Robyn Mendenhall Gardner was amazed when what started off as a monthlong ab workout challenge between friends and family caught fire on the Web.

The Montana mother of eight told Shots she came across a 30-day ab fitness plan online and, after having a tough time finishing it, turned it into a Facebook event to motivate herself.

Within days, the Facebook challenge went viral. At last count, more than 2.7 million people had signed up. And Gardner's challenge attracted attention from major media outlets, including Good Morning America.

The challenge features a daily series of progressively longer sets of crunches, planks, sit ups and leg lifts. Participants have taken to the Facebook event's wall, reporting their progress and encouraging each other to stick with it. The challenge lasts through the end of June.

But, don't count on the 30-day challenge to give you the abs of Heidi Klum or Channing Tatum in a month.

Why not?

"You can't selectively burn body fat," says Dixie Stanforth, a member of the University of Texas, Austin's kinesiology faculty and the Fitness Institute of Texas who has worked as a personal trainer for 26 years. "If you could, people who chew gum would have really thin jaws."

What it will do, though, is tone the abdominal muscles, and it might serve as an exercise starting point for people who aren't otherwise very active, Stanforth and other fitness experts tell Shots.

"To become leaner, abdominal work would be a piece of that puzzle," says Brian Focht, an associate professor in the human sciences department at Ohio State University. "But you'd have to add other components — such as more well-rounded training, aerobic activity and changing dietary habits — to really lose weight."

The challenge might be too strenuous for some people. Stanforth cautions that particularly the crunches and sit-ups could be hard on a person's back. Similarly, Focht says people might need to modify the workout to suit their needs and ability levels.

Gardner, who recently lost 59 pounds, echoes a similar warning on her event page. She's careful to note that participants might need to modify the exercises or consult a doctor. She also encourages participants looking to lose weight and strengthen their core to integrate diet and additional exercise into their routine.

"Just being able to do as much as you can is an accomplishment, especially for people who haven't done a crunch or a sit-up in 20 years," she tells Shots.

Overall, she says she has been overwhelmed by the response. She says she has received thousands of messages, some from as far away as Nepal, from people seeking advice.

Gardner says she hopes the challenge helps her tone up and encourages her to continue daily ab workouts.

Indeed, for those who complete the program, Michelle Segar, director of University of Michigan's Sport, Health and Activity Research and Policy Center, says the trick is building on the success. "In general, when you ask people to do a short-term activity, you're going to get a short-term behavioral result," says Segar, who researches fitness motivation.

She suggests that participants consider ways the competition improves their daily lives. Does it give them more energy? Do they feel stronger and more confident?

Encouraged by the attention her 30-day challenge has drawn, Gardner hopes to host other challenges that focus on different muscle groups. For now, though, Gardner is working to complete the task at hand. She says she has stuck with the challenge so far, and she plans to see it through.

"How can I not be motivated now?" Gardner laughed.

is a writer and editor who lives in Washington, D.C.

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