Catastrophic injuries—defined as serious injury to the spine, spinal cord, or brain—and fatalities are uncommon in football; both have become less common since the 1970s, although a small number of them still occur each year. Both concussions and catastrophic injuries can be caused by helmet-to-helmet collisions as well as impact against the ground or other players' knees; in other cases, they can be caused by players who have sustained a head injury returning to play, which can place the player at risk of sustaining a severe injury. Despite the decrease in catastrophic injuries, a greater number of NFL players have reported major injuries and shortened careers since the 1970s, in part due to the increasing size and speed of players and the use of artificial turf.