Millennials have a new shopping preference that should terrify Lululemon and Athleta

Young Americans may have fallen out of love with yoga pants.

The dominant apparel trend of athleisure seems to have reached its peak in the second quarter of 2017.

Athleisure is the trend of buying athletic apparel for its unique properties - sweat-wicking, stretch, comfort - without necessarily intending to use it to work out.

Athleisure has been huge for the last several years. According to The NPD Group, US activewear apparel sales totaled $45.9 billion in 2016, up 11% from 2015 and far outperforming the traditional apparel sector overall.

Though athletic brands Lululemon, Under Armour, and Nike initiated the trend, other retailers such as Gap, J.Crew, and Forever 21 jumped on the bandwagon in hopes of boosting sales. When we ventured out to the streets of New York City earlier this year, nearly everyone we saw was dressed in athleisure, citing the trend's comfort and versatility as the reasons they loved it.

But signals abound that athleisure could be on the way out.

Quo Vadis Capital founder and president John Zolidis told Bloomberg that the athleisure fashion trend came to a "screeching halt" at the beginning of 2017.

UBS analysts said that as the "athleisure cycle moves past its peak," denim has started to make a comeback. They cited product experts and fashion magazines as evidence of denim's resurgence.

This spells trouble for retailers who have made big investments in athleisure.

Lululemon, who was a major propagator of the trend earlier in the decade, just had its highest rate of product going on sale in the last four years, according to Credit Suisse. The bank said that the brand has reached "maturity," and that athletic apparel growth is decelerating.

Sports retailers Dick's Sporting Goods and Foot Locker both saw big second-quarter misses that sent stocks tumbling.

Not everyone agrees that athleisure is dying, however.

NPD sports industry analyst Matt Powell notes that though the trend is suffering, so is all of retail apparel. In terms of sales, the gap between sportswear and the rest of the apparel industry is still as wide as it ever was - they're just both shifting downward.

"There is no indication that the athleisure customer is spending their money on other footwear and apparel," Powell wrote.

Instead, Powell says that since so many brands rushed to cash in on the trend, the dollars are being spread thin for retailers. That has created a bubble in retail apparel, which is now "bursting."

Powell says athleisure won't die, but it is "very sick."