Debby Fritts, left, gets tips from Charles Anderson before a rowing exercise class at Rowbot Fitness in Smyrna, Ga., recently. Anderson and his wife, Aubrey, opened the indoor rowing gym, adding to the trend in this type of cardio fitness.

ATLANTA — The treadmill and elliptical are great for cardio, but rowing?

“It's a great workout,” Charles Anderson said. “It literally works every major muscle group.”

Anderson and his wife, Aubrey, recently opened Rowbot Fitness, an indoor rowing club in Smyrna, Ga., that fitness experts say is part of growing trend in group exercise.

Although he uses rowers in most of his clients' workouts, Brooks Conway, a trainer at Quest Gym in Duluth, Ga., said that the gym sells more rowers (at $925 a pop) than it uses.

Still, Conway said, interest in indoor rowing “is definitely growing” and almost all of his clients, especially those interested in losing weight, use one of Quest Gym's two machines in their workouts.

At Rowbot Fitness, which opened last month, rowing is the only exercise going.

The hourlong classes, held most days of the week, take students through speed and distances drills on indoor rowers, called ergs.

“It's a pretty small industry with only about 50 in the country,” said Charles Anderson, a senior software engineer. “Five years ago there weren't nearly that many but it's picking up steam.” Rowing reclaimed Anderson's attention in 2008 after he put on nearly 60 pounds.

“I wasn't exercising anymore and spent a lot of time in front of the computer and TV, neither of which did much for my waistline,” he said. “I went from 190 to over 250 pounds over five years.” A former Georgia Tech rower, the 29-year-old Anderson said he knew the benefits of the sport and the vast amount of calories — more than 750 per hour — he could burn doing it. He lost the extra pounds in eight months.

Then in 2009, while attending the Atlanta Erg Sprints at his alma mater, Anderson said he met Lowell Caylor, owner of Greenville Indoor Rowing in South Carolina.

“That was the spark for starting my own business,” he said, and the Andersons launched Rowbot Fitness on July 23.

During a recent workout, half a dozen people went through an hour of sprints with Aubrey, setting the pace. “Put a lot of power behind those strokes,” she said as the machines whirred and “Invincible” by OK Go played in the background.

Charles Anderson said every time you take a stroke, an estimated 26 per minute for an hour, you use the leg, core and arm muscles, but the perceived rate of exertion is low because you're using all your muscles as opposed to a select few.