He may be a go-to action hero, but Sylvester Stallone is feeling a little fragile today.

Ordering a cup of tea "with a strainer", the Hollywood tough man reveals: "I've been complaining for the last 20 minutes because of my back."

Stallone, who rose to fame as the star of the Rocky and Rambo films, is back on screens as Expendables leader Barney Ross, in the third instalment of the successful action franchise. He took a hit from the "back blast" of a rocket-propelled grenade during filming, which required some surgery.

"I took a fall on my back and had some metal put in there, so if I'm squeaking, deal with it - it's not my shoes but my back. But I'm getting better," he says, laughing.

Sly, as he's often known, isn't as intimidating in real life as his screen personas. While he can beat up rivals, fly helicopters, drive tanks and fire weapons in movies, in person he's witty and warm, though it's clear he takes work seriously.

The 68-year-old actor jokes that he uses the "intensity" of his injuries to "grade the quality" of his movies.

"When I shot Stop! Or My Mom Will Shoot, I never got hurt," he quips of the 1992 box office flop. "Though Rambo and The Expendables, I break my neck and my spine and I dislocate both shoulders.

"I've fallen through a tree in First Blood," he continues. "I was really proud of the scene in Rambo III, where there was no CGI and we literally almost got hit by a helicopter on horseback. It's like a weird sporting event, when you do these kind of films. You know you're going to get injured."

But it was his 15-round hand-to-hand battle with Dolph Lundgren in 1985's Rocky IV that remains his proudest moment from his 40-year career. "I've got to say, dealing with Dolph Lundgren was brutal. Really brutal. He was so unbelievably powerful, he almost killed me."

For the third Expendables film, Stallone, who also co-wrote and co-produced the movie, has brought in some newcomers, all of whom he hand-picked.

Harrison Ford, Mel Gibson, Antonio Banderas and Wesley Snipes (making his big-screen comeback since serving a three-year sentence in prison for tax offences) join the returning cast, which includes Jason Statham, Arnold Schwarzenegger, Terry Crews and Lundgren, alongside a younger generation of stars; Kellan Lutz (better known as Twilight's Emmett Cullen), mixed-martial arts expert Ronda Rousey and former welterweight boxing champion Victor Ortiz.

"I felt like we were at a point where we needed an upgrade and retool, to bring actors who are known for their physicality but also are dramatic actors," Stallone explains. "I welcome them and think it's a necessary evil - let them get beat up for a while.

"With Ronda, there was some debate over how she's a fighter and not an actor. But I said, 'She's a new kind of entertainer, she's a hybrid'. And I could not be more right. This girl is one of a kind - she's very sexy, she's charming, yet still raw and learning."

Ford, who plays CIA head Max Drummer, proved the most challenging actor to sign up, Stallone reveals. "It's very time-consuming. It takes a long time to convince some people. You're calling them for favours on your birthday and on the phone for four hours," he says. "You're like, 'It's my birthday. Do me a favour!' You do a lot of begging, it's wild."

Gibson, who plays villain Conrad Stonebanks, was slightly easier to convince. "Mel wanted to know about the part. Originally I wanted him to direct. Everything is so personal with him and he said, 'I wouldn't mind playing a villain'. I said, 'Sold!'"

With 17 stars in the cast, there was inevitably some friendly rivalry on set.

"Everyone here has healthy egos and athletic ability - some are very good actors and you're going to be competitive," New York-born Stallone admits. "No one wants to be the one sitting there, looking like a piece of chewed string at the end of scene, so you do your best."

And with the poster for The Expendables 3 looking like a who's who of action stars, the bar has already been raised for the next film.

"To have a cast like this is unprecedented. It's a circus, it really is a high concept. And the thing now is, what's next? Do you keep raising the bar? Pretty soon, there'll be no room on the poster. It should be a fold-out poster," Stallone quips.

He's previously said he'd like Clint Eastwood and Jack Nicholson to join the gang, and Chinese martial arts star Jackie Chan and Dwayne 'The Rock' Johnson have also thrown their names in the ring.

Stallone would welcome them all. "Those two would be great additions. The Rock would be fantastic."

Crews, who plays barrel weapons specialist Hale Caesar, has described the star-studded cast as Stallone's "version" of Marvel's Avengers superhero team.

"It wasn't as if I said, 'We gotta go up against the Avengers', because The Avengers is a spectacular franchise. But this is our version, and you could branch off," Stallone explains. "The young Expendables could go on their way, Antonio's character Galgo could have his spin-off and Hale Caesar... And then you bring us back for the 'big' event against the major villain.

"That was the pipe dream. It may still come to fruition," he adds. "And we could face off against The Avengers - we'd be the earthbound Avengers!"

With a fourth film sounding like it's already in the works, the married father-of-five (one of his sons, Sage, died in 2012 aged 36) teases that the plan is to make another two or three, although he'd be very keen to go up to Expendables 10.

"Yep, you heard it here first! It all depends on this one yet, but I can see maybe two more. We'd like to continue because it's an on-going experiment."

Stallone has also confirmed a female-led version, The Expendabelles. The script is in its final stages, with filming expected to begin in 2015.

"With The Expendabelles, you've got a situation here where we're in uncharted waters. To put all women together, would that really work?" he says. "When we do it, it can't just languish there and use the (Expendables) name. It has to be something that would hold its own."

Mind you, Stallone has some work to do himself, when it comes to holding his own with modern gadgets and gizmos.

"The young ones are really good with technology," he says. "Me, I'm just learning how to work a clothespin [clothes peg].

"Paper clips baffle me. How can they possibly work?"

EXTRA TIME - THE YOUNG GUNS :: Kellan Lutz (Smilee) - Kellan, 29, best-known for playing vampire Emmett Cullen in the Twilight films, had to learn motocross skills to portray the former Navy SEAL who, despite his name, never smiles.

:: Ronda Rousey (Luna) - The only female of The Expendables, Luna was a nightclub bouncer before being recruited by Barney. In real life, Rousey, 27, is the world's top female mixed martial artist.

:: Victor Ortiz (Mars) - Welterweight boxing champion Ortiz, 27, makes his big-screen debut as the sharp-shooting, lightning-fast boxer and lethal soldier Mars.

:: Glen Powell (Thorn) - American actor Powell, 25, plays drone pilot and hacker Thorn, who is also a death-defying free-climber.

:: The Expendables 3 opens in cinemas on Thursday, August 14