The two companies are fighting for a piece of the fast-growing sports bra market, which analysts say accounts for more than US$1 billion in sales each year in the US. Last year, Lululemon executives said third-quarter bra sales grew more than 20 per cent.

Patent lawyers say lawsuits of this type are rare in the fashion industry, mostly because few retailers are willing to go through the trouble - or expense - of securing design patents for their products.

“This is a long, expensive process - we’re talking at least a year and a half, and several thousand dollars - just to get the patent,” says Laura Ganoza, an intellectual property lawyer in Miami. “That’s a lifetime in the fashion industry, where the lifespan of an article of clothing is a season, if you’re lucky.”

Lululemon, which has more than three dozen design patents, “clearly sees this bra as a product that’s going to have a long shelf life,” she adds. In order to secure the patent, the company has to prove that the sports bra’s straps are an original, “non-obvious” design that has ornamental value (as opposed to being strictly functional).

“The bar for obtaining a design patent is high, which is one reason you don’t see many cases like this,” says Christopher Larus, an intellectual property lawyer in Minneapolis. “If this case moves forward, I would expect there will be a fair amount of focus on whether Lululemon’s designs are truly novel, or whether this is something that’s been done before.”

Sports bras and bralettes have been driving much of the recent growth in the bra industry, which for years has been dominated by Victoria’s Secret. But recently, the company’s sales have been plunging, creating an opening for online start-ups, sportswear companies and others to step in.

“When you have one player that owns the majority of the market, as Victoria’s Secret did, you start to see a real loss of innovation,” says Heidi Zak, co-founder of direct-to-consumer bra company Third Love. “The status quo is what people get used to, and companies don’t see a need to create anything different. We’re finally starting to see that change.”

Third Love, for example, offers half-sizes that range from AA to G cups, and is expanding up to size K. (Victoria’s Secret, by comparison, maxes out at DDD.) Other start-ups, like True & Co. offer bras without wires, elastic or padding.

The hottest athleisure wear in Hong Kong, from Alala to Zara Sport

“There’s a realisation that consumers are looking for a new kind of bra - whether new fabrics or approaches to sizing - and companies are rushing to tackle that problem in one way or another,” retail analyst Sucharita Mulpuru says. “And, it seems to me, that Lululemon is trying to protect its designs and potentially scare off competitors.”

This isn’t the first time the company, which last year made US$2.34 billion in revenue, has taken a competitor to court. In 2012, Lululemon sued Calvin Klein for allegedly copying the waistband design on its US$98 Astro Pant. The companies later settled out of court for an undisclosed sum.

“What Lululemon is doing here is staking its turf,” Jeremy de Beer, a law professor at the University of Ottawa, told Reuters at the time. “The business strategy is to deter other people from even trying to copy designs, because it’s going to cause them legal problems.”