NFL training camps began this week and featured some players wearing “Guardian Caps” which are used to help prevent head injuries. 

The caps are a soft-shell layer that goes over a player’s helmet. The caps can reduce head injuries by 10 percent or up to 20 percent if all players involved in a collision are wearing them.

The NFL ruled that players at certain positions who are at greater risk of head injuries have to wear the guardian caps at the start of training camp through the second preseason game.

The main positions affected are tight ends, linebackers and offensive and defensive linemen.