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Concussions have become a hot topic in hockey and football, as evidence mounts of their potentially debilitating long-term impact, but received relatively little attention in MMA, note the authors of the paper, published in the American Journal of Sports Medicine.

“This draws attention to the fact that relevant questions need to be asked of a sport for which the objective at some level is to knock them out,” said Michael Hutchison, a U of T kinesiology professor and lead author.

“The [knocked out] person is rendered unable to defend themselves, and then they’re getting multiple strikes to their head. That’s probably not good for one’s health.”

The UFC — the sport’s dominant promotional company — is still reviewing the “technical medical document” and is not ready to comment on it yet, said Steve Keogh.

In barely a couple of decades, MMA has won a wide following in numerous countries, with one marketing company last year estimating its worldwide audience to be 300 million people.

Fighters are allowed to kick and punch, as well as employ wrestling-like techniques on the ground in bouts often conducted inside a chain-link-fence “octagon.” Some events pay cash rewards for the “knockout of the night.”

Despite its popularity, the sport has long faced criticism, with several medical associations calling for its banning, and some provinces and states doing so.

There is little empirical data, though, on its relative safety.