The story behind those soft-shell puffy Guardian Caps players have been wearing
Football helmet - Wikipedia
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
Some CFL players are wearing foam covers on helmets
Why are NFL players wearing padding on helmets at training camp?
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
CFL players adjust to their bigger heads, helmet covers worn in practice
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
NFL awards grants to four projects in helmet safety development program - ESPN
Bombers set to don Guardian Caps” on helmets at training camp
Chicago Bears: Why players are wearing padded helmets
New York Jets: 2022 Helmet Life-Size Foam Core Cutout - Officially Lic – Fathead
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
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