If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
Kirk Cousins spotted wearing a fancy new helmet at OTAs
NFL safety: How VICIS' position-specific helmets have become all the rage - The Athletic
NFL hopes to reduce head injuries with helmet experiment – The Morning Call
Finally: First QB Helmet Designed To Reduce Concussions Has Been Approved by NFL
Seeking to reduce head contact, NFL mandates use of Guardian Cap helmet for early portion of training camp - ESPN
New NFL Helmet Cuts Quarterback Concussion Risk—But Only 7% Better Than Older Models
Study: Many NFL players wore low-rated helmets – The Denver Post
First QB helmet designed to help reduce concussions approved