Professional mixed martial arts competitors are twice as likely to suffer head injuries than football players, according to study published in The American Journal of Medicine. “One-third of professional mixed martial arts matches end in knockout or technical knockout, indicating a higher incidence of brain trauma than boxing or other martial arts,” The Associated Press reports.

The University of Toronto study reviewed 844 videos from 2006 to 2012; the videos featured professionals competing in mixed martial arts. The research revealed that “108 matches or nearly 13 percent ended in knockouts. Another 179 matches, or 21 percent, ended in technical knockouts, usually after a combatant was hit in the head five to 10 times in the last 10 seconds before the fight was stopped,” The AP continues. The findings are especially problematic given the rising popularity of mixed martial arts — particularly among children, teenagers, and young adults.

The sport combines techniques from several different combat-based sports or disciplines, including karate, judo, and kickboxing. Competitions are legal in all states except New York. Medical experts recommend imposing stricter rules to keep competitors safe and banning young children from participating altogether. “Researchers proposed introducing rules like in boxing where a fighter gets a 10-second count and evaluated after a knockdown,” The AP explains. “They also proposed more training to help referees to identify fighters who are defenseless or have lost consciousness so they can stop fights more quickly.”

Professional fighter Tony Way disagrees that the sport is inherently unsafe.

Fighting is always monitored, Way tells Fox News, “They’ll stop if gets rough.”

Researcher Michael Hutchison confirms children should not participate in mixed martial arts, but admits more studies are necessary to make decisions about the sport as a whole.