Built By Data: NFL Helmet Innovation
NFL players wear Guardian Caps to reduce head trauma during training camp
NFL training camps: Guardian Cap players are wearing over helmets, why
Guardian Caps: Why some NFL players are wearing them in training camps
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
NFL makes Guardian Caps mandatory for many positions through second preseason game - NBC Sports
What are Guardian Caps? NFL players sport specialty helmets for 2022 minicamp
What are those funny-looking soft caps on NFL helmets during camp?, Sports