What is an illegal knee in MMA and why was Aljamain Sterling declared the winner against Petr Yan at UFC 259?

In the UFC, hitting a grounded opponent on the head with knees or kicks is considered illegal and can amount to disqualification. A grounded opponent has been defined as a fighter being in any position in which anything except the soles of his feet is touching the ground.

MMA is the only combat sport that allows fighters to hit each other even when either opponent is down. However, in most organizations like the UFC, the use of knee strikes and kicks on a downed opponent is illegal while hand and elbow strikes to the head are considered legal.

At UFC 259, the UFC bantamweight title changed hands in an unprecedented manner after defending champion Petr Yan illegally kneed Aljamain Sterling in the penultimate round of the fight. Yan was ahead on the scorecards of two judges when the incident happened and could have won the fight.

Instead, it was Aljamain Sterling who took home the title in a DQ win. Yan's knee strike was dangerous and looked absolutely intentional. Sterling's knee was on the ground when Yan violently kneed him on the temple, leaving the fighter unable to continue.

Khabib Nurmagomedov reveals the reason behind Yan's illegal knee strike on Sterling at UFC 259

Following the fight, Sterling clearly looked concussed and also distraught because he didn't want the fight to end the way that it did. He broke into tears inside the octagon and threw away the title before leaving the cage. In the post-fight interview with Joe Rogan, a disappointed Sterling apologized to his fans and promised to run it back after recovering.

Daniel Cormier revealed that Khabib Nurmgomedov told him Yan fell prey to bad advice from his cornermen during the fight. When Sterling was down, Yan apparently asked his team whether he should hit him with the knee. Since Yan and his teammates spoke in Russian, Nurmagomedov understood what was going on.

According to Khabib, Yan's cornermen asked him to go ahead and hit Sterling with a knee which ultimately cost him his title on the night.