Christina Aguilera does up to 500 sit-ups a day.

The singer is preparing for her role in new musical film Burlesque, which will see her frolicking around in skimpy costumes.

Christina is determined to ensure her body looks toned and terrific, so has asked her personal trainer Tee Sorge to increase the intensity and length of her workouts.

"She's tremendously happy and feels really strong," Sorge told In Touch Weekly magazine. "She lifts weights two to five times a week. We want to make sure she doesn?t lose her muscle mass or too much weight.

"We work the entire upper body by doing everything from bicep curls to tricep dips. Christina does three sets of each exercise and uses weights ranging from five to 15 pounds. Her favourite thing is sit-ups. She does 300 to 500 sit-ups a session."

Sorge has also helped Christina improve her diet. While he admits she has always been "good" about what she eats, the star is now following a strict diet.

"We took out all the whites, like white rice and white sugars," Sorge said. "She?s sticking to whole grains and lean proteins."