Retired NBA great Tim Duncan is known for a few things — his five NBA championships, his two MVP awards, his three NBA Finals MVP awards. But the 19-year veteran who played his entire career in San Antonio probably is best remembered for his calm demeanor and quiet approach to the game.

Lace up the gloves and get him in the ring, though, and Duncan is a whole different person. At least, that is according to his martial arts trainer, Jason Echols, who gave a lengthy interview to Bleacher Report about training the Spurs legend as a kickboxer.

“He’s an animal,” Echols told the website. “It’s a different thing than what you see out on the court. The more intense the training gets, the more calm he becomes, which you’ve seen on the court. The way he executes the movements, he’s a f–king monster.”

Duncan first began training with Echols in his San Antonio-area gym around 2008, when Duncan was 32 and in the middle of his sure-to-be Hall of Fame career.

The fact Duncan still was striving for NBA titles at the time apparently did not stop him from diving head first into his new hobby.

“When he walked in the door, I could see that he was very hungry to learn martial arts, and his movement, his absorption of the martial arts was just phenomenal,” Echols said. “You could really see the athleticism that existed in him already. It caught on to these movements, and he was able to absorb them better and pick it up better than the average person because of his athleticism. … It’s incredible to watch him move. Now the world is seeing it.”

That athleticism comes with a few caveats, not for the 6-foot-11 Duncan but for his 5-10 trainer, who said even when Duncan is not going at full power, it is difficult to get hit by him.

Echols has even suffered the occasional injury just from the sheer force of Duncan’s size and power.

“One time, I tore my bicep just trying to move with him at a higher-level pace,” he said. “He’s so big and his elbows are like spears and his bones are really sharp. Just interacting and punching with him, I hit him with a right hook and his elbow caught the inside of my bicep and tore it right off just from moving around.”

So what’s the solution? Obviously to bring in someone his own size — someone like fellow Spurs legend David Robinson.

“I’ve had the pleasure of working with David Robinson, too, and he’s another incredibly humongous, crazy, amazing guy that’s been coming through,” Echols said. “They’ve done a bit of [sparring] already. David is at a level where he’s not as advanced as Tim is. He started more recently than Tim, but to watch those two guys move around together is definitely a spectacular sight to see.”

Asked whether Duncan, whom Echols called a “roaring lion” more than once in the interview, could enter the octagon and compete in MMA, the trainer was more dubious.

“He would be a legit competitor, for sure,” Echols said. “He has a desire to learn and compete. He has the heart for it. He most definitely would be a competitor. I would highly encourage him not to. Being a retired Spurs player, I wouldn’t want him going out there and getting punched and kicked by some of those guys, but Tim would be a competitor.”

So, while you may never get the chance to see Duncan on a UFC pay-per-view card, if you’re in the San Antonio area and pop into a certain martial arts gym, you may get to watch a Spur spar. But maybe think twice before hopping on the mat with him.