Now that the muzzle can come off, and Sean Sherk is retiring from the sport he’s been a part of since very close to its inception, he can explain some things. Things like why he turned into a boxer circa UFC 84 after being a grinding pestle for all those years before.



And, maybe more important, what took so long for the former UFC lightweight champion, who last fought at UFC 119 three years ago, to hang up the gloves?



"Well, it’s a hard decision to make," he told MMA Fighting after announcing his retirement in a statement through his Training Mask brand. "I’ve been a part of this industry almost since the beginning, and I wrestled competitively since I was seven years old. So we’re talking 25 years of competition, probably longer than that - walking away from something you love isn’t easy to do.



"I just think it was time for the door to be closed, time for me to move on with some different things. I know the injuries aren’t going away, they’re not going anywhere and they’re not getting any better. It was just time for some closure."



The culprit behind everything are Sherk’s hips. While training for his UFC 84 fight with B.J. Penn, he says he injured his hips and was never right again. That fight, on the heals of being stripped of his lightweight belt for testing positive for steroids after defeating Hermes Franca at UFC 73, saw Sherk undergo a shift in fight night philosophy.



Suddenly, the cardio juggernaut went from a relentless dictator of wills to becoming a vague boxer who was all too happy to stand and trade. The question that he was asked a million times since has been, "why did you stop shooting?"



"Now that I’m retiring I can tell the truth," he says. "I know it’s always been one of the top questions. Obviously I couldn’t say anything because otherwise opponents would pinpoint that stuff, but I had MRIs done [ahead of the Penn fight] and found out that both of my hips were torn and that I was going to need surgery. The doctor basically told me at that point in time, ‘if you have surgery on this you’ll never be 100 percent again - you’ll lose your mobility, you’ll lose your quick twitch and some of the explosion, and you’ll lose some of the agility.’



"And to me it just wasn’t worth it, so I said, you know what, I’ll just deal with the pain. I said, I’ll just deal with this as long as I can. And that’s what I did. Gradually over the years my hips got worse and worse and worse. About two weeks ago I was told I needed hip replacement surgery. So that was the deciding factor right there. I went from needing surgery to fix torn labrums to needing total replacement."



At the height of his career, Sherk -- who fought earlier as a welterweight -- was one of the most nihilistic, game-plan smashing wrestlers the game had known to that point. Standing a squat 5-foot-6 and with a reach of only 67 inches, he went by the nickname "the Muscle Shark," which was apt for a guy who could have been the original prototype for Urijah Faber’s Team Alpha Male team in Sacramento.



Through his first 15 professional fights he worked as a machinist full-time in Minnesota, and he brought to the cage with him that same sense of cold industrial production. He continued to work part time while facing some of the biggest names of his era -- names such as Matt Hughes, Nick Diaz, Benji Radach and Georges St-Pierre.



Like St-Pierre, he wasn’t a glorified collegiate wrestler, but he had the wrestling moorings from his high school days. He also had the dogged persistence of one, which he honed over the years through various wrestling competitions. In MMA, he was always willing to eat punches to drag the fight into his domain, which he did more often than not.



Sherk captured the vacant 155-pound belt against Kenny Florian at UFC 64 by taking him down punishing him for five rounds. The bout was as memorable for the blood as it was for his dominance. Florian cut Sherk open with a couple of sharp elbows from guard, but Sherk continued to plant his head into Florian’s chest and work his ground and pound. It was 25 minutes of vicious pace, and at the end of it Sherk found himself wearing the lightweight belt.



It was the crowning achievement of his career.



"Winning the UFC title, that right there changed my life," he says. "I was working a part-time job all the way up until I fought Kenny Florian for the lightweight title. I was still working -- I had to, there just wasn’t enough money in the industry to sustain having a house, having two kids, having the amount of bills that I have. I had to work between fights to pay my bills, and I trained my butt off to make sure I won those fights, because there’s obviously a big difference in pay when it comes to winning and losing.



"When I won that fight and won that world title, that just changed everything for me. That was the most definitive moment of my career by far."



The fallout was a fight later when Sherk defended that belt against Franca. After his victory, it was revealed that he had tested positive for the steroid Nandrolone, and the California State Athletic Commission suspended him for a year. Though he was able to get the suspension reduced to six months through an appeal, the UFC stripped him of the title.



Sherk lost his return fight against Penn via TKO. Yet, even with his hips deteriorating, he still appeared in the Octagon three more times. The silver lining of the injury was it gave him a chance to showcase his boxing skills in the twilight of his career, which paid off in more ways than one.



Not only did he go 2-1 in his final three fights in the UFC, he took home fight of the night bonuses twice -- in his victories over Tyson Griffin at UFC 90 in Chicago, and over Evan Dunham at UFC 119 in a razor-close decision. In between he dropped a unanimous decision to eventual champion Frankie Edgar.



"Those ‘fight of the nights’ are a big deal," he says. "I started boxing when I was a kid, and I’ve been doing this steadily since 1993. So, I had a lot of years of boxing under my belt, and it was good to show that because there was a long period of time where everybody called me ‘one-dimensional.’ I wasn’t one-dimensional -- I showed one dimension but I wasn’t technically one-dimensional. And it was gratifying. It’s nice to go out there and have 15,000 people standing up watching you fight. That’s really exciting."



Sherk retires with a professional record of 36-4-1. He says he accomplished just about everything he set out to do in MMA. Just about, that is, as there are a few things he came up short on.



"My game plan was to compete until I was 40, and my last competitive fight was when I was 37, so I didn’t accomplish that goal," he says. "The other thing was I wanted to win that world title one more time. I wanted to be one of the few guys in history who was able to win the world title twice, and I was not able to accomplish that goal either. But I can look back on my career and be happy with what I accomplished."



In retirement, Sherk says he’ll continue to work at the Minnesota Martial Arts Academy with his longtime coach and training partner Greg Nelson, who has been with him from start to finish over the last two decades. Other than that? He says he’ll stay as busy as he did when that cage door latched.



"I actually started flipping houses this year," he says. "So that’s kind of my new endeavor."