ST. LOUIS, Mo. — Beyond the quiet stacks at the Schlafly Branch of the St. Louis Public Library, past the tables of computers and across the way from the checkout is a large meeting room. The windowless space is carpeted and has beige walls; slate blue chairs ring the perimeter.

On a recent Tuesday, patrons stride in after work, stop at the registration table and sign in for the evening’s class.

After a few minutes of chitchat, the class gets a cue that it’s time to start.

“Everybody get up!” commands singer Robin Thicke, as “Blurred Lines” trumpets through a CD player that has a microphone nosed up to its speaker. “Woo!”

By the time he gets to the “hey, hey, hey!” refrain, the 30 or so women are on their feet, shimmying to the beat in parallel lines across the room. “Step for Fitness” is underway.

At the library.

The class is one of several fitness-themed programs held in branches of the St. Louis Public and St. Louis County library systems. Aerobics, dance, yoga and tai chi are as much a part of the systems’ missions as the memoirs, novels and nonfiction tomes that pack their hushed shelves.

“Built into our strategic planning is to provide community gathering spaces,” says Jennifer McBride, the communications manager for the St. Louis County Library. “Fitness programs fall under that umbrella.”

With several of the county’s branches undergoing renovations, McBride expects there soon will be an expanded menu of free fitness classes, including more programs targeting seniors and the return of Zumba to a branch this fall.

Some workout classes found their way to the library when they had nowhere else to go. The Golden Warriors Fitness Troupe, a gentle exercise program for older adults, outgrew the Pine Lawn Community Center about five years ago. The Natural Bridge Branch now hosts a loyal group of warriors each week, McBride says.

Step for Fitness was in the same predicament several years ago when it overflowed from its space at a nearby church.

The Schlafly branch, in the city’s Central West End, welcomed the dance class as part of its mission “to improve people’s lives and provide resources to our families,” says John Koniak, communications coordinator of the St. Louis Public Library. “Being healthy is a part of that.”

Step is one of the library’s most popular offerings, drawing about 400 people each month to its back-to-back weekly sessions, says Leandrea Lucas, Schlafly’s regional branch manager.

“It’s a combination of both avid library users and some who come just for the class,” she says.

“It’s something to see when everyone is moving in sync with one another. It’s a beautiful thing.”

On this night, Dani Narayan leads the first dance. The teachers are seasoned class members who rotate duties throughout the evening.

Narayan, 66, has been dancing about 10 years. “I always liked to dance, but I never liked partner dancing, she says. “My husband won’t dance.”

In line dancing, the steps are variations of left-right, forward-back combos, with kicks, shoulder shakes and hip swivels sprinkled through.

Narayan has covered her tennis shoes in layers of rolled-down knee-high stockings, a trick to keep the soles from catching the carpet. A couple of other women pull shower caps over theirs. There are also heels, sandals, wedges and sneakers.

All those feet more or less find their rhythm as the second song comes on, an “Everybody Dance Now-Rock This Party” mashup. The strutting starts, with two side steps followed by a pivot. Wrist circles add a touch of attitude.

“Anybody need to learn it?” Narayan asks, taking a quick survey of the room. “Looks like everybody’s got it.”

By now, even with the air conditioning blowing at full tilt, the women have started to sweat.

Angela Howard takes a quick break. Howard, 56, also leads the class sometimes.

“It’s the only exercise I love to do. I hate gyms,” she says.

But here at the library, people let loose, whooping and egging on their friends. “It’s like a drug,” Howard says. “You have a bad day; you get here, and you’re in a good mood.”

She points out a woman in a wheelchair who is swinging her arms and snapping her fingers to the music. “Even if you’re in a chair, you can do something,” says Howard. “It’s all about moving.”

As the session approaches its midpoint, Betty Johnson, 67, takes over. Clad in an oversized navy T-shirt that says “Own it.” in light blue letters, Johnson introduces a new dance, the Nola Bounce.

After she demonstrates the sequence — “half-diamond, up, out, 1-2-3, half-turn” — the women watch and tentatively mimic the moves.

“Just fake it till you make it,” Johnson encourages as her silver hoop earrings swing in time with her steps. “Put a little ‘mmmpp’ into it! Put your flavor into it!”

Everyone is game. Four lines form. They Nola Bounce through Pitbull’s “We Are One” with just a couple of missteps here, a dropped turn there.

“We’ll try it again next week,” says Johnson. “It just takes practice.”