEK STATE PARK -- Three miles north of the intersection of Quince Orchard Boulevard and Clopper Road, Gaithersburg. This lush and serene park offers a hilly, challenging course for intermediate and advanced skaters who have their braking skills under control. The park roads offer smooth pavement and a wide area for skaters, bikers and joggers to share.

BALTIMORE/ANNAPOLIS TRAIL -- Severna Park. A great place for beginners, this scenic trail runs along the Severn River on the Chesapeake Bay side. The wide path is very smooth and not very crowded. The hills slope gently and there is plenty of shade for a hot, sunny day.

VIRGINIA

WO&D TRAIL -- The Cuspis Trail is strictly for advanced skaters, but the other paths, including Four-Mile Run, are great for beginners and intermediates. Not sure if you can handle it? Walk the trails and get an idea of the inclines and the crowds. The pavement is smooth and joggers and bikers are generally polite.

MOUNT VERNON BIKE TRAIL -- This trail will challenge even the most advanced skaters with its steep hills and twisting turns. Skating south of the Belle Haven Marina is much more challenging than skating north. But either way, watch out for the cyclists -- the trail is narrow with little room for maneuvering at the last second.

THE PENTAGON -- Strictly after hours and on the weekends, the Pentagon parking lot serves as the nation's biggest skating rink. Although the pavement is very rough in spots, it is generally a great surface for practicing stopping, turns and tricks. This is the perfect place to practice strokes and speed control.

WHERE TO LEARN

AMERICAN IN-LINE SKATING INC. -- Sponsors hockey tournaments, leagues, clinics, camps and private lessons. For information on upcoming classes call Greg Keim at 703/765-7186.

CARAVAN SKATE SHOP -- 10766 Tucker St., Beltsville. 301/937-0066. Free clinics every Saturday 1 p.m. Rentals available. Private and group lessons through the Caravan In-Line Skating School.

FAIRFAX COUNTY PARK AUTHORITY -- Classes held at recreation centers throughout Northern Virginia. Package of five lessons costs $45. Call South Run Recreation Center at 703/866-0566; Springhill Recreation Center, 703/827-0989; George Washington Recreation Center, 703/780-8894; Lee District Park, 703/922-9841; and Providence Recreation Center, 703/698-1351.

INNER SKI -- Free one-hour lessons every Saturday at 10 a.m. at locations in McLean and Gaithersburg. Free rental with lesson. Call 703/556-9200 or 301/948-1494.

POWER ROLLERS SKATING SCHOOL -- Detailed lessons for beginning, intermediate and advanced skaters held at various locations throughout the Washington area. Youth roller-hockey league also available. Call 703/560-7586.

R.G.S. SPORTS -- 3036 Annandale Rd., Falls Church. Free one-hour clinic every Sunday at 11 a.m. Rentals available. Call 703/533-7200.

SKATERS PARADISE -- 1602 Belleview Blvd., Alexandria. Free clinic held monthly, call for date and time. Rentals available. Private lessons can be arranged. Call 703/660-6525.

SKI CHALET -- Free lessons every Thursday from 6 to 8 p.m. and every Saturday from 11 to 1 p.m. at the Arlington and Gaithersburg locations. Rentals available. Group and private lessons also available. Call 703/521-7777 for more information.

SPORTING EDGE -- 10036 Darnestown Rd., Rockville. Free clinics every Saturday at 10 a.m. and 11 a.m. Rentals available. Private lessons can be arranged. Call 301/279-5996.

WINDSURFING UNLIMITED -- 4913 Cordell Avenue, Bethesda. Free one-hour clinics every Wednesday and Thursday at 6:30 p.m. Rentals available. Call 301/951-0705.

SKATING CLUBS

BALTIMORE STREET SKATERS -- Offers clinics, races, skate tours, street hockey and skiers' cross-training. Call 410/666-9463.

CARAVAN ROLLERBLADE CLUB -- Meets at Caravan Skate Shop in Beltsville every Saturday, emphasis on learning the sport and safety. Call 301/937-0066 for meeting times.

WASHINGTON AREA ROLLERSKATERS -- Sponsors a roller-hockey league for adults and offers group skating tours in and around the city for all skill levels. Also sponsors a free clinic every Saturday at noon in the parking lot north of Military Road on Beach Drive in Rock Creek Park. Call 202/466-5005.

UPCOMING SKATING EVENTS

SATURDAY -- In-Line Skate Challenge '94, 9 a.m. at the Fairfax County Police driver training track, off Route 50 on Willard Road in Chantilly. The event features 10K and 5K races and a roller-hockey tournament in several age divisions; benefits Virginia Special Olympics. Race-day registration starts at $22. Call 703/631-7882 or 215/723-7356.

SUNDAY -- Courthouse Criterium, Judiciary Square, Fifth and E streets NW, 5K skating event. Race registration starts at 7 a.m. (fees $12 to $20), racing at 8 a.m. (Metro: Judiciary Square). Call 202/986-2488.