“Alli Elliot shimmies from side to side as she follows her virtual aerobics instructor. The woman on the screen calling out exercise commands is tough, but perky. "I like that the instructor is always smiling," said Alli, a 13-year-old cheerleader at Arlington Middle School. "When you look at her, it's like she's smiling at you. It makes you want to do it better." The program, on a life-size screen in an aerobics studio, is like a personal training session without the pricey fees. Most importantly, Alli, with her busy cheerleading schedule, can use the studio at anytime. Aerobics Anytime, at 5959 Airline in Arlington, is one of the newest additions to the 24-hour gym scene. The studio, developed by Eads entrepreneur Don Furr, merges exercise with computer science. Clients use an electronic key system to scan into the space whenever they wish. A variety of weights and steps are available to increase the intensity of their workout sessions. When clients are ready to begin, they step onto a marked space on the floor that triggers an "air-powered switch" and a 20-minute workout video appears on the screen.