Concussions are particularly concerning, as repeated concussions may increase a person's risk in later life for chronic traumatic encephalopathy (CTE) and mental health issues such as dementia, Parkinson's disease, and depression. Concussions are often caused by helmet-to-helmet collisions, impact against the ground or other players' knees, and upper-body contact between opposing players. However, helmets have prevented more serious injuries such as skull fractures. Cervical spine injuries can be catastrophic, but have sharply declined since the mid-1970s due to rule changes and improved workout regimes, equipment, and coaching.