If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
Guardian Caps debut at NFL training camps around the league
NFL hopes to reduce head injuries with helmet experiment
NFL players beginning to adopt protective Guardian Caps over helmets at practice - NBC Sports
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Why are NFL players wearing padding on helmets at training camp?
NFL hopes to reduce head injuries with helmet experiment - Seattle Sports
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries
Guardian Caps leave some NFL players and coaches with concerns: 'You feel like a bobblehead'