Second, swimming is an excellent exercise for conditioning your cardiovascular system. Although you won't go as far in a 20-minute swim as you will during 20 minutes of walking, running, or cycling, the cardiovascular benefits are essentially the same.

Third, swimming seems to enhance joint flexibility, especially in the neck, shoulders, hips, and midsection area. Much of this is due to the repetitive twisting movements as your body turns from side to side during the crawl stroke.

Of course, exercising in water is a totally different experience than land-based activity. For one thing, swimming requires a horizontal body position, that places less stress on your heart with respect to blood circulation. The gentle pressure of the water against your skin also enhances blood circulation.

Because water is an isokinetic medium, the resistance varies in proportion to your muscle force. That is, when you give low muscle force against the water you encounter relatively low resistance from the water, and when you give high muscle force against the water you receive relatively high resistance from the water.

Another advantage of aquatic exercise is the cooling effect of the water. Unlike hot weather activity on land, you are unlikely to overheat when swimming. What could be better than a cool pool, lake or ocean in which to exercise on a hot summer day.

With so many reasons to enjoy swimming, why do so few adults spend time in the water? The major problem for some adults is lack of confidence in their swimming ability. This can be easily overcome by a few lessons or a refresher swimming course. However, I find that most adults do not understand how to swim long enough for fitness benefits. To make a land activity analogy, they start out running rather than walking. In other words, instead of starting with a slow swimming speed and gradually picking up the pace new swimmers tend to sprint a lap or two, exhausting themselves and ending their exercise session prematurely. By so doing, they miss both the enjoyment and the fitness benefit of this fine physical activity.

Although swimming is excellent exercise, it should be performed in a relaxed manner. Your arms and legs should feel loose and your movements should be balanced and coordinated. If you feel tight or tense, simply slow down and let the water work with you rather than against you. Because the water buoys up your body, you can swim very slowly and still maintain your top-of-the-water body position.

By the way, unless you want to swim competitively, do not compare yourself with other swimmers. Bodybuild plays a major role in swimming speed, but it has little effect on your fitness outcome. For example, people who are wide from side to side and thin from front to back seem to skim over the water surface like a surfboard. On the other hand, people who are narrow from side to side and thick from front to back encounter much more water resistance and must work much harder at a given swimming pace. The solution is to swim for yourself at a comfortable speed, and let the faster swimmers use the high speed lanes.

As an aerobic conditioning activity, you should aim for at least 20 minutes of continuous swimming. However, until you develop sufficient cardiovascular endurance, you may vary the pace throughout your 20-minute swimming session. Consider the following training progression for beginning a fitness-oriented swimming program. For best results, stay within the guidelines that gradually increase the swimming distance and the number of faster-paced laps.

Sample Beginning Swimming Fitness Program

Week 1
Day 1: Swim 100 yards (4 pool lengths) at a slow pace.

Day 2: Swim 150 yards (6 pool lengths) at a slow pace.

Day 3: Swim 200 yards (8 pool lengths) at a slow pace.

Week 2
Day 1: Swim 200 yards alternating slower-paced and faster-paced laps.

Day 2: Swim 250 yards (10 pool lengths) alternating slower-paced and faster-paced laps.

Day 3: Swim 300 yards (12 pool lengths) alternating slower-paced and faster- paced laps.

Week 3
Day 1: Swim 300 yards alternating 1 slower-paced with 2 faster-paced laps.

Day 2: Swim 350 yards (14 pool lengths) alternating 1 slower-paced with 2 faster-paced laps.

Day 3: Swim 400 yards (16 pool lengths) alternating 1 slower-paced with 2 faster-paced laps.

Week 4
Day 1: Swim 400 yards alternating 1 slower-paced with 3 faster-paced laps.

Day 2: Swim 450 yards (18 pool lengths) alternating 1 slower-paced with 3 faster-paced laps.

Day 3: Swim 500 yards (20 pool lengths) alternating 1 slower-paced with 3 faster-paced laps.

Week 5
Day 1: Swim 500 yards alternating 1 slower-paced with 4 faster-paced laps.

Day 2: Swim 550 yards (22 pool lengths) alternating 1 slower-paced with 4 faster-paced laps.

Day 3: Swim 600 yards (24 pool lengths) alternating 1 slower-paced with 4 faster-paced laps.

Week 6
Day 1: Swim 600 yards alternating 1 slower-paced with 5 faster-paced laps.

Day 2: Swim 650 yards (26 pool lengths) alternating 1 slower-paced with 5 faster-paced laps.

Day 3: Swim 700 yards (28 pool lengths) alternating 1 slower-paced with 5 faster-paced laps.

Week 7
Day 1: Swim 700 yards alternating 1 slower-paced with 6 faster-paced laps.

Day 2: Swim 750 yards (30 pool lengths) alternating 1 slower-paced with 6 faster-paced laps.

Day 3: Swim 800 yards (32 pool lengths) alternating 1 slower-paced with 6 faster-paced laps.

Week 8
Day 1: Swim 800 yards in the following manner - 10 faster-paced laps, 1 slower-paced lap, 10 faster-paced laps, 1 slower-paced lap, 10 faster-paced laps.

Day 2: Swim 800 yards in the following manner - 15 faster-paced laps, 2 slower-paced laps, 15 faster-paced laps.

Day 3: Swim 800 yards without any slower-paced laps.

Succeeding Weeks
Gradually increase your distance until you are swimming comfortably for 20-30 minutes.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA. He is author of several fitness books including Building Strength and Stamina and Strength Training Past 50.