The stability ball - an extra-large, inflatable orb designed to improve balance while targeting specific muscle groups - has grown in popularity since it's mainstream introduction in the late '80s and early '90s.

The stability ball can be adapted for many uses, including developing core strength, improving posture and facilitating stretching, among others. Its application is particularly widespread in the physical therapy industry, where it was first put to use nearly 30 years ago.

Thanks to fitness professionals' interest in the stability ball and its numerous benefits, there have been several exercise programs developed over the past few years for just about every need, desire and body part.

The stability ball and your core

So much of the exercise we do, such as running and cycling, focuses on the lower body. Not much attention is paid to the trunk, or core, of the body. It is the muscles of the core - the abdomen, chest and back - that stabilize the rest of the body.

Think of your core as a strong column that links the upper and lower body together. Having a solid core creates a foundation for all activities, and is especially important when you add a heavy load, such as weights to your workout.

It is important when you are strengthening the core that you create balance between the muscles of the abdominal and the back. Many people will naturally have an imbalance between the strength of their abdominal muscles and the lower back muscles. Exercising with stability balls helps to develop and strengthen those muscles.

Infomercials and magazine advertisements seem to be targeting the individual who wants to strengthen their abdominal muscles. Although end results such as ''toning'' and ''shrinking'' aren't totally accurate, a handful of the techniques the equipment supports are valid.

However, you don't need an ab rocker or a special track with handles to concentrate on this area. The stability ball is well equipped to help you concentrate on your capricious core.

Here are three exercises that can be performed with a standard stability ball and target all three sections of the abdominal muscles:

Supine trunk curl

Start with the top of the ball beneath the center of the back. Press the lower back into the ball and tighten the abdominals as you curl the rib cage toward the pelvis. Slowly return to the starting position.

Supine oblique curl

Start with the top of the ball beneath the center of the back, then stagger feet and rotate hips to one side. Anchor the lower hip to the ball and move the rib cage at a diagonal direction toward the legs (for example, right elbow to left inner thigh). Make sure your neck and pelvis are stable.

Forward transverse roll

Kneel on the floor and place your forearms on the ball, making sure your hips and arms form a 90-degree angle. From this starting position, roll the ball forward as you extend your arms and legs simultaneously. Contract your abdominals to help support your lower back, which should not be strained.

Roll as far forward as possible without compressing the spine, drooping shoulders, or rounding the torso. Return to starting position.

The benefits of balls

Besides providing balance training, stability balls work the trunk in almost every exercise that is performed. By concentrating on the abdominal section, your posture will improve and you will find that you are generally more balanced and aware of your body movements. Your core will be more prepared to support the rest of your body in whatever activity you choose to do.

How to choose a ball

It is important to buy the right size ball and maintain the proper air pressure. The firmer the ball, the more difficult the exercise will be. The softer the ball, the less difficult the exercise will be.

If you are just beginning, overweight, an older adult, or you are generally deconditioned, you may want to consider using a larger, softer ball. When sitting on the ball, your knees and hips should align at a 90-degree angle.

Following are general guidelines for buying the right size stability ball:

	Under 4’6” (137 cm): 30 cm ball (12 inches)
	4’6”–5’0” (137-152 cm): 45 cm ball (18 inches)
	5’1”--5’7” (155-170 cm): 55 cm ball (22 inches)
	5’8”--6’2” (173-188 cm): 65 cm ball (26 inches)
	Over 6’2”(188 cm): 75 cm ball (30 inches)