“Brad Nelson must hate tires. He raises a 10-pound sledgehammer high over his head, then bashes it into a 2-foot-thick tractor tire. He whacks it over and over, as grim as if he were bashing Osama Bin Laden himself. ‘You get more workout in less time’ said Nelson, between whacks. ‘And it's something fresh and fun.’ Nelson's business, Kinetic Edge in Woodbury, is one of a growing number of boutique fitness centers with imaginative — or even bizarre — approaches to fitness. New, creative ways to sweat are popping up faster than resolutions on New Year's Day — including workouts based on kickboxing, rap music, ballet and indoor triathlons. ‘We are always looking for new ways to spice up the exercise routine,’ said Kara Thompson, spokeswoman for the International Health, Racquet & Sportsclub Association. One hot trend, she said, is Budokon — a blend of yoga and martial arts. BalleCore, another offshoot, blends ballet and yoga. Zumba — aerobic dancing to Latin music — is mainstream enough that there was, for example, a Zumba fundraiser in Anoka last month to combat violence against women.