Of all the possible ways for us to get from here to there under our own power, swimming is by far the least efficient. Sure, we can float, tread water or paddle on our backs with relative ease. But to move through water at even a slow walking pace requires significantly more energy. In fact, world champion swimmers can barely travel at 5 miles per hour, whereas a suitably motivated dolphin can hit 25 mph.

The principal reason is that water is about a thousand times denser than air. Unlike running, cycling or other common forms of human locomotion that transfer most of our muscle energy into forward movement, swimming requires us to expend way more than 90 percent of our energy simply overcoming fluid resistance.

Of course, the density also provides a few perks. It means we can float, more or less, near the surface, depending on our proportion of body fat and the volume of air in our lungs. [See box on Page H7.] And it allows us to position ourselves horizontally so we can at least move through the water like a sleek racing boat or torpedo, rather than an upright statue or barge.

What determines the way we move? At its simplest, swimming is yet another manifestation of Newton's Third Law of Motion: Our hands, arms and legs push against the water; simultaneously, the water pushes back in an equal and opposite reaction, propelling us forward.

But there's much more to it than that. Many scientists contend that the principles that govern our movement through this aqueous medium are, strangely enough, the same ones that keep an F-15 in the air and allow motor boats to plow through the water. Fluid dynamics tell us that our motion depends on two basic factors: propulsive forces that we exert and resistance, or drag, that we encounter.

No matter how streamlined we try to make ourselves, the drag is formidable. That's why sheer strength won't make you a better swimmer and why a little knowledge of physics can come in handy.

Olympic swimmers are able, at best, to convert only 9 percent of their energy into forward movement while swimming the front crawl or freestyle, the fastest and most efficient of the four competitive strokes. (On average, the butterfly, backstroke and breaststroke follow in speed and efficiency.) Most of the rest of us use as much as 98 percent of our energy overcoming water resistance.

What a Drag

The main reason for this spectacular inefficiency is that our forward momentum is limited by form drag, the resistance due to the shape of our bodies moving through the water. As we move ahead, water pressure builds up in front of us; at the same time, we produce a large turbulent area behind us, which has a much lower pressure. That pressure imbalance creates a force -- from high pressure to low -- that tries to push us backward. Form drag increases with the square of the velocity, and depends critically on the frontal area of the moving object.

That's why boat designers build racing hulls long and narrow, instead of short and stubby. And it helps explain why many of the world's fastest swimmers look more like professional basketball players than weightlifters. American swimmer Matt Biondi and Russian swimmer Alexander Popov, Olympic gold medalists and world record- holders in the 100-meter freestyle, are perfect examples. They are thin, long-limbed and stand at 6 feet 7 inches and 6 feet 6 inches, respectively.

To minimize form drag, competitive swimmers concentrate on pushing their torsos down in the water, while keeping their hips up. This compensates for the natural buoyancy of the upper body, allowing them to swim horizontally -- instead of diagonally, with their legs dangling downward from the surface. That reduces the head-on or frontal area affected by drag.

Many good freestyle and backstroke swimmers have also learned to roll their bodies from side to side, lifting one shoulder out of the water with each stroke. This results in a narrower body profile. It also lengthens their reach, and hence total body length, making possible a longer and more efficient glide. (If you've ever paddled two similar-shaped kayaks of different lengths you may have noticed that the longer kayak, despite its greater weight and displacement of water, glides farther with each stroke of the paddle.)

In addition, the side-to-side roll produces a twisting force that adds a rotational component in the direction of travel. This helps to propel swimmers forward, like the twisting of a screw through wood, and allows them to use their hip and torso muscles, as well as their arms and legs.

Competitive swimmers also try to minimize frictional drag, the "stickiness" of an object as it moves through the water. A baggy swimsuit or excess body hair create rough surfaces that increase frictional drag. So many swimmers use tight-fitting suits and often shave the hair on their bodies before big races.

