Step aerobics is so early-'90s. These days, the hottest class in gyms across the country is kick-boxing. The number of kick-boxers rose 65 percent to 3.8 million from 1998 to 1999, making it the fastest-growing sport, reports the National Sporting Goods Association. (For those keeping score, participation in step aerobics fell 2.6 percent to 8.2 million.) Wind surfing lost the most enthusiasts, with participation sinking 24 percent last year. And for the first time ever, fewer Americans in 1999 laced up their inline skates (24.1 million vs. 27 million in 1998). Maybe they're just cruising around on skateboards instead.