CFL players adjust to protective helmet caps
NFL players don Guardian Caps, designed to protect from concussion
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Guardian Cap debuts during minicamps in effort to reduce avoidable head contact
Guardian Caps Guardian Sports
NFL says positions wearing Guardian Caps saw 52% decrease in concussions - NBC Sports
Robert Saleh dislikes the new Guardian Caps: And he has a point
Guardian Caps NFL: What player positions are required to wear them?
Football helmet - Wikipedia
NFL expands rule against misuse of helmet; Guardian Caps required in regular season, postseason
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77