Catastrophic injuries have been on a steady decline since the 1960s, due in part to rules banning dangerous forms of contact such as spearing, face tackling and butt blocking. However, catastrophic injuries are still caused by helmet-to-helmet collisions, as well when players hit their heads against an opposing player's knee or the ground. Returning to play after sustaining a head injury earlier in the game also places players at risk for an even more severe injury. Many states are requiring teams to prevent players who have shown any signs of a concussion from returning to a game, while other steps such as more aggressive enforcement of safety rules and better condition of the neck muscles have been suggested. Additionally, coaches are being urged to train players to block with their shoulders instead of their heads.