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Summer, typically slow in the business world, proved a season of entrepreneurial inspiration for Nadine Gelberg.

As she strolled the beach in Ventnor, the 43-year-old Chestnut Hill resident noticed a trend - beyond sweat and pumping arms - among the power-walking women she passed: They were wearing big, baggy T-shirts.

"I thought, 'Couldn't we all look way better in fitness apparel?' That was kind of the 'Aha!' moment."

What clicked was the connection she drew between the kind of fitness wear she had in mind and an area of academic study she pursued, along with a Ph.D., at Pennsylvania State University. That involved the way innovation can increase participation in physical conditioning.

"I couldn't play tennis if they didn't have composite rackets," Gelberg offered as an example, noting that at 5 feet tall and 100 pounds, she's too slight to swing a wooden racket for long. "A lot of innovations provide participation opportunities."

Through helping to launch the Xenith football helmet, designed to decrease concussions, as well as working in sports market research and small-business consulting, Gelberg said, she "built up the courage to launch something on my own."

Tapping personal savings and the generosity of friends and family for start-up costs, in January she introduced her line of loose-fitting, yet fashionable, tops and bottoms made from an ultrasoft blend of bamboo (which is thermal-regulating and naturally antimicrobial), Spandex, cotton, and polyester on www.devigi.com.

(Devigi means "to compel" in Esperanto, a language "nobody speaks . . . so things are trademark-able," Gelberg said.)

The pants - currently only for women, as are most tops - have a double waistband, to tighten and smooth an area all those baggy T-shirts on the beach were trying to hide.

Producing her line locally was important to Gelberg. She's using CSH Inc. in Port Richmond, adding one more stitch in the growing made-in-the-U.S.A. apparel movement.

"It wasn't a political statement," she said. "I think things work out better when you can do things face-to-face, especially when you are at the start of a learning curve."

In CSH's Steven Levin, she found a veritable partner - and an empathetic small-business owner.

"My philosophy has always been try and help people out when they're getting started," said Levin, who has been in business as a clothing contractor since 1985 and is working with bamboo-based fabric for the first time because of Devigi.

The apparel industry is fraught with problems, Levin said. Survivors must be "determined and focused" with a good idea, he said, adding that Gelberg seems to fit all three.

In the one-hand-washes-the-other world of local sourcing, that bodes well for him, Levin noted: "Whatever is good for her is good for me."

Gelberg considers distribution her biggest challenge. To fully appreciate what she's selling, she said, it has to be at least felt, if not tried on. There are no fitting rooms in the online marketplace.

So Gelberg has opted to follow another fashion trend, the pop-up store. As temporary locations - in vacant storefronts, art galleries, health clubs, and parks - they afford start-ups the chance for multiple retail outlets without the expense. No long-term leases are required, no building maintenance.

Pop-ups have allowed Gelberg, essentially a one-person operation with help from family, to display her creations from Virginia to Massachusetts.

"You're not wedded to one geographic location and space to grow your business," Natalie Nixon, director of Philadelphia University's Strategic Design MBA program and a professor of fashion-industry management, said of pop-ups. "You can go after a target market."

Initially, Gelberg's was women in their early 30s to late 40s - "the active mom who may be running a 5K but not a marathon. If you can run a marathon, you can wear any apparel."

She has been surprised to learn her clothing - $38 for tank tops, $89 for bottoms, $95 for covers - holds appeal for teenagers (to lounge in) and women in their 50s and 60s (who prefer it for the gym and air travel).

But it's way too early to declare success. Gelberg said online sales were "spiky" - nine one month, 22 the next, more than 200 in March - depending on exposure. A spot on NBC's Today apparently triggered the March surge.

Gelberg's priority now is finding "a more stable growth path . . . figuring out the distribution, so it's less me-focused for point of sales."

"It's not an effective growth strategy," she said. "I can't be in all places."

Diane Mastrull: >Inquirer.com

Nadine Gelberg explains the thought behind her fitness clothing line (bamboo based) and her retailing model (pop-up shops). www.inquirer.com/gelbergEndText