Guardian cap' to prevent head injuries mandated for practices, CFL says
Guardian Cap: What are and why the NFL asks this safety measure in helmets
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Guardian Cap Use a Concern for Watt, Saleh Early in Training Camp - Sports Illustrated
NFL training camps: Guardian Cap players are wearing over helmets, why
NFL players don Guardian Caps, designed to protect from concussion
NFL says Guardian Caps reduced concussions by more than 50%
Truth About Guardian Caps
NFL hopes to reduce head injuries with helmet experiment - Seattle Sports
What are Guardian Caps and how are they helping football teams?