Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Guardian Caps: What are they and why are NFL players wearing them? - Sactown Sports
Guardian Cap: What are and why the NFL asks this safety measure in helmets
If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
NFL players experimenting with 'Guardian Caps' to prevent head injuries
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NFL players don Guardian Caps, designed to protect from concussion
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