But routines involving high-impact maneuvers -- particularly if adopted too quickly -- could spell both short-term pain and long-term trouble.
"The dangers may come in when one does explosive movements or high-impact activities," Woodruff said. "Some individuals' joints may not tolerate those types of activities."
And a good workout shouldn't push the body to the point of exhaustion every day.
"You shouldn't feel like you want to throw up or go back to bed," said Hollywood personal trainer Valerie Waters, whose clients include Jennifer Garner and Cindy Crawford and who generally does not structure her routines to run longer than one hour. "There needs to be some balance."
Michael Dupper, assistant professor in the Department of Health, Exercise Science and Recreation Management at the University of Mississippi, agreed that musculoskeletal and overuse injuries await those who try to do too much, too soon -- a strategy he terms the "fast food" approach to fitness.
"Most folks who use the 'fast food' approach to exercise usually do not look at it as a permanent behavioral lifestyle change, but a short-term 'wedding preparation' or 'class reunion' approach to getting in shape for one significant event in their lives," he said, citing Oprah Winfrey's high-profile training regimen in preparation for her run in the 1994 Chicago Marathon.
So, what does Dupper think of Spears' three daily workouts, complete with a 1,000-crunch quota?
"Great idea; however, if you are not used to three daily workouts, it can be brutal, physiologically and mentally," he said, calling the regimen a "short-term solution which would be almost impossible to maintain for any great length of time."
Dupper was somewhat more forgiving of Paltrow's killer regimen, as long as she has been able to gradually work herself up to its strenuous demands. But as for Madonna's routine, he said some discretion may be in order.
"Madonna is 50 years old, I believe," he said. "I would have to be concerned with amenorrhea [absence of menstrual bleeding] and osteoporosis based on these workout schedules."
Granted, these routines aren't for the common man or woman. But then, the common men and women aren't scrutinized by their peers, the press and the public on a 24/7 basis. The pressure to be in top form is incredible, not unlike that placed on a professional athlete. And maybe that's how everyone ought to think of them.
"I don't think it's fair to call out Madonna because clearly she's not held to the same standards as the rest of us," Waters said. "We have to acknowledge that what she's doing on stage is hardcore. It's a highly aerobic routine done in high heels that goes on not just for one night, but for months, and for hours each night. You've got to consider her as a pro athlete in her 50s."
So, Madonna: Keep kickin' and setting the bar for Hollywood's hardcore elite.