Football is a dangerous sport. Even with the best protective gear, sprained knees, pulled muscles and an occasional broken bone are part of the game. But seven years of research into the impact of head trauma in high school players points to new dangers that parents will find extremely alarming.

"We are seeing changes in brain activity even without a diagnosed concussion, even without any sign or symptoms showing up and that that occurs in a large population of our subjects," said Larry Leverenz, a clinical professor of health and kinesiology at Purdue University.

More than half of the players participating in the trials showed signs of altered neurological function and dramatic changes to the wiring and biochemistry of their brains, according to a series of studies published by the Purdue Neurotrauma Group. They focused their research on pre-concussive head injuries, which up until now went largely ignored due to a lack of symptoms such as dizziness or disorientation associated with a concussion.

"It's not just the neurons that get damaged, it's the glial cells, it's the vasculature," said Eric Nauman, a mechanical and biomedical engineering professor at Purdue University.

"Basically in football and woman's soccer about half the team is experiencing these kinds of things, these kinds of changes. Some of them heal and some of them don't by the time they start playing their next season, and that was the thing that really got us nervous," he added.

More On This...

The researchers placed sensors on the athletes to record impact forces and coupled that data with brain scans and cognitive tests to track neurological function over the course of the trial.

They found hits to the head that up until now were considered less dangerous may be the most dangerous of all because they go unnoticed, occur more frequently and cause damage that could result in long-lasting neurological problems.

Based on their results, researchers are developing equipment that better protects the head from high-force impacts.

"You're not going to change the game. You are not going to get rid of the game, at least. So how can you make changes that keep the spirit of the game there, keep players enjoying, keep fans enjoying the game but at the same time be safe?" Leverenz said.

They say the technology to make these games safer exists. But to get them out of the laboratory and on to the field requires a general consensus that these sports are a lot more dangerous than previously thought.