Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Why are NFL players wearing padding on helmets at training camp?
Guardian Cap Debuts at Training Camps League-wide
CFL players adjust to protective helmet caps
NFL makes Guardian Caps mandatory for many positions through second preseason game - NBC Sports
Padded helmet cover shows little protection for football players - Scope
Guardian Caps NFL: What player positions are required to wear them?
NFL: Concussions dropped by more than 50% among players who wore Guardian Caps during summer
Coverage of the NFL in Guardian Caps
NFL announces more steps to reduce head injuries
NFL executive: Guardian Caps could one day be used in games