When heads collide, think of the brain as something with the consistency of three-day-old Jello in a bowl. Injury occurs when the brain sloshes and twists inside the skull.
UCLA Neurosurgeon Dr. Neil Martin says over the years, numerous hits can lead to the progressive degeneration of the brain, a condition called chronic traumatic encephalopathy.
"It starts with behavioral and mood disorders like depression. It can include thoughts of suicide, loss of impulse control or flair ups of temper," said Martin.
The condition entered the spotlight last year when Dave Duerson of the Chicago Bears, who suspected he had CTE, shot himself in the chest and left his brain to research.
"That was the thing that just was so concerning when I heard that Junior Seau had committed suicide and shot himself in the chest. It was just so reminiscent," said Martin.
CTE is determined after death because there isn't a way to diagnose it while you're alive. It's even been found in the brains of athletes who've never suffered any known concussions. This leads to the alarming conclusion that cumulative, non-concussive blows can be as damaging as a concussion.
"Many of the football players who after death, who've had an autopsy where they found this problem of CTE, have not had a history of multiple concussions; they've had a history of many, many hits," said Martin.
Why some athletes develop CTE and others don't is unknown. Research is underway to determine if some people might be genetically predisposed to getting CTE.
"What would really be great is if we had a definitive, diagnostic test for CTE. Then we could identify it at an early phase," said Martin.
But by the time an athlete reaches the college or pro level, it might be too late. That's why experts tell parents to start monitoring injuries while kids are young.
"I would definitely steer them away from contact sports, boxing, football, hockey, and steer them towards other sports," said Martin.
While helmets do offer protection, Martin feels they may offer players a false sense of security since athletes may think they can tackle even harder. Some student sports programs, like Servite High School in Anaheim, will ask their players to undergo brain function testing so they can see if anything changes after a concussion.