The whistle blows: TWEEET! “On the line!” barks Aaron Biggs, a fitness instructor at Twin Lakes Recreation Center, 1700 W. Bloomfield Rd. Twenty exercisers of various ages and fitness levels shuffle into place along the edge of the indoor soccer arena. They stand at attention, silently awaiting instruction on their one-hour workout. “Right off the bat, I let people know there’s no horseplay, no side talking,” explains Biggs, 24. “If you’re not giving one hundred percent, I’ll get in your face. I’m not afraid to raise my voice.”

Biggs’ class is one of several fitness boot camps that have cropped up in Bloomington. These classes utilize a circuit-training format that moves participants from one station to the next, keeping their heart rates elevated throughout the session. Classes incorporate both body-weight exercises and equipment, from standard pushups and squats to the Jacob’s Ladder machine seen on TV’s The Biggest Loser. It’s common to see a mix of exercisers from teens to retirees and first-time exercisers to triathletes. Outside of Biggs’ class, though, you’re unlikely to encounter a drill sergeant.

“We’re not going to yell at you,” says Ryan Ketchum, 29, fitness director at Force Fitness and Performance, 3205 W. Fullerton Pike. “We’re here to motivate, challenge, and coach you, and provide you with the safest, most effective program we possibly can.”

Ketchum explains that the circuit-training model offers several benefits: Participants simultaneously improve both strength and cardiovascular fitness; they can complete a workout in as little as 40 minutes; and the group format allows for a social atmosphere and a lower cost compared to personal training.

For Adam Schaeuble, 31, founder of Next Generation Personal Training & Fitness at 1709 N. College, what really sets boot camp apart is its results. In the past two years, his 500 clients have lost a collective 8,100 pounds. “That’s a live number,” he explains, “meaning if someone gains weight I drop the total.” Both Schaeuble’s and Ketchum’s boot camp programs incorporate dietary guidelines; Schaeuble also provides daily check-ins on each participant’s food and exercise goals. “We will hold you accountable,” he says.

Much of boot camps’ success, Schaeuble says, is due to participants’ ability to inspire one another. “I have an 80-year-old woman who comes out in her Snoopy socks and knocks out full pushups. So when that college guy sees her, maybe he’s motivated to do full pushups that day.”