For Various Reasons, N.F.L. Players Do Not Wear Cups - The New York Times
What are NFL players wearing on their helmets at training camp and how do they work? - The Mirror US
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Guardian Cap Use a Concern for Watt, Saleh Early in Training Camp - Sports Illustrated
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
Guardian Caps', NFL's Most Recent Attempt At Preventing Head Injuries
What are Guardian Caps and how are they helping football teams?
Guardian Caps debut at training camps throughout NFL
NFL hopes to reduce head injuries with helmet experiment
Why are players wearing padded helmets at NFL training camps?
What are the padded helmets players are wearing at NFL training camps?