Out of 91 former NFL players, 87 were tested positive for a brain disease called chronic traumatic encephalopathy, also known as CTE.
These former football players donated their brains to science after their deaths.
Although it is not yet clear why some haven't developed the disease, researchers at U.S. Department of Veterans Affairs and Boston University believe that this brain damage is linked to repeated head trauma and concussions.
This doesn't mean, however, that 96% of all NFL players were likely to develop CTE, according to Dr. Robert Cantu, co-founder of the website Concussion Legacy Foundation, where the study was published.
CNN reports the NFL had taken several actions to keep players safe through pre-game and on-field medical assessments and post-game assessments to those involved in hard hits during play. The American football league also reduced the number of kick returns by moving the kickoff spot forward by 5 yards.
"We are dedicated to making football safer and continue to take steps to protect players, including rule changes, advanced sideline technology and expanded medical resources," according to the league's satement. "We continue to make significant investments in independent research through our gifts to Boston University, the (National Institutes of Health) and other efforts to accelerate the science and understanding of these issues."
NFL's Head, Neck and Spine Subcommittee chairman, Dr. Russ Lonser said in May that for the past three seasons, there was a 40% reduction in helmet-to-helmet hits and 25% reduction in concussions.
According to Cantu, this post-mortem diagnosis is not only common to athletes.
"There are a number of cases in people who never saw an athletic field," he said, such as former military members, autistic children who often banged their heads, people who suffered gram mal seizures, abuse victims and circus performers.
CTE is a progressive degenerative brain disease that impairs neural pathways that control a person's memory, judgement and fear.
