The 4 Minutes singer - who was photographed with shockingly sinewy arms last week, but later insisted the images had been digitally altered - achieved her shape by lifting weights in a room heated to 27C.
However, after a year on a programme used by dedicated bodybuilders, the 50-year-old star is now looking for a new fitness goal.
A source told Britain's Grazia magazine: "It's Madonna's birthday next week and she's showing no signs of taking it easy.
"There aren't many women her age who have a body like that but even Madonna may have to admit this time she might have to tone it down.
"She asked her trainer Tracy Anderson what she could do to stop her arms looking so sinewy. Tracy told her to do more stretching exercises to make them look leaner and Madonna has decided to follow her advice."
Despite her change of heart, friends believe Madonna - who previously ignored Tracy's advice to "go easy" on training - secretly likes her arms.
The source added: "I think that, aesthetically, there's a big part of her that likes the way her arms look. It shows she's in control."