For Carwin, the problem will result in his second major surgery in a year. On November 15, doctors will perform a procedure that will drill out bone and give his spinal cord some room to keep a disc from pressing against his nerves.
The surgery is scary, but as Carwin tells it, not as scary as the moment that led to it.
He had noticed that anytime he was doing jumping exercises, his legs would lock up briefly, but he reasoned that his muscles were just taking longer than usual to warm up. But on one recent day, he was working on wrestling takedowns, and suddenly fell flat on his back. It was at that point when he realized that his legs had gone completely stiff.
"Honestly, I thought I was almost paralyzed," he told MMA Fighting.
The feeling came back about 15 seconds later, but Carwin knew something was very wrong with his body. A follow-up MRI revealed damage that put the former UFC interim light-heavyweight champion on the shelf and in a predicament. His doctor gave him two options. Either he could go through surgery, or he could retire.
While it wasn't a snap decision, it wasn't a hard one either.
After consulting with his doctor, Carwin (12-2) spoke with both his wife Lani and manager Jason Genet. To him, it sounded like the surgery was inevitable at some point as his symptoms weren't likely to just go away on their own. It was possible he could retire and need the procedure a year down the road, so, he reasoned, why not just do it now and continue on with his fight career? It's not like he's afraid of the hard work he'll have to put in to return. This is the guy who nearly won a UFC championship while holding a fulltime job as an engineer. If anyone was going to embrace this uphill task, it's him.
"If I didn't believe I still had a title run left in me, I would retire," he said. "I feel like I still have some things to prove. I have unfinished business. I love the doubters."
Carwin is not likely to return to the octagon until around the summer of 2012. At that point he'll be 37 years old, coming off two surgeries in less than two years, and looking to break a two-fight losing streak. So there will likely be many doubters to be found.
It wasn't long ago though when he was the next big thing, an undefeated 12-0 with 12 first-round finishes heading into his heavyweight championship fight with Brock Lesnar at UFC 116. In that bout, he seemed on his way to lucky 13, punishing Lesnar in a first-round barrage that nearly caused a stoppage on more than one occasion. But Lesnar withstood the ferocious onslaught and rebounded with a second-round arm-triangle submission.
After surgery in November 2010 that addressed neck and disc damage that was a result of wear and tear throughout his athletic career, Carwin came back almost a year after his last fight, ironically replacing a recently dethroned Lesnar in a No. 1 contenders bout with Junior dos Santos at UFC 131. The performance undeniably exhibited Carwin's toughness (he was outstruck 104-22 according to FightMetric), but for him, it ultimately proved to be the most disappointing of his career.
Carwin offers few excuses for the loss, saying that it's the one fight he'd like to take back.
"The Junior fight just wasn't me," he said. "I don't think mentally it was me. When I went out there, I didn't press and get in his face like I typically would. That's one time to perform, and I just didn't perform to my best. I could sit here and look at things here or there I could have done better, but the fact of the matter is that I didn't perform to my best."
That memory will at least partially motivate Carwin forward as he works though rehabilitation and finds his way back into the gym. At least for a short time, it had already been doing so. Shortly before his injury occurred, the UFC had given Carwin an indication that he would be on the year-end event at UFC 141, and he had been in the gym gearing up his training.
By the time he returns a few months from now, the division is likely to look quite different. A title fight with Cain Velasquez hoping to defend his belt against dos Santos is on the horizon, and at the end of the year, Brock Lesnar and Alistair Overeem will tangle in a bout that could produce the next top contender.
"The heavyweight division is exciting," he said. "I can see any of those four guys holding the belt. It sure makes for some exciting matchups."
But there's even more depth likely on the way. Top 10 heavyweight Fabricio Werdum is likely to be shipped in soon, and by the time Carwin returns, Strikeforce might have folded up shop and brought its talented big men to the UFC's octagon.
Carwin has no predictions on the world he'll walk back into when his body is healed up, but he knows the division has its challenges.
The only challenge for him now is getting healthy. Doctors have told him that he should return to "as normal as whatever athletes can be" after years of wear and tear. There is no guarantee, just the promise in his mind. Asked if he has anything he'd like to say about his future, the soft-spoken heavyweight laughs. There will be no big proclamations forthcoming.
"I appreciate being part of the sport and the opportunities it's given me in life," he says.
And with that, Shane Carwin goes back to work.