Making Waves

Perhaps the most pernicious form of drag is wave drag. Unlike form drag, which pulls us from the back, wave drag pushes from the high-pressure area we produce in front of us. When we swim, splash or make any sort of movement in the water, we create waves. As we propel ourselves through the pool at faster and faster speeds, the waves that form in front of us grow larger and larger, building into a wall of water that limits our forward movement. Unlike most forms of fluid drag, which grow as the square of the velocity, wave drag increases as the cube. So when you double your speed through the water, the retarding force of wave drag increases eightfold.

Ted Isbell, a professional engineer who is also the swimming coach for Channel Islands Aquatics in Oxnard, Calif., says many top swimmers discover that they can move faster by kicking underwater for a considerable distance immediately after a turn (instead of rising quickly and swimming on the surface) because wave drag is largely eliminated underwater.

Isbell thinks underwater dolphin kicking -- an undulating motion with both legs together -- may be one area where short, stocky competitors could have an advantage over their taller, thinner rivals who have a decided advantage at the surface. Because an object moving underwater is affected largely by frictional drag, he says, minimizing its surface area reduces the resistance it will encounter: "Since a short, fat object has less surface area than a long, slim object, ship designers have designed submarines with short, round, fat hulls, while they have designed surface ships with longer, slimmer hulls."

Another factor affecting wave drag is how smoothly one moves on the surface. Because waves carry energy, the waves that a swimmer generates from any jerky or unnecessary movements in the water represent losses of energy. That's why the muscular guy swimming in the next lane, thrashing his arms around like windmills in an effort to speed up, doesn't seem to be making as much forward progress as he should.

In fact, scientists studying athletes competing in the 1996 Olympics found that swimmers who used fewer strokes per lap were actually the ones most likely to win. Popov, the current world record-holder in the 100-meter freestyle, concentrates so much on maintaining perfect form and minimizing the number of strokes he uses to cover each pool length that he never seems to be swimming hard when training.

As a result, he has been able to minimize both form and wave drag, which become increasingly more important at race speeds. His coach, Gennadi Touretski, bases his unorthodox method of training on the basic principles of hydrodynamics and on watching fish swim. Fish and other animals, Touretski says, increase their speed by covering more ground with each movement, rather than increasing their rate of movement. When Popov won his first Olympic gold medal at the 1992 Olympic Games by beating Biondi in the 50-meter freestyle, he covered the distance in 33 strokes compared with Biondi's 36.

Terry Laughlin, founder and director of Total Immersion Swimming, a swimming-improvement program based in New Paltz, N.Y., tells swimmers that by counting their strokes they can eventually learn to eliminate needless movement and maximize their efficiency.

"Stroke counting is important because it's the clearest marker of how well you're using your energy," he says. "Only about 2 percent of the human race swims with instinctively long strokes. The rest of us have powerful instincts telling us to swim faster by stroking faster."

Popov and other elite swimmers "can easily swim the length of a 25-yard pool in seven or eight strokes," says Laughlin. "Whenever I count strokes during lap swim at the local Y, the average stroke count is 25 or higher."

When Push Comes to Shove

If increasing stroke rate won't make you faster, what about pulling harder? Or, like a kayaker with a bigger paddle, pushing more water behind you? When I began swimming competitively back in the 1960s, coaches viewed swimmers as paddleboats propelled by the backward push of water. The most efficient way of moving forward, they told us, was to grip the water with your hand and pull straight back from your head to your hip.

High-speed underwater films of the best competitive swimmers at the time, however, revealed that their hands followed a weaving, S-shaped path. Then in the 1970s, James "Doc" Counsilman, head coach at Indiana University who achieved fame as Mark Spitz's coach, filmed swimmers with flashlights on their hands in a darkened pool and discovered that most of the hand and foot motions of competitive swimmers were up and down or side to side, rather than front to back.

