Some say it started with horses. For decades, trainers have used ponds and pools to give thoroughbreds a superb cardiovascular workout without putting stress on the animal's legs.

In recent years, professional athletes have used the rehabilitative virtues of water exercise to maintain their conditioning while recovering from injury. Some, like the Washington Redskins, use sophisticated Aqua Arks -- 8-foot-deep tanks with water jets, fastening straps and a life vest -- to partially keep the body above the water line. Others don flotation devices and "soggy jog" across pools.

"It's harder than it looks," says Jim Breen, professor of sports medicine at George Washington University. "Overcoming water resistance gives you a great workout."

For a less intense but no less effective workout, more and more people are choosing to walk, not run, in the water. Just as walking on land is gaining ground, walking in water is making a huge splash this summer.

"Water walkers far outnumber lap swimmers at many pools," says John Spannuth, executive director of the U.S. Water Fitness Association. "Lots of people can't swim or don't like to swim or don't want to get their hair or face wet. Nearly anyone can water-walk their way to fitness."

Spannuth became involved in water walking in 1986, when he was senior aquatics director at a YMCA in Oklahoma and saw a man of about 70 walking in chest-deep water. The man's doctor had advised him to walk for exercise, but arthritis made walking on land painful so he walked in the water. Spannuth roped off half a lane for water walkers and soon expanded the area to 1 1/2 lanes. By the end of 1988, more than 2,000 people were regularly water walking at that Y. Today, he says, many fitness centers are building shallower and constant-depth pools just to accommodate the growing number of water walkers and others interested in non-swimming water exercises.

Walking in water adds an extra dimension of strength training that doesn't occur with the same exercise on dry land. Since water is about 12 times more resistant than air, it can intensify exercise and strengthen muscles. "Preliminary findings indicate that water walking causes significant improvements in the strength of the upper thigh and muscles on the insides of the leg," says physiologist Gilbert Gleim, director of research at the Nicholas Institute of Sports Medicine at Lenox Hill Hospital in Manhattan. Studies show, too, he says, that water walking gives similar cardiovascular benefits to walking on land. The added resistance also boosts walking's calorie-burning power.

"When you walk in water at mid-thigh level, at a 3-mile-per-hour pace, you'll burn almost twice as many calories as you would doing the same thing on land," says James A. Nicholas, director of orthopedics at Lenox Hill and team orthopedist for the New York Jets. For example, he says, running at a 10-minute-per-mile rate in thigh-high water translates to a speed of nearly 6 minutes per mile on land. This makes water exercise "the best way to enhance aerobic strength and conditioning in a non-impact environment," he says.

To take the plunge into water walking, experts suggest:

1. Start shallow, at mid-shin level if you're out of condition. If you're in a pool that doesn't get that shallow, start as shallow as possible and walk slowly. Over time, walk deeper and faster. Your goal is to settle on a comfortable level, say waist-to-chest deep, and a speed that you can maintain for at least 20 minutes without becoming breathless. (Consult a physician before starting any new exercise program if you are a man over 45 or a woman over 50, or if you have any risk factors for heart disease.)

2. Lean forward slightly, keep your back straight and your stomach muscles pulled in.

3. Walk all the way through your foot, toe to heel. Resist the tendency to stay on your tiptoes in water; this will work just the calves.

4. Walk or jog an equal number of laps forward and backward to place an equal emphasis on all muscles.

5. Include a warm-up and cool-down in your routine. Begin and end with slow movements and stretches in the water.

6. Increase intensity by moving faster, lifting your knees high, using your arms above water for cardiovascular endurance and under water for muscle toning and endurance.

7. Walk sideways, but don't turn around after each length. Face the same direction while walking both lengths or you won't work both legs equally.

8. Get funky. Try some dance moves or practice your basketball footwork.

9. Try toys. Buoyant barbells, empty milk jugs with the caps on, water-skiing belts and other devices can add resistance and punch to a water-walking workout.

10. Step out in the new water footwear, like aqua socks, if you're walking in open water or on an uneven surface.

11. Drink plenty of fluids. You can become dehydrated even after working out in water.

For more information, send a self-addressed, stamped envelop to the U.S. Water Fitness Association, P.O. Box 360133, 9851-D Military Trail, Boynton Beach, Fla. 33436.

Bodyworks appears on alternate Tuesdays.