Though the muscles that can be displayed at the beach seem most important, like bulging biceps or killer quads, the frequently ignored muscles in your back and mid-section need attention, too, and exercise balls are just the way to give them a good workout.

"The muscles of the 'core' are the true workhorses of the body, as they are essential to posture, spinal alignment and support, balance, and all movement -- both everyday movements as well as exercise," says Jude Sullivan, a clinical exercise physiologist at the University of Wisconsin Health Sports Medicine Center. "The muscles of the core are located deep in your pelvis, abdomen, trunk, and back, and they can be targeted directly through the use of exercise balls."

Exercise balls, which are large, vinyl balls with varying degrees of inflation, are also known as Swiss balls, physioballs, spine balls, or theraballs. They are inexpensive, easy to use and, unlike other forms of exercise and stretching, they can be fun.

On the Ball

"Exercise balls are used mostly as a support for strength and stretching exercises," says Richard Cotton, a spokesman for the American Council on Exercise. The kicker, he says, is that "the instability of the support activates muscles that would normally not be stimulated by traditional, well-supported exercise positions."

By introducing the concept of instability into an exercise or rehabilitation program, exercise balls find the core muscles in the body and put them to work by forcing a person to maintain balance and proper posture while doing an exercise -- or risk falling off the ball.

"Exercise balls challenge your core strength," says Sullivan, who is also a certified strength and conditioning specialist. "When sitting on an exercise ball, you suddenly become aware of your trunk area by virtue of sitting on an unstable surface. This increased awareness, in theory, will educate the user to better use their core muscles to maintain their position, essentially counteracting the instability of the ball."

Mastering the Bounce

When first attempting to sit on an exercise ball, it may look easy, but it's not. First-time users should get help from an exercise physiologist, certified trainer, or physical therapist to learn how to properly use the ball. But the good news is that exercise balls come with a low-risk for injury.

"Exercise balls are very forgiving," says Sullivan. "You can't break them, and more importantly, they won't likely hurt the user."

To begin, you should find an exercise ball that is appropriate for your size.

"The strength of an exercise ball is really important, as is the size of the ball," says Cotton.

Exercise balls come in a range of sizes; to be appropriate for a person's size, it should allow the hips and knees to relax at a 90-degree angle, while the back is in a neutral position.

According to Spine-Health.com, to find a "neutral lumbar spine position":

• Slouch slightly while on the ball by rounding the lower and upper back.
• Begin bouncing lightly.
• Allow your body to automatically find your straightened posture, which will allow you to maintain your balance and continue bouncing. The straightened posture, which occurs during the "up bounce," is your neutral lumbar spine position.

When you feel comfortable practicing "the bounce," you've found your center of gravity, and even changing it slightly, according to Spine-Health.com, will require you to correct it to stay on the ball. This exercise challenges core muscles and is a good introduction to using an exercise ball.

When you've got this down, you can introduce more advanced exercises into your regimen.

Exercise Ball Exercises

There are a variety of exercises that can be done on the exercise ball, from spine rotation exercises, to mobility and stretching techniques, to pelvic isolation exercises.

"A crunch is a good start for the exercise ball," says Cotton.

Simply lie back and rest your calves on the ball, and do a stomach crunch. The difference between doing a ball crunch and a crunch on the floor is that the ball crunch forces you to maintain balance while you perform the exercise, targeting the abdominal and back muscles more directly.

"Also, a squat rolling the ball down the wall, with the ball between your back and the wall, targets your leg, back, and stomach muscles," says Cotton.

Other exercises include doing a push-up with your hands on the floor and your legs extended on the ball, and leg lifts with the ball squeezed between your ankles. Keep in mind, though, that even sitting on the ball or practicing the bounce is beneficial since it forces proper posture and maintaining a center of gravity.

Ball Beware

Before hopping on and going for a roll, there are a couple of things to be aware of when using an exercise ball.

"If a person has an issue related to a sudden, unexplained loss of balance, such as sudden blood pressure changes potentially leading to light-headedness or dizziness, he or she should avoid using an exercise ball," says Sullivan, since balance plays a key roll in this device.

Another rule to follow when using an exercise ball is to use it on a soft floor.

"Don't use the exercise ball on or near any hard surfaces like a concrete or tile floor, which could cause injury if you fell off," says Cotton. And, as with any exercise, "Progressing gradually in the use of the exercise ball is very important."

On a Roll

Exercise balls allow you to target muscles that are seldom used but essential to health and fitness.

"The ultimate goal of the exercise ball is to challenge core strength," says Sullivan.

With a stronger "core," overall athletic ability is enhanced, and with that comes better posture and balance.

With a cost of between $10 and $200, depending on size and what comes with the exercise ball, such as videotapes, air pumps, or other equipment, it can be a bargain compared with costly exercise machines. Better yet, they are easy to use and remind you that there is more to your body than a six-pack and shapely calves.

Don't Forget Medicine Balls

Medicine balls are similar to exercise balls. Smaller in size and heavier in weight, they are used to strengthen the core of the body.

"Medicine balls, also called heavy balls, promote muscular strength and endurance, work the core muscles, and can simulate some athletic moves," says Cotton. "They can target the forearms, biceps, triceps, and support muscles, including the abs, upper and lower back, shoulders, and legs."

As with the exercise ball, work with a certified trainer, physical therapist, or exercise physiologist when you are first beginning to use the medicine ball.

A good starting exercise is to find a partner and simply play catch with the medicine ball. This strengthens the arms, and chest, and back area.