During an appearance Tuesday on the Tonight Show with Jimmy Fallon, WWE legend Dwayne “The Rock” Johnson described the legitimate physical pain he endured in a particular scene for the upcoming movie The Smashing Machine.

Johnson, who plays the role of MMA legend Mark Kerr in the film, said he suffered a concussion during the scene that portrayed Kerr’s first career loss (beginning at the 9:40 mark of the video):

The Rock noted that Kerr’s 2000 fight against Kazuyuki Fujita in Pride Fighting Championships is featured in the movie. In that particular fight, Kerr absorbed a ton of punishment and lost by unanimous decision, marking his first defeat in MMA.

Johnson said that both he and The Smashing Machine director Benny Safdie made it clear to the actor playing Fujita that he needed to actually hit The Rock in order to properly pay homage to the fight.

Despite his respect for The Rock, the actor agreed, and Johnson said he was knocked “loopy” and had a legitimate concussion as a result.

However, The Rock suggested it was all worth it, saying, “If you’re going to make an MMA movie, you’ve got to go all the way. And that’s what we did with this.”

The Rock has been among the biggest actors in Hollywood for several years, but every indication is that his portrayal of Kerr could prove to be the best performance of his career.

He went to great lengths to ensure the success of the movie, telling Fallon that he put on 30 pounds in order to look as much like Kerr as he possibly could.

That allowed Safdie to use Johnson in fight scenes rather than bringing in a stunt double, adding to the authenticity of the film.

Arguably the most highly anticipated MMA movie of all time has nearly arrived, as The Smashing Machine will officially hit theaters on Friday.