Exercise junkie: Madonna's body shows signs of strain

But Madonna, 48, has never been one to duck a challenge.

She is pulling out all the stops to reach a physical peak for her headline spot at the Live Earth concert next month. And her efforts are showing.

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Rippling fit: Madonna shows the effects of her rigorous regime

The honed physique she has maintained for years looked more muscular than ever as she dined out with friends in New York, showing off her bulging biceps in a vest-top outfit.

The physical contrast to the 25-year-old who rocketed to stardom in 1984 was stark.

Friends say she feels under huge pressure to give an impressive performance at Wembley Stadium on July 7, where she tops the bill at the British leg of a string of concerts across the world to raise awareness of global warming.

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Madonna's arm is covered in large veins, which can be a sign of too much exercise

"The state of the planet is something she feels very strongly about," said a friend. "She feels that at Live Earth she will be speaking to the planet. And she obviously wants to look her best.

"The eyes of the world will be on her and she is very conscious of her advancing age. She has thrown herself into the fitness regime like never before."

Sources say Madonna has been using the so-called gyrotonic expansion system, a wooden contraption of levers and pulleys which elongates muscles. She also practises pilates and yoga and does between 200 and 400 sit-ups a day.