Welcome to "Changing Habits," trainer Debbie Eisenstadt Mandel's exercise class designed for nuns.

"People think it's all harmony and serenity and gliding around calmly spreading the word, but nuns get very stressed ministering to people all the time and it helps them to work out," said the petite but very bouncy Mandel.

"We are not cloistered nuns - we are care givers up the gazoo, out and about the whole time," said Sister Peggy Tully, 60, who's still comforting the relatives of her eight parishioners who died on 9/11.

"We get e-mails, voice mails, people asking to see us, and we are expected to be signs of joy and hope - but I cannot be joyful without energy, so I work out."

Once a week, Mandel puts the sisters of St. Gregory's of Amityville through their paces at a Bellerose, Queens, health center, with an hour of stretches and, coming soon, dumbbells.

Then it is squats, push-ups against the wall and abdominal work, all done to relaxing, slightly ethereal music and shouts of "burn" or "wooooo!"

"I do exercise combos at home for my spine and arthritis, but it is much more fun doing it with Debbie and the other sisters," said Sister Annette Sledzaus, 86, clad in sweats and clutching a water bottle.

"It makes the time fly," she added, as she lifted a 6-pound medicine ball as easily as a prayer book, with Mandel's encouragement.

Mandel then has the sisters hold their arms out in a crucifixion style she calls "the iron cross," followed by a stretch she calls her version of the Resurrection, prompting groans of exertion from the class.

Confident the nun workout trend will soon spread across the country, Mandel is working on a book called "Changing Habits: The Sisters Work-Out," aimed at women of all professions and faiths.

"Nuns have problems with obesity, heart disease, diabetes and cancer - just like other women."

There was one notable difference from the average aerobics class, though: After the work-out, Mandel and the nuns piled into the next-door chapel for meditation beneath stained-glass windows.

"People say to me, 'Did you tell the nuns you are a Jew?' and I just laugh and say, 'I surrrrre did! I like to mix it all up," she says, shrieking with amusement.

Mandel's late father survived Auschwitz and her mother escaped the Holocaust by posing as a Polish farm girl, until she escaped to Italy and eventually move to the United States.

Mandel, who was born in a Catholic convent in Rome, then raised in Brooklyn, believes her parents' survivor spirit not only helped her thrive, but inspired her to energize others.

She also works with breast cancer patients and Holocaust survivors, stressed-out college professors and exhausted moms.

"Many women, not just nuns, give and give their energy and never say no until they become sick and tired," she said.

"Then the doctor prescribes pills - but there is a better way."