NFL training camps: Guardian Cap players are wearing over helmets, why
NFL players wear Guardian Caps to reduce head trauma during training camp
History of The Football Helmet - Atavus
Football Helmet Buying Guide
Guardian Caps Guardian Sports
Colleges add Guardian football protective helmet cap - Sports Illustrated
Vicis expands helmet line with new soft 'cap' for practices, 7-on-7 and flag football – GeekWire
An Inside Look at the Riddell Axiom Helmet - Boardroom
Riddell SpeedFlex Football Helmet
Guardian Caps leave some NFL players and coaches with concerns: 'You feel like a bobblehead'
Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure