The stretching area of your average health club these days looks more like a nursery school for grown-ups. There are weighted balls, enormous bouncy balls of various sizes, foam tubes and smaller, squishy balls. The machines and free weights can be intimidating enough, but the friendlier-looking balls just seem mystifying.

But these inexpensive devices can be very effective in strengthening muscle, improving balance and reducing muscle tension.

Dan Druckhammer, a physical therapist at Olympic Rehabilitation Specialties in Fremont, says that balls can help bring back the flexibility sedentary adults had as children.

"We tend not to challenge our body in that manner," he says. Balls and rollers, he said, can reawaken muscle strength. "Sitting in a car or desk chair doesn't provide that stimulation."

Here is a brief overview of how to work out, and relieve tension, using these playful tools.

STABILITY BALLS

What they are: These are the largest, bouncy fitness balls that are pumped full of air. Sizes range from 45cm (for those 4-foot-10 and under), 55cm (4-foot-11 to 5-7), 65cm (5-8 to 5-11), 75cm (6 feet and taller).

Used for: Improving balance, strengthening the body's stabilizers and relieving back tension.

How they work: Because you have to focus on balancing on the ball as well as doing whatever other exercise you're doing, you develop strength in the core stabilizers of the body — mainly the abs, the obliques and lower back.

How to use them: Almost any exercise can be advanced, simply by using a ball as a substitute for your weight bench or your chair. For example, you can sit on the ball and do shoulder presses or bicep curls, which engage the abdominal and other core muscles to keep balance while working the other muscles. They are also good for doing advanced crunches and abdominal exercises. Many desk workers have begun using balls as a chair (start slowly, gradually increasing your time sitting on the ball before you ditch your office chair entirely), which really works the muscles in your hips and low back. Some people enjoy lying backward over the ball as a way to stretch out your spine (think of the bridge position in yoga).

Special tip: Some fitness clubs, including the Ballard Health Club, are now offering "Ballates," a form of Pilates using the stability balls. Be careful of low-back pain during crunches; sometimes people need to start on the floor before they progress to an unstable position. If you are just beginning, start slow by just sitting on the ball to get the feel of it. Depending on your fitness level, you may want to be close to something or someone to catch your balance if needed. As with any new exercise, be sure to get proper instruction.

Available at: General sporting-goods stores and online fitness stores.

Price: $19-$40.

MEDICINE BALLS

What they are: Smaller and far denser balls, about the size of a basketball, which come in various weights, from 2.2 to 24 pounds. They have been used for fitness more than a century, but today's higher-tech versions are usually made of durable hard rubber.

Used for: Strength training.

How they work: Like free weights, they add resistance to a movement but are more versatile than weights.

How to use them: Medicine balls are easy and fun to use. They can be held with both hands while doing almost any exercise; for instance, holding one while doing squats increases the intensity of that movement. They may also be tossed.

What size to get: Depends on the exercise and how much resistance you want; better to start lighter and work your way up.

Special tip: The balls are especially good for people with arthritic hands and who have difficulty gripping dumbbells.

Available at: General sporting-goods stores and online fitness stores.

Price: Depends on material and size, ranging from about $10 to $90. Sold individually and in sets.

FOAM ROLLERS

What they are: Tubes of hard plastic foam. From 1 to 4 feet long, some are round and some are cut in half, leaving one side flat and the other side round.

Used for: Focused relief of muscle pain and tension, and for improving strength and balance.

How they work: Applying pressure to foam rollers mobilizes the soft tissue of the body to release tension.

How to use them: Rest your body weight on a roller, and then roll, then rest, over areas of tension — for instance, the calf or the hamstrings. To improve posture, use the foam roller on the mid- and upper back muscles, the chest muscles and under the arm on the sides of the chest (imbalances in these muscles can cause the shoulders to roll forward, creating a hunched posture). Then follow up the muscle manipulation with upper back and shoulder exercises to pull the shoulders back. Another move is to lie on the roller lengthwise (basically resting the length of your spine along the roller) to stretch your chest, straighten the middle spine for improved posture, and help realign postural muscles.

To improve balance and strength, use the rollers cut in half. Stand with one under each foot, and do squats while challenging your balance.

What size to get: 3 feet long with a 6-inch diameter is the most effective for large areas; also available in 1-foot-by-6-inches for smaller areas. A 4-inch diameter increases the amount of pressure. The 1-foot rollers cut in half are good for balance exercises.

Special tip: These may not be effective for releasing tension in the smaller muscles along the spine and around the shoulders and neck. These rollers can be too firm and intense; you may need to start with a softer device, like the Miracle Ball Method balls.

Available at: The Foam Shop, www.performbetter.com, www.spriproducts.com.

Price: $15-$30.

MIRACLE BALLS

What they are: Two squishy plastic balls, each a bit larger than a softball.

Used for: Gentle approach to muscle manipulation and reducing pain and tension. The balls are especially great for the neck, shoulders and rib cage, because they are more malleable.

How they work: They are gentle and small enough to allow the user to completely relax and to let their weight sink into the balls. This allows the balls to penetrate the muscles and release tension. These balls are not designed for exercises. In fact, it is coined an "un-exercise" program. Creator Elaine Petrone says, "There is a very clear distinction between releasing muscles and engaging muscles." She says that even yoga and Pilates, which have strong relaxation components, are still focused on engaging muscles.

How to use them: Place balls in areas of muscle tension (be sure that you feel comfortable with their placement), breathe deeply, and allow the weight of your body to sink into the ball. The balls come with an easy-to-follow guidebook.

Special tip: According to Petrone, the balls can be used as a way to stop "intellectualizing" the movements of your body and to really allow your body to feel. "Notice the shifts when the muscle tension releases," she says. "Your body will give you a tremendous amount of information if you let it." Avoid the sore backs and necks of long trips; the balls are small enough to pack in a suitcase.

Available at: Many bookstores, www.amazon.com and www.elainepetrone.com.

Price: Two balls and instruction book: $17.95.

TENNIS BALLS

Used for: Reducing muscle pain and tension.

How to use them: Put two tennis balls in a tube sock, tie the end, and lie down on balls on either side of the neck or along spine for tension relief (the tube sock softens the balls up a bit and prevents them from rolling away).

How they work: Muscle manipulation.

Special tip: Be cautious with the amount of pressure you apply; they are firmer than you think and could be too intense for some people. Tennis balls do not have much give.

Available at: Sporting-good stores.

Michele Kulbel is a Seattle-based writer and personal trainer: mkulbel@hotmail.com