Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries : r/nfl
Guardian Caps added to help reduce football concussions - The Witmarsum
Vicis expands helmet line with new soft 'cap' for practices, 7-on-7 and flag football – GeekWire
History of The Football Helmet - Atavus
Steelers debut new, safer helmets for off-season training
The Extra Yard: The Ins and Outs of an American Football Helmet - Stories
NWT Guardian Cap Soft-Shell Protective Helmet Cover Navy
Guardian cap' to prevent head injuries mandated for practices, CFL says
EXPLAINER: NFL using helmet caps for player safety, research
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Padded helmet cover shows little protection for football players - Scope
The Extra Yard: The Ins and Outs of an American Football Helmet - Stories