It's a Wednesday evening at the Pop Physique studio in downtown Baltimore, and a dozen women — most clad in leggings, T-shirts and socks — are rotating their hips while trying to hold an exercise ball between their thighs.  

"Great job, guys!" says instructor Smithy Onattu, directing her students via a headset as a playlist with songs such as Lana Del Rey's "Florida Kilos" and "Tumblr Girls" by rapper G-Eazy pumps through the art-filled space.

Over the course of an hour, the group will tackle a series of exercises: planks and push-ups, plies and other ballet moves. They'll stretch while using bright orange straps and do demanding lower body work on yoga mats.

"Squeeze, squeeze. And pulse and pulse," says Onattu, as the women study her and watch themselves in a full-length mirror. "Squeeze from your seat as you lift up."

Pop Physique offers an hourlong, ballet barre-based class that draws on dance, Pilates and other disciplines.

Already popular on the West Coast, it's one of the newer fitness facilities that have hit the Baltimore market in recent years. It's also representative of a new workout space trend growing in the city and taking hold across the country: boutique gyms.

Boutique gyms offer fewer members and a more intimate setting than so-called big-box health clubs. They're typically run independently rather than being corporate-owned and provide specialized targeted exercise programs.

Think barre, Pilates, yoga, boot camps, cross training and group cycling. Personalized instruction is emphasized.

"The boutique fitness facility industry has emerged as a trend in the past five years," says Stephen Tharrett, a co-founder and principal of Club Intel, a Dallas-based brand strategy company that offers data and insight into the $22.4 billion dollar fitness club industry.

The growth in boutique fitness can be attributed to several key cultural, socio-demographic and economic trends, according to Tharrett.

"One is the rise of the niche fitness consumer who is seeking a personalized fitness experience that aligns with their unique interests and needs," he says, noting that millennials who tend to value innovation and entrepreneurship are helping to drive the momentum. "Another is the changing mind set of consumers that has moved from quantity is better to more personal is better."

The so-called big-box fitness segment has seen increasing pressure to cut prices because of the growth of budget clubs and slower membership growth because of the presence of boutique fitness centers, according to Tharrett.

Membership revenues for the big-box market players (several calls to large chain gyms in Baltimore seeking comment for this article were not returned) have remained relatively flat the past few years, but revenues from other sources (i.e., personal training, small group training, spa services) have shown steady growth.

That growing popularity of specialized offerings has led to boutique studios such as Pop Physique, the brainchild of Jennifer Williams, a former professional ballerina and Pilates instructor, and her husband, Deric, a brand developer. The couple launched their enterprise in Los Angeles in 2008.  

Local devotees have been flocking to the Charles Street franchise since its opening in the summer of 2012.

There are about 2,000 students, according to a company spokeswoman. It's $20 for a single class, and new clients can get 30 days of unlimited classes for $100.

"It's something different," says Lauren Kohr, 25, of her decision to try a boutique fitness class.

A community relations staffer for a national charity, she has been coming to Pop Physique "off and on" for about six months. "It's challenging, but the atmosphere is not intimidating."

Amber Rose, Kohr's friend and co-worker, appreciates the relatively small class sizes. Most important, the 28-year-old volunteer manager believes her body looks and feels better. "And it really tones the booty," she says, laughing.

While Pop Physique has the look of a ballet studio, other boutique facilities evoke elements of a traditional gym.