NEW YORK, United States — Nike's latest sports bra is an engineering marvel. Using the Flyknit technology normally reserved for sneakers, the bra is made up of just two panels of fabric — other Nike Inc. bras can have up to 41 separate sections meshed together — and promises sturdy support on the field or in the gym. It took researchers more than 600 hours of biometric testing, recording movement on cameras and creating body maps to develop the latest weapon in a battle to win over women.A sports bra arms race has broken out among athletic wear makers. In May, Lululemon Athletica Inc. released a new cross-back design called Enlite, with built-in cups and a bonded underband, in a fabric that's more like the brand's sweat-wicking yoga pants than a traditional bra. Last year, Victoria's Secret launched stretchy athletic gear line Victoria Sport. Startups such as Knix Wear and OMsignal have entered the fray, too, providing alternatives to the dominant big brands.Sports bras, as an apparel category, have grown more than 20 percent year-over-year to about $3.5 billion in the US in 2016, according to data from A.T. Kearney. At the same time, people were buying fewer traditional padded bras, so bra-makers have scrambled to adapt. Young people are especially devoted to sports bras. A 2015 survey by consumer research firm NPD Group Inc. showed that 41 percent of millennials wore a sports bra over the previous week, while that number was just 21 percent for older women.Today's bras have cups and different levels of compression for different activities — all in an effort to account for how women's bodies move during physical activities. New fabrics add features such as moisture wicking and ventilation and anti-stink properties. The big question is whether efforts to make bras more comfortable are truly innovative or just another gimmick."You don't see a lot of disruptors because of the technical specifications and the fact that it really needs to perform more than any other garment," said Liz Dunn, chief executive officer at retail and brand consulting firm Talmage Advisors. "If you've ever ventured out in a bad sports bra, you never want to do that again. It's a bad situation."Why is it so difficult to make a good sports bra? Unlike such other essentials as shirts and boxers, performance is a necessity. Because there's no muscle controlling their movement, womens' breasts can cause discomfort during physical activities. Researchers are at the beginning stages of studying how breasts move during exercise and how best to support them. These efforts are complicated by the fact that there's long been stigma around studying breasts. Then there's the fact that no two breasts — even on the same woman — are exactly alike in placement, size and density. So even for brands that want to provide the perfectly engineered bra, the science just doesn't exist.You don't see a lot of disruptors because of the technical specifications... It really needs to perform more than any other garment.Designers of new iterations of the sports bra seek to make them look better than the old ones and move with the wearer. That's welcome news for women who are used to wrestling stodgy undergarments on and off, feeling tightly compressed and restricted.But coming up with a better design isn't simple. The first general exercise bra arrived in 1977; it comprised two jock straps sewn together. Now, with new fabrics and technology, bra makers are going back to the drawing board. Intimate wear companies were caught off guard by the proliferation of sports bras, and athletic companies took advantage, said Manik Aryapadi, a principal at the retail practice of A.T. Kearney. Labels such as Victoria's Secret and Hanes didn't have the research and development investment or capabilities of a Nike or Under Armour Inc.Women’s growing tendency to wear athletic clothes as regular attire sparked retailers to begin rethinking the sports bra in 2013, when Victoria's Secret introduced its own characteristically sexy line. The company's sports bras initially were designed to let women show off their cleavage at the gym. The brand's CEO at the time, Sharen Jester Turney, said the company wanted to solve the "uniboob problem, where your sports bra makes you look straight across — no one likes that,” she said in an interview with Bloomberg in 2013.Athletic apparel companies soon followed with more scientific additions. In 2015, Under Armour announced a line featuring three levels of support and gel inserts that added comfort. A year before, Nike had launched its Nike Pro bra collection with an online fitting tutorial. The line was a collaboration with Loughborough University, one of the few academic institutions that conducts research on the garment.Lately, bra competition has turned uglier. This month Lululemon sued Under Armour for patent and trademark infringement, taking issue with four bra styles sold by its rival. Lululemon holds more than 30 design patents. The spat is centred around bra straps. Lululemon patented different configurations of straps, intertwining four thin straps on the back of a sports bra. The yoga wear retailer says Under Armour is selling similar designs, adding in the complaint that "Under Armour’s unauthorised acts as described herein have caused and will continue to cause irreparable damage to Lululemon and its business unless restrained by this Court."Lululemon declined to comment on the suit. Representatives for Under Armour did not respond to a request for comment.Regardless of the suit's outcome, the pair are sure to keep cranking out new bras as the industry begins to attract the mass-market labels. As athletic brands continue to chip away at the industry leaders, mid-tier players such as Target Corp. and H&M Hennes & Mauritz AB will capitalise by selling cheaper versions, just as they did when leggings became trendy. Some sports bras at Old Navy can be bought for as little as $8. Now it's only a matter of time until Amazon.com Inc. shows up to the party, said Aryapadi.By Kim Bhasin and Lindsey Rupp; editor: Francesca Levy.