By Jonathan Stempel

(Reuters) - Adidas AG <ADSGn.DE> <ADDYY.PK> on Monday escalated its legal battle against Skechers USA Inc <SKX.N> over athletic sneaker designs, filing a lawsuit accusing Skechers of stealing its "Springblade" concept for its similar "Mega Flex" shoe.

In a complaint filed in U.S. District Court in Portland, Oregon, Adidas said Skechers willfully infringed two patents related to Springblade, a three-year-old design whose midsole includes blades meant to help propel runners forward.

Adidas accused Skechers of developing its Mega Flex shoes, including Mega Blade 2.0 and Mega Blade 3.0, as "takedowns" that copy the Springblade technology without the cost of creating it.

"This pattern of unlawful behavior and freeloading in the industry is outrageous and must end," Adidas, based in Herzogenaurach, Germany, said in a statement. "We will take every legal measure possible to protect and defend our innovations."

Skechers spokeswoman Jennifer Clay said the Manhattan Beach, California-based company does not comment on pending litigation.

Adidas' lawsuit, the company's second lawsuit against Skechers in federal court in Oregon in less than a year, seeks an injunction against any infringements and triple damages.

The lawsuit is one of many in the footwear industry seeking to protect patent and trademark rights.

Last September, Adidas accused Skechers of illegally copying the design for its classic white Stan Smith tennis shoes. Adidas won a preliminary injunction in February, and a jury trial is scheduled for May 2017, court records show.

Another athletic footwear giant, Nike Inc <NKE.N>, persuaded the U.S. Supreme Court in 2013 to end a smaller rival's effort to void its trademark for the popular Air Force 1 sneakers.

The case is Adidas America Inc et al v Skechers USA Inc, U.S. District Court, District of Oregon, No. 16-01400.

(Reporting by Jonathan Stempel in New York; Editing by Leslie Adler)