SCREEN tough guy Sylvester Stallone has a surefire way of knowing how good a movie is – by his physical state at the end of shooting. “Usually I grade the quality of a film by the intensity of the injury,” he says.

“I do Stop Or My Mom Will Shoot and I never got hurt. I do Rambo or The Expendables, I break my neck, my spine, dislocate both shoulders – and think, ‘this is a great movie’.

“So on this one I ended up really taking a fall on my back and had to have a metal plate put in there. So if I’m squeaking, it’s not my shoes it’s my back”.

That’s the likeable thing about Stallone, he’s never lost his sense of humour. He takes his work seriously as a writer, director and actor but can see the funny side even when the joke’s on him.

He and the producers are laughing all the way to the bank with The Expendables. This, his third franchise after Rocky and Rambo, assembles a bunch of old-time action hero actors and demonstrates that age doesn’t weaken their ability to punch, kick, fight and shoot with the best of them.

The first two episodes have taken half a billion dollars at the global box office. Now comes The Expendables 3 in which young blood is added to mix and the violence toned down to 12A standards but in which the old-timers still show they’re not ready to draw their pension just yet.

Stallone is back leading the gang with the likes of Arnold Schwarzenegger, Mel Gibson, Wesley Snipes, Antonio Banderas, Jet Li, Dolph Lundgren and Harrison Ford representing the old guard.

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British tough guy Jason Statham is back for a third round along with newcomers Twilight’s Kellan Lutz, the number one female martial artist in the world Ronda Rousey and WBC welterweight boxing champion Victor Ortiz.

A handful of Expendables are in London for the premiere and tell us about their injuries. Sometimes it’s difficult to tell if they’re joking as the conversation resembles locker room banter more than a serious Q&A.

“I got injured in the first take and carried on through the movie and didn’t say that to anybody because I didn’t want for them to think I was getting older,” says the actor who swashed his buckle as Zorro. “It was just running up the roof to the helicopter. On the first shot I get a pain in my right knee and now it’s gone.”

Sly turns to him and says, “That’s it, a pain in the knee? I wake up with that, it’s like breakfast”. Injuries are an inevitable part of the job with filming like a weird sporting event. “It’s like any sport, you’re going to get injured. There’s no getting away from it,” he adds. “Everyone normally shows up very fit and goes downhill from there. That’s the way it is.”

Making the violence less graphic than in past Expendables films was a deliberate decision to broaden the audience: “I thought also the amount of violence, the amount of warfare in this movie if it was graphic it would be nauseating, almost too much after a while. Even though I personally enjoy it, it would be pushing the envelope and also diminish the humour.”

The introduction of new faces was another deliberate act in recognition of the need to upgrade and retool the franchise. “So I brought in actors who are known for their physicality but also incredibly dramatic actors, nominated actors, actors that have done films almost in every genre.

“Also the age factor must be served so we brought in the new generation. It’s almost a parental thing. All children think they know the answers, get into trouble, the parents save them and then they save the parents. So it’s kind of a family situation.”

Banderas saw his contribution as bringing comedy to the story. “The character was designed in a way to allow me to do that very clearly. So I start playing with that idea in mind. I improvised a lot. He became a compulsive talker. This weapon of mass destruction of talking. “When I saw my fellow actors start rolling eyes I said, ‘yes I am there’. They wanted to strangle me, basically every scene.”

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There was some debate, admits Stallone, to bringing Ronda Rousey into the Expendables. “They said she’s not a fighter and I said no, she’s a new kind of entertainer. A lot of us are hybrids. We all have other sides to us. I couldn’t have been luckier or more right about Ronda. She’s very sexy and can also tie you into a knot and be charming. Ortiz is a real world champion. You have world champions and professional athletes in the cast.

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“I don’t think it could be accomplished if it was all actors. There are some skills there that take an entire lifetime to learn. We were blessed in getting to employ those skills in the movie.”

Plans are under way to expand the franchise with an all-female mercenary group called – what else? – The Expendabelles, with plans to start shooting next year.

Stallone relates an imaginary scenario: “Are the Expendabelles really part of my character’s divorce with Sigourney Weaver and she inherited half the Expendables. I lost my house and the mercenaries.”

Stallone imagines that, as in the Marvel world, Expendables characters could feature in their own spinoff films. And why not, he has a reputation for being right. When he started doing franchise films he was ridiculed for it. Now they’re the norm.

He’s also not afraid of a challenge, like squaring up to Dolph Lundgren in the ring for Rocky 4. “That was brutal, really brutal. He was so unbelievably powerful. I can’t describe it. He almost killed me. Cliffhanger was amazing because I hate heights and was 4,000 ft up a clif. I don’t even like being in cowboy boots, that’s too high. But the idea of sustaining a 15 round fight, that’s really one long stunt. That’s what I’m proud of because I know I could never come close to doing that again today.”

Like playing a team sport, having such a strong bunch of performers in one film leads to competitiveness. As Stallone says, “Nobody wants to be the one sitting there looking like a piece of chewed string at the end of the scene. You do your best.”

The Expendables 3 (12A) opens on Thursday, August 14