Anna Mercer, left, leads the adults-only Raunchy Ride spin class at Thrive Studio.

Warning: Things are about to get raunchy.

As The Pussycat Dolls blast and strobe lights bounce off the black walls, Anna Mercer addresses the roomful of spandex-clad men and women straddling stationary bikes.

"I've missed you, you sexy little things," she greets them.

This is Raunchy Ride, an adults-only spin class at Thrive Studio on River Street in Coolidge Park. Thrive is a new fitness club on the North Shore, a cafe-turned-yoga and cycling studio owned by Mercer and her friend Kim Gavin. They also do personal training, massage therapy and nutrition consulting.

For seven years, Mercer has been Gavin's trainer. They'd meet, train, and then go out for coffee and bagels. Eventually, the routine became tiresome.

"After doing this over and over and over again, we were just fantasizing about 'wouldn't it be really great if ...'" Mercer said.

Through observing different studios in larger cities, they came together to make their fantasy a reality.

"There are a lot of goals, but the main reason we started Thrive is to have a place where people could find their fitness needs met and their nutritional needs met," said Gavin.

Group classes include Kick Your Buddha, an advanced yoga class; Thrive and Shine, an early morning spin class; and, yes, Raunchy Ride. It's their most popular class, which Mercer said is surprising. She wasn't sure people would appreciate it.

"We're taught to be on our best behavior as children," she said, "and that kind of sticks."

Or not.

As the raunchy riders warm up, Mercer asks them to describe their day in one word. "But it has to be dirty," she insists slyly.

Kelly, 45, says her day was "orgasmic."

Mercer gives the participants pet names like "sweet pants" and flirts shamelessly with the men and women in the room.

"Who wants Josh to take his shirt off?" Everyone claps. He doesn't acquiesce. Yet.

"Elizabeth," Mercer drawls as Justin Timberlake's "SexyBack" begins to play, "this song reminds me of you."

They lean right and left on the RealRyder stationary bikes, which can move from side to side to engage core muscles and give a stronger indoor cycling workout.

Mercer moves throughout the room checking everyone's form, literally.

"Your pants are nice and tight," she tells Jasen.

It's not just the temperature in the room that can make one flush a little pink. This is not a class for the delicate of mind or body.

"No (wimps) in here," Mercer insists.

She said she's experienced too many facilities that are too reserved, too polite, too careful to not offend.

"No one has to play nice here," she said.

She said the owners are considering opening a raunchy yoga class.

Everyone takes the weights off the handlebars of their bikes, pedaling as they punch forward -- left, right, left, right -- with the weights.

"Tell me how much you (flippin') love this," Mercer shouts.

Josh -- that's Josh Harris, a yoga instructor at Thrive -- finally pulls his shirt off and Mercer leads the room in an appreciative catcall.

"She abuses me as much as she wants," he says, laughing.

They speed up.

Mercer begins to wrap things up, leading the class in a series of stretches atop their bikes. Everyone is breathing heavy and glistening with sweat.

And then it's over.

"Ooh," Mercer sighs, "I need a cigarette."

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