IN THIS ISSUE: Watch Out, Jen! How Angie Got Fit to Kick Butt

Jennifer Aniston may be getting into shape to steal Brad Pitt back, but she'll have Angelina Jolie's Salt muscles to contend with!

OK! GALLERY: ANGIE TOTES THREE OF HER BROOD TOY SHOPPING

For much of the nine years since Angelina played a sexy action hero in Lara Croft: Tomb Raider she's been criticized for being stick thin.

But now, the 5-foot-7-inch superstar is back in fighting shape thanks to her physically demanding role as CIA agent Evelyn Salt.

OK! GALLERY: GOLDEN COUPLE BRAD & ANGIE GLOW AT SALT PREMIERE

While playing Salt, who is accused of being a Russian spy, Angelina endured stunts that required her to run, jump, climb, ride a motorcycle, fight off enemies and handle heavy weapons — most of which she performed without a stunt double!

To prepare, Angelina studied martial arts, including Muay Thai (a kickboxing fighting form from Thailand) and Krav Maga (hand-to-hand combat). Her best fighting moves were combined to create the film's action sequences.

OK! NEWS: LIEV ON ANGIE'S ETHIC — "SHE WORKED HER BUTT OFF"

Besides learning to fight, Angelina adjusted her diet (she carbo-loaded!) and worked with stuntwoman and trainer Eunice Huthart in the gym to build muscle. Angelina did tw0-hour weight-training sessions up to five times a week.

No wonder Salt co-star Liev Schreiber was impressed by Angie's fitness. "She's very, very tough to chase," Liev confessed.

FOR MORE ON ANGELINA JOLIE AND HER WORST NIGHTMARE AS JENNIFER ANISTON GETS THE ULTIMATE REVENGE, PICK UP THE NEW ISSUE OF OK! IN STORES EVERYWHERE THURSDAY!