Britney Spears has beefed up security and signed up for the hardcore Israeli combat techniques Matt Damon uses in THE BOURNE ULTIMATUM after growing paranoid following a batch of crazed fanmail. The pop star is reportedly terrified someone is out to harm her and her young sons Sean Preston and Jayden James - and she's taking steps to make sure she can take care of herself. A source close to Spears says, "She has beefed up security, informed them of such mail. She is doing her best to make a comeback but she is terrified about the negative attention she is getting." And now she's fighting fit after signing up for Krav Maga training. The insider adds, "She realises she does not have the stamina she used to, after having two kids. She is tired, her body does not work the same. She is putting in overtime into working out strength training, and she's serious about martial art Krav Maga." The fighting technique, which has been practiced by the Israeli special forces for decades, has become one of Hollywood's trendiest martial arts in recent years - Jennifer Lopez and Angelina Jolie have all recently studied Krav Maga for film roles like Enough and Tomb Raider. Fighters claim that as well as helping agility and toughness, Krav Maga also helps raise self-esteem and mental awareness. John Aldcroft, who teaches the martial art at London's British Academy of Krav Maga, says, "It's definitely perfect for Britney Spears because she obviously has a lot of aggression in her and she needs to get that out. Krav Maga will enable her to do that with a degree of control. "It will also help her with personal safety issues, especially when it comes to the paparazzi, who seem to crowd her at all times."