Richard Sherman -- Insane Leg Muscle ... Blows NFL Star's Mind!!!

Richard Sherman is not easily impressed ... but the NFL superstar had to pick his jaw off the ground after an encounter with a legendary leg muscle that blew his mind!!!

The quad belongs to strength and conditioning stud Mike Barwis -- who famously trains some of the top athletes in the world including Ndamukong Suh and UFC stud Rashad Evans.

Barwis -- whose new show "American Muscle" debuts on Discovery on July 9th -- was kickin' Sherman's ass at his training center in Detroit recently when the Seahawks DB came into contact with Mike's leg.