A new study on the long-term risks of playing football found more than 40 percent of retired National Football League players examined showed signs of traumatic brain injury, which is often a precursor to the degenerative brain disease, chronic traumatic encephalopathy.

The rate of TBI found in the retired football players is "significantly higher" than that of the general population, Dr. Francis Conidi, one of the study's authors, said in a statement.

"This is one of the largest studies to date in living retired NFL players and one of the first to demonstrate significant objective evidence for traumatic brain injury in these former players," Conidi said.

The study also found that the longer a subject played in the NFL, the higher his risk for TBI -- though there was no relationship between the actual number of concussions and TBI.

Until last month, top NFL officials had repeatedly denied there was a link between playing football and brain injuries.

Researchers used a powerful method of brain scanning technology known as diffusion tensor imaging, combined with thinking and memory tests on 40 retired NFL players, the majority of whom had been retired from the league for less than five years.

According to a breakdown of the 40 living retirees:

The average age was 36

Subjects ranged in age from 25 to 56

Subjects played an average of 7 years in the NFL

During their career, subjects sustained an average of 8.1 concussions

31 percent of subjects experienced "sub-concussive" hits in their career

It was not immediately clear how the study's subjects were chosen.

The DTI scans measured the flow of water molecules in the brain's white matter -- the deep tissues that connects the different regions of the brain -- and found damage in 43 percent of the subjects.

Half of subjects also demonstrated "significant problems" with executive function: 45 percent had learning or memory problems, 42 percent had attention and concentration problems and 24 percent had spatial and perceptual function problems.

"This research in living players sheds light on the possible pathological changes consistent with chronic traumatic encephalopathy that may be taking place," Conidi said.

Symptoms of CTE include cognitive impairment in things like memory and multitasking; mood problems like depression and apathy; behavior changes like aggression and impulse control; and, most rarely, motor problems like body tremors or difficulty making facial expressions.

CTE, which can thus far only be diagnosed after death by examining the brain, was diagnosed last year in 87 out of 91 former NFL players who chose to have their brains examined posthumously.

Growing concerns about the long-term impact of playing football have prompted changes to how some college programs are running practice, and even several young players to retire early.

Most recently, 30-year-old NFL safety Husain Abdullah -- who suffered five concussions in his seven-year career -- announced his retirement, citing his desire to have “a sound mind” in the years ahead.