JUSTIN Clarke has spoken emotionally of his devastation at being forced into retirement because of concussion.

The young Brisbane Lions defender copped a knock at training more than two months ago, and after losing parts of his memory and barely being able to exercise since, made the announcement on Thursday.

After seeing three independent specialists who all recommended he retire, the decision was equal parts easy and heartbreaking for the 22-year-old.

Any small knocks in the future would lead to the same symptoms, or possibly permanent damage, Clarke was told.

As in his 56-game career, Clarke was resolute when fronting reporters on Thursday morning, but fought back tears as he discussed the process.

"I'm devastated that not only will I never be able to play footy again, but I won't be able to play contact sport again either," Clarke said.

"The worst thing is I won't be able to go out with my mates and experience that feeling of comradeship and all that it entails in the future.

"It's something I guess as a 22-year-old you shouldn't really have to worry about."

The South Australian, taken at number four in the 2011 NAB Rookie Draft, said he could barely remember anything from the first three weeks after the incident.

An academic star who scored 99.95 per cent on his Australian Tertiary Admission Rank, Clarke is a pilot and studying aeronautical engineering.

He said the loss of memory scared him more than anything.

"It's been an emotional time, particularly when I was first told about that memory aspect," he said.

"The scariest thing is I was on my way to uni and I completely forgot the route I was taking.

"It takes me through the Gabba area, and I should know where I was going – what I was going to do, the roads I was going to take – and that's the type of thing I've always been good with.

"I found myself at an intersection and I had no idea where to go.

"I didn't know whether I was meant to go straight or right or how I was going to get there.

"That scared me a lot. I didn't like that feeling because it's not a feeling I've ever experienced before.

"That really rattled me."

Clarke said he had been advised he should regain his full faculties, but for the time being he is working at half capacity and can only do a brisk five-minute walk before getting tired.

He has to make lists to compensate for his sketchy memory.

After Jonathan Brown and Matt Maguire, Clarke is the third Lion in the past two years to be forced into retirement because of concussion.

He said he loved the sport, had no regrets and would do it all again.

"I think the game itself in terms of the processes around concussion is fantastic," Clarke said.

"Footy's a great game and it's been a risk I've always been willing to take. It's such a minimal risk and it's just a freak accident that's occurred."

Coach Justin Leppitsch stood alongside his defender for the 15-minute press conference, saying Clarke's health came before anything else.

"It's been a hard road for Clarkey … to say goodbye to the game you love," Leppitsch said.

"I went through this with Browny, but Browny had won three premierships and been a star of the game and fulfilled his dreams.

"It's a little bit different, whereas Justin has missed out on that, he's lost that opportunity to achieve those things and that's what makes it sad.

"He was going down the path to do that and his career has been cut short.

"That does make it all the more sad.

"I think we all know Justin has got a bright future off the footy field, so that's the silver lining."

Clarke said he would pursue his studies, which he hoped to complete in four years before possibly travelling abroad to work in his chosen field.

He said the finality of telling his teammates on Thursday morning had been the most difficult.

"I've loved playing AFL footy," he said.

"Being able to experience the highs and lows that entails has been something that has been really special.

"I just wish it hadn't been prematurely cut off like this."