By Dorene Internicola

NEW YORK (Reuters) - Paddle boarding, an ancient form of surfing, is gaining popularity as a high-intensity, low-impact platform for group fitness classes from calisthenics to yoga.

Experts say it may feel like walking on water because paddle boarding works on the flat water of a lake or pond and doesn't involve waves.

"Paddle boarding is a form of surfing. A paddle board is basically a surfboard that's thicker, more buoyant, so it can hold you standing up in flat water," said Scott Bumbalough, founder of Maui B's Stand Up Paddle Boarding, which is based in Florida.

"For fitness classes we take a one-hour lap around the lake," he said, which in this case is Lake Ivanhoe in Orlando, Florida. "Buoys are placed at intervals and at each buoy we do an exercise such as squats, push-ups, squat thrusts on the board."

Bumbalough encountered paddle boarding, which probably dates back to ancient Polynesia, while living in Hawaii, where it was a popular way for surfers to train when the surf was down.

"It's a high-intensity, low-impact workout," said Bumbalough. "The class probably works about 85 percent of the muscles in body."

Bumbalough said he brought paddle boarding to Florida in 2007. Since then it's spread throughout the nation as a hugely popular sport that can be done anywhere there's safe water.

Peace, serenity, and six-pack abs are among the many benefits that have accrued to Nani Sadowski, a healthcare consultant, since she started exercising on the paddle board two years ago.

"I have a very stressful job, so it's very relaxing to be on the water," said Sadowski.

She also does paddleboard yoga, which she explained is a complete practice from sun salutations to the final resting pose done on the water.

"I prefer yoga on paddle board because you have to focus that much more on your balance," said Sadowski. "You look down and see the water below you. Any fear is more a question of mind over matter."

Shirley Archer, a Florida-based fitness expert and spokesperson for the American Council on Exercise, has tried paddle boarding and finds it is a very effective core workout that requires balance.

"You can't fake it in the water ... It provides you with immediate feedback," Archer, who is a certified yoga and Pilates instructor, explained. "Yoga on the paddle board simply expands on this concept because many yoga postures require good balance."

She said that paddle boarding, while ideal for those seeking a new challenge, would likely work out only for people who enjoy the outdoors and who don't mind getting wet.

Sadowski said she has only fallen off once, perhaps because there is one instructor for each five students in her class.

"They watch us like hawks the whole time," she added.

Her devotion to paddle boarding has transformed her body.

"People always comment on how toned my arms are, how great my posture has become," she explained. "When I was off work for a couple of weeks I went out every day for two hours," she said.

The result? I'm 38 years old and I actually got a six-pack."

(Editing by Patricia Reaney)