Jonathan Messick led about a dozen sweating, straining students on Saturday in his Boot Camp Fitness Class at Thoroughbred Park in downtown Lexington. Messick, a personal trainer for Bodyfitworldwide.com, said he works with anyone who shows up at 9 a.m. Saturdays for an hour of intense exercise. The regimen this Saturday included sprints the length of the park, sit-ups, air-boxing, side-squats and no cheating whatsoever. "Abs tight!" Messick barked during the sit-ups. "Focus on the core!" The class is $10 but half of the proceeds is donated to charity, Messick said.