There has been a lot of discussion on the effect routine concussions have on American football players, with some recent research suggesting that up to 40 percent of retired National Football League players could be suffering from traumatic brain injury, or TBI.

A new study now suggests that football players are damaging their brains even without receiving a concussion. As reported in Science Advances, medical researchers have witnessed structural changes to the brain due to routine hits that happened over just one season of play.

The study followed two groups of athletes. The first group had 38 college footballers who play for the University of Rochester. Each player’s brain was scanned in an MRI before and after a season of play. They also had their helmets fitted with an accelerometer that captured the force that the players experienced during a hit.

"We measured the linear acceleration, rotational acceleration, and direction of impact of every hit the players sustained, which allowed us to create a three-dimensional map of all of the forces their brains sustained," lead author Adnan Hirad, a graduate researcher at the University of Rochester, said in a statement.

The team found that although just two people received a concussion over the season, over two-thirds of the players had decreased structural integrity in the brain. They also noted that 59 percent of the 19,128 hits sustained by all players happened during practice. Not unexpectedly, these hits had a slightly lower acceleration compared to hits during games.

The second group had 87 athletes from other contact sports. The team wanted to use an independent approach to validate their football findings. Twenty-nine people in this group had a clinically-defined concussion. These people underwent MRI scans and had blood samples taken in the first 72 hours after the injury.

The team detected reduced structural integrity of the midbrain just like in the football players group. The blood work also revealed an increase in tau proteins in the blood. This protein is linked to neurodegenerative diseases, and TBI has previously been shown to lead to an increased risk of dementia.

"Public perception is that the big hits are the only ones that matter. It's what people talk about and what we often see being replayed on TV," said senior author Brad Mahon, an associate professor of psychology at Carnegie Mellon and scientific director of the Program for Translational Brain Mapping at the University of Rochester.

"The big hits are definitely bad, but with the focus on the big hits, the public is missing what's likely causing the long-term damage in players' brains. It's not just the concussions. It's everyday hits, too."

The team points out that more research is needed to translate these findings into concrete directives. The future investigations need to be larger in scale across multiple age groups and throughout contact sports.