Feb. 13 (UPI) -- "Heading" the ball is a fundamental part of soccer, but concerns over safety have changed the way the game is played at the youth level in many countries, including the United States.
Now, a study published Thursday in JAMA Ophthalmology adds to mounting evidence that "headers" can cause significant brain trauma, even in older players.
The findings suggest these injuries extend beyond concussions to include more "subtle" effects on cognitive and even eye function, researchers say.
"If one sustains soccer headings repetitively in a short time frame, eye-brain connection may experience impairment," study co-author Keisuke Kawata, an assistant professor of kinesiology at Indiana University, told UPI.
As many as 4 million American youths play soccer. In 2017, the U.S. Soccer Federation placed restrictions on heading the ball for players 11 years of age and younger, over safety concerns.
The act of heading in soccer has drawn significant attention in recent years, due in large part to recent research highlighting the risk for injury, particularly concussions. The long-term effects of concussions on athletes in several sports -- including ice hockey and American football -- has also been the subject of intense scrutiny, both in the media and the medical community.
The findings of Kawata and colleagues follow on the heels of a landmark study published last year in the New England Journal of Medicine, which documented the brain health of Scottish soccer players who suffered repeated concussions. In addition, a study published in JAMA in January linked declines in memory with repeated heading in soccer.
For the study, the Indiana University-based researchers compared the cognitive effects of heading a soccer ball in 40 adults with those of 38 controls, who only kicked the ball. A JUGS soccer machine was used to "pass" the ball to participants -- from a distance of approximately 40 feet -- at a speed of roughly 25 miles per hour. Participants in each group headed or kicked the ball 10 times.
The authors recorded the head acceleration -- as a measure of stress on the head -- of the participants in each group, and noted that the head acceleration was nearly 10 times greater among the "headers" as compared with the kickers. All participants were evaluated immediately after heading or kicking, two hours later and 24 hours later using the King-Devick test, which assesses for eye movement disorders.
The authors found that the participants in the kicking-only group performed better on the test at all three assessments than those in the heading-only group, suggesting that head trauma sustained by the headers may adversely affect eye function.
"This study does fill an important knowledge gap in terms of acute soccer heading effects," Kawata said. "I think there is sufficient evidence to suggest that even mild heading, repetitively for long-term, can increase risks for later-onset of brain disorders."