What happens to your mind when it's released from its usual sensory onslaught? The U.S. space program studied this question back in the 1950s by placing volunteers in flotation tanks in dark, soundproof laboratories. Some scientists expected brain activity to shut down but instead discovered that many volunteers said their minds came alive with visions from their own imaginations.

But then, nearly any lap swimmer could have told them this would happen. One of the sport's main attractions is its Zen of quiet, meditative tranquillity -- back and forth, back and forth -- that lets the mind float off to peaceful levels of creativity and well-being. Released from 90 percent of their body weight and sometimes freed from physical limitations that make land movement difficult, many people find water activity uniquely calming and refreshing.

"The water lets people relax in a way that most can't achieve on land," says Kenneth Frey, director of the Institute of Physical Therapy in Manhattan. "You lose the tension of constantly fighting gravity, and your muscles work in a much more integrated fashion."

Swimming consistently ranks as one of America's most popular exercises, and hot weather boosts its appeal, particularly since it's one of the few activities that cools you down while boosting your heart rate.

The second most effective aerobic exercise after cross-country skiing, "Swimming involves all of the major muscles in the body and, as a result, it gives you more of a total conditioning effect than many other sports," writes physician Ken Cooper in "The Aerobics Program for Total Well-Being" (Bantam, 1982).

Swimming's long, sinuous motions, along with the increased range of motion your body has in the water, elongates your muscles while strengthening them.

"We call them banana muscles," says Millard Freeman, the national aquatics director for the YMCA of the USA. "Weight lifters develop bulgier apple muscles, while swimmers develop long, tapered muscles."

Lap swimming is particularly popular among people over 30 in sedentary jobs, Freeman says, because "it helps them get strong and heart-healthy, with little risk of injury." Some swimmers have switched over from high-impact sports like running, he says, and others "cross-train" by alternating swimming with weight-bearing activities, such as racquet sports and aerobic dance. Some people started swimming for fitness during pregnancy or injury and got hooked.

Heart patients may find swimming particularly appealing (with their physician's approval), says Jane Katz, professor of health and physical education for the City University of New York and author of several books on swimming. "When your body is horizontal and immersed in water, your heart is actually larger than when you're vertical on dry land and it has to pump against gravity. As a result, between 10 and 20 percent more blood is pumped with each heart contraction. This effect gives you the potential to work harder and longer than you ever could on land."

Also, your pulse rate doesn't usually get up as high in the water as on the land, she notes. Exercise heart rates are typically 10 to 15 percent lower during water workouts, since the heart does not have to work as hard in cool water as it would on land to dissipate exercise- generated heat.

Start fitness swimming as you would any new activity, "slowly and progressively," Katz says. "Begin with a warm-up -- bobbing and moving easily in the water. Then go into your main set." For a beginner, this could mean swimming one lap and resting, then swimming a second lap and resting. As you advance, try to stay in the water for a total of 30 minutes, resting when you need to, with the goal of resting less frequently until you're swimming a full 30 minutes. (Consult a physician before starting this or any new exercise if you are a man over 45 or a woman over 50 or if you have any risk factors of heart disease.)

Challenge yourself and vary your workouts by trying:

Medleys. Swim one length of each stroke.

Intervals. Swim one or two fast lengths, then one slow length.

Beat the clock. Use a waterproof watch or pool clock to calculate how long it takes you to swim one length. Then try to swim it faster.

Pyramids. Attempt a long swim, say 36 lengths, by swimming one length, then resting, then swimming two lengths, then resting, three lengths, resting, four lengths, resting, five lengths, resting, six lengths, resting, then work your way back down to one length. The decreasing numbers will give you a psychological edge to finishing.

Distance per stroke. Count how many strokes it takes you to travel one length. Then try to travel the same length with one less stroke.

Rest period. Get in the pool 10 minutes before the lifeguard blows the whistle for the "adults only" swim. Jog and jump around in the water to get your heart rate up so that, as soon as the kids get out, you can kick off for 15 peaceful minutes of lap work.

Countdown. Swim 11 lengths, then rest. Then swim 10 lengths and rest, nine, eight, etc. By the time you're done, you'll have swum 66 lengths, a swimmer's mile in a 25-yard pool.

Next time: Water-walking.

Bodyworks appears on alternate Tuesdays.