In response to the growing understanding of this danger, the National Football League (NFL) has revised its safety regulations. Players who have suffered a head injury on the field must undergo a "concussion sideline assessment"—a series of mental and physical fitness tests—before being allowed back in the game. In an effort to diminish the amount of head and neck injuries on the field, NFL officials have begun enforcing stricter penalty calls for helmet-to-helmet contact, leading with the head, and hitting a defenseless player. Furthermore, as of 2010, if a player’s helmet is accidentally wrenched from his head during play, the ball is immediately whistled dead. There is hope that these new regulations, coupled with advances in helmet design, will reduce the number of concussions player endure, and thus curb the number of CTE cases.