The following is an excerpt from Bounce Your Body Beautiful: 6 Weeks to a Sexier, Firmer Body by Liz Applegate, Ph.D. Prima Publishing, © 2003. Used with permission.

Exercise balls are not only fun, they're fast becoming the hottest trend in fitness. In this workout you'll learn easy-to-master exercises that will firm your belly, butt, thighs, and every other inch of your body, in just 30 minutes a day.

• Find out 4 reasons why fitness ball workouts work

• Learn 3 things you need before you begin

• Try weeks 1 and 2 of the 6-week workout

4 Reasons Why Fitness Ball Workouts Work

Why does the fitness ball provide such amazing results for so many women? Let me count the ways.

1. Fitness balls provide a more efficient workout compared to weight machines and dumbbells. In fact, a recent study from the University of Waterloo in Ontario, Canada, found that people who did curl-up abdominal exercises on a ball rather than on the floor doubled the tension on their rectus abdominus -- the large muscle that forms your "six pack" -- and quadrupled the activity of their obliques -- the muscles that form the sides of your abdomen and waist.

The secret? The inherent instability of the ball's round surface. When you work out with dumbbells and a bench or with exercise machines, you primarily use only the muscle you are focusing on. When you do a biceps curl, you only work your biceps. But when you do that same move on a fitness ball, you must use the muscles in your legs, abdomen, buttocks, and back just to stay balanced. These muscles keep you from falling off the ball. With no other exercise equipment do you work so many muscles at once.

As recent ball convert Julie in Pennsylvania put it, "No other exercise has ever worked my butt while I'm targeting my chest or upper body."

2. Fitness balls encourage you to be consistent. Because you can keep your fitness ball at home, deflate it and pack it in a suitcase, and generally use it anywhere you find yourself, you'll never have an excuse to skip a workout.

"I'll be taking my exercise ball with me on vacation this summer, since it's so simple to transport and the exercises are so easy to remember and fun to do," said my good friend Karen, a veterinary intensive case unit nurse. "It's portable, economical, user-friendly, versatile, fun, and fabulous. I love it!"

We've all seen how often people set up their home gyms in a dreary basement or garage, not because these dark, lonely places inspire fitness, but because a weight bench and dumbbells take up a lot of room and don't really go with the living room decor. Not so your fitness ball. It fits in just fine anywhere, allowing you to do your workout in the living room or even in the backyard. On a beautiful day, you can roll the ball outside and do your moves as you breathe in the fresh air.

My friend Alisa does her fitness ball exercises while she watches Oprah. "I get a better workout than I otherwise would at the gym," she tells me. "At the gym I'm always thinking, 'How fast can I get this over with?' But when I'm at home, I tend to add extra exercises or do more sets or reps in order to drag out my session to match the full hour of the Oprah show."

3. Everyone benefits from using fitness balls. There are no cumbersome weights to maneuver. Anyone -- from an 80-year-old woman to a young child -- can exercise with a fitness ball. And, instead of hoisting heavy barbells and dumbbells for resistance, you work with small "heavy balls" that everyone can lift. These "heavy balls" are handheld, water-filled balls that offer lighter resistance than other weights, thus making your workout safer. These smaller balls also increase your grip strength. They allow you to put your muscles through a greater range of motion, and they encourage you to complete more repetitions at a lower weight, a big factor in sculpting the long, lean muscle look that many women seek.

4. Balls make working out feel fun and natural. Fitness balls and the handheld balls bring back fond childhood memories, whereas dumbbells tend to make women feel unfeminine and tense. Just about every kid in America has bounced or thrown a ball, whereas few women grew up with a set of dumbbells.

As one of my clients, Lori, put it, "It's a blast. I just feel good balancing on the ball."