The disabled aren't the only ones to benefit from water exercise.

Aquatic exercise is one of the fastest growing segments of the fitness industry, and a new wave of "vertical" water workouts is sharing pool space with traditional "horizontal" swimming programs.

"Ten years ago, water exercise was pretty much for older adults," says Julie See, president of the Aquatic Exercise Association, a Florida-based organization that has 18,000 certified aquatics-fitness professionals, compared with just 1,000 in 1992. "Now you'll find people of all ages and ability levels doing all kinds of workouts in the pool."

Among the most popular, she says, are classes such as aquatic kickboxing and aquatic boot camp, deep-water running wearing a flotation belt, water walking and Eastern arts such as tai chi and yoga.

Also, in recent years, sports-medicine experts have embraced aquatic exercise as an ideal way for injured athletes to maintain cardiovascular fitness while rebuilding strength and flexibility in a low-stress environment.

While water workouts typically are easier on the body than land-based exercise, aquatic activity can be as strenuous as you want to make it, says Pat Cook, aquatic-program supervisor for the Fairfax County (Va.) Park Authority.

"Water itself is like a piece of exercise equipment, because it provides about 12 times the resistance of air," Cook notes. Just as your muscles get stronger from working against the resistance of an iron dumbbell, working against the resistance of water also has a strengthening effect.

Water workouts offer a built-in symmetry, Cook says, because every forward pull has a backward pull.

"This means you naturally get a balanced workout in the water," she notes. For those who want an extra strengthening effect, Cook recommends adding resistance with innovative new "pool toys" designed for water workouts, such as webbed gloves and aquatic dumbbells.

Plus, she says, "In a land-based aerobics class, there's a lot of pressure to keep up, but you can hide in the water and just do as much as you're able to without anyone knowing."

Water's buoyancy counters gravity's downward pull, "which takes the weight and strain off your joints," notes William Bullough, aquatics director for the Montgomery County (Md.) Recreation Department. "This makes water exercise low- or no-impact, depending on how deep you go." And, in summer's heat, you can get a super workout without breaking a sweat.

One of the simplest, fastest-growing forms of aquatic exercise is water walking, says John Spannuth, president of the United States Water Fitness Association in Boynton Beach, Fla.

Water walking is as easy as putting one foot in front of the other, say aquatics experts, who offer this advice:

How to water walk

-- Consider wearing shoes made for water walking or use an old pair of sneakers to protect your feet from the bottom of the pool.

-- Warm up by moving around in the water slowly and stretching gently for about five minutes.

-- Pick a depth that is challenging, yet enjoyable. Chest-deep water is recommended, but if this is too difficult, walk in hip- or waist-deep water.

-- Don't lean forward. When walking in water, your ear, shoulder and hip should be in a perpendicular line to the water's surface.

-- Vary how you walk. For example, walk forward, backward and sideways, or march with knees high.

-- Move your arms through the water for added intensity.

-- Avoid the tendency to stay on your tiptoes, and be sure to walk all the way through your foot. When walking forward, step heel to toe. When walking backward, step toe to heel.

-- Increase the intensity by making bigger or faster movements or by adding resistance, such as by cupping your hands instead of keeping them open or in a fist.

-- Recognize that water's cooling effect may result in heart rates that are about 10 to 15 beats lower when exercising in water compared with the same effort on land.

-- Cool down with stretching and easy movements in the water for about five minutes.<>