Robyn Platoni spins her way across the varnished wooden floor, her roller skates positioned for a perfect landing on jump after jump. The youngster, wearing a sparkling red costume and a radiant smile, wows the audience with her precision and strength. As Robyn finishes, many spectators rise to their feet and let out a roar. Public interest in roller-skating has generally waned since the disco '70s. But among the 2,300 amateur competitors like Robyn at the 59th United States Roller Skating Championships, roller-skating is definitely a happening sport. And some skaters, such as Robert and Barbara Angelotti, say that the recent surge in the popularity of in-line skating has heightened awareness of the entire sport - the traditional quad skate included. Competitors at the two-week-long nationals, which ended Aug. 18 in Syracuse, N.Y., leapt, whirled, and raced from as early as 5 a.m. to as late as midnight each day. About 350 skating clubs from 45 states were represented - including clubs from Alaska and Hawaii. While recreational skaters out for a roll down the sidewalk may think traditional quad skates are passe compared with in-lines, the competitors here were serious about both. And in the artistic events - such as the Elementary Girls Singles Final in which Robyn competed and won first place - quads were the only ones on the floor. True, quads may be a bit clunky and unwieldy at times, even though the skates' wheels and bearings are now high-tech. In-line skates, in fact, aren't even an option yet for artistic skating at nationals, although suitable models are in the works. All things considered, quads have their own merit in artistic events: Skaters exude solidity and intensity from their firm footing. The tried-and-true skate has broad appeal, as evidenced by the 65 artistic events at nationals for age groups ranging from 8 and younger to those 45 and older. There is ''a lot more competent, a lot more refined technique'' that has developed over the years, says Brian Moynahan, a coach from Los Angeles. A ''complete package'' approach to artistic skating has evolved, he says - meaning that performance techniques, costumes, and, in the case of freestyle events, choreography figure heavily into skaters' presentations. The Freshman Dance Elimination was a case in point. The 17 couples, all young teenagers, glided beautifully through the compulsory dances. But to ensure they caught the judges' attention, the skillful skaters donned elegant costumes that were accented with eye-catchers such as velvet, sequins, and feathers. And many couples, before beginning the compulsory moves, skated an opening flourish, which could consist of no more than seven steps. Judges certainly face a difficult task in differentiating these high levels of technique and showmanship. But Judy Graves, a member of the artistic judges committee, says that although judging can be grueling, it is ''purely a labor of love.'' Ms. Graves helped to evaluate the artistic performances, which were divided into the categories of singles and pairs free skating, dance skating, and figure skating. A panel of five evaluated each event, with each judge scoring the competitors on a scale of 100. Graves says one of the key points of evaluation is a skater's edges - the curves traced on the floor by a skate. Smooth, deep edges (curves) are the mark of a good artistic skater. Interestingly, some skaters cite more intangible qualities such as self-motivation and dedication as necessary for successful skating. And many agreed on the importance of coaches. ''They're lifesavers,'' says Heather Hadges, who skated the Freshman Dance Elimination with Tony Potempa. Jeannie Saya, a coach from Santa Rosa, Calif., says, ''The process of learning is more important than the result.'' Also valuable, she points out, are the opportunities that competitions create for children to talk with adults - opportunities that many youngsters don't regularly have with nonrelatives. ''The adults and the children have a lot of conversations about motivation and attitudes,'' she says. Saya, who skated competitively for 17 years and has also been a judge, met her husband through the sport. Part of her 21 years spent coaching has been devoted to working with her husband, son, and daughter; both her husband, Eric, and son, Nicholas, made the nationals this year. Such involvement may seem intense, but it's actually rather common in the roller-skating world. And even if all family members aren't skaters, they're sure to be found in the audience. SCORES of families and friends made the trek to Syracuse, loudly cheering and clapping for every successful (and occasionally unsuccessful) step. Even such hearty supporters, however, filled only a fraction of the large auditorium where the championships were held, emphasizing the low public profile roller-skating has. But the United States Amateur Confederation of Roller Skating (USAC/RS), which organizes the championships and other meets each year, remains committed to furthering the sport. USAC/RS is hoping to have in-line speed-skating and hockey events included in the 2000 Olympics; roller hockey was already an exhibition sport at the 1992 Games in Barcelona, Spain. Yet nationals alone provided enough satisfaction for most skaters - for first-time national competitors such as Tony and Heather, the Freshman Dance team; and for skaters like Cassie Davis, a third-place winner in the Elementary Girls Singles Final. (At age 12, she was skating in her sixth nationals competition.) And the key to making it that far? Practice, practice, and more practice. The Angelottis, who won first place in the Classic Dance Final, follow a fairly typical routine: They skate four to five days a week, which on weekends means a 1-1/2-hour drive to the rink where their coach works. But even so, says Mr. Angelotti, they can't skate ''as often as we like to.''