That provided evidence to scientists that swimmers were moving themselves forward primarily through lift, the same principle that keeps airplanes aloft. Airplane wings have a curved upper surface, so air moving over the top travels a bit farther before reaching the back of the wing than does air that flows directly under the wing.

Daniel Bernoulli, an 18th century Swiss scientist, discovered the principle that faster-moving fluid has lower pressure. In airplanes, this puts the lower pressure above the wing and higher pressure below. The resulting force causes the plane to rise, or lift.

Bernoulli's principle also applies to liquids, but for swimmers the lift is horizontal, rather than vertical. This is because lift propels a body in the direction perpendicular to the force of drag resulting from the high- and low-pressure differences. And the drag forces involved in propulsion of swimmers, according to some sports scientists, are produced primarily by the up and down motions of the hands and feet.

Some sports scientists also contend that swimmers move forward like propellers through the water by changing the pitch of their feet and hands as they move through their S-shaped stroke pattern. About 80 percent of the propulsive power in freestyle comes from the arm strokes, they estimate, while about 20 percent comes from the kick. The downward motion of the kick, like the downward sweep of a dolphin's tail, produces a pressure difference above and below the foot. As the foot moves over more distance, which can be accomplished by bending the knees and increasing ankle flexibility, propulsive force is enhanced.

The downward motion of the hand, which is pitched at an angle as it enters the water, allows water to travel faster over the knuckles than the palm -- thus producing a low-pressure area above and a high-pressure area below.

According to Ernest Maglischo, a scientist and swimming coach at Arizona State University, as a swimmer's hands and feet change direction and pitch, the water flowing around them is also changing direction, so that they act like the blades of a propeller.

Or Maybe Not

A number of other scientists don't buy these explanations. They say the surfaces of the hands and feet aren't sufficiently large or curved to produce enough lift to propel swimmers forward at the speeds they are swimming. And because swimmers don't spin their arms in the shoulder joints and complete only a half-stroke at a time, they say the propeller analogy isn't valid either.

While some scientists have gone back to the idea that propulsion is due simply to backward motions of the arms, most others admit that they have no idea. "To be honest, I don't think anyone has a handle on it," concedes Isbell.

However swimmers may be propelling themselves by the actions of their hands and feet, Laughlin contends that it is aided in the freestyle and backstroke by "core body propulsion," achieved by rolling their hips and shoulders with every stroke.

"Swimmers are dynamic," says Laughlin, "not static," and "body rotation in swimming generates power just as it does in baseball, golf or tennis (or in throwing a javelin or a punch): A biomechanical chain reaction occurs, in which the legs propel the hips, which power the torso, which drives the last link in the kinetic chain -- the shoulders and arms.

"The most powerful movements don't start and stop in any one joint; when we employ precise body mechanics, power ripples through our bodies like it does through a cracked whip until it finally arrives at the point where it's released.

"But there is one key difference in how swimmers use the kinetic chain compared to land-based athletes. On land, the chain reaction starts with twisting the body away from the direction of the swing while the legs are anchored to the ground, an action known as elastic loading, similar to a rubber band being stretched. The hip cock acts like the handle of a whip, throwing the energy upward through torso, shoulders and arms with increasing speed and power.

"Since swimmers cannot anchor their feet to the ground, the hips cannot act as a whip handle, making it essential that you focus on moving the entire torso. When we are swimming with maximum effectiveness, it is torso rotation that thrusts the recovering hand forward into the water at the same time that it drives the propelling hand back.

"We increase stroking power not by lifting weights, but by shifting from passive body roll to dynamic body rotation, pressing into service the stronger muscles of the torso that feed power to the arms."

For most of us, however, swimming can't be perfected by thinking about every movement we make. It has to be drilled into our nervous systems by constant practice and, most importantly, by a good coach.

"The best swimming is more art than science, as exemplified in the world's best swimmers, who demonstrate a grace, economy and flow in their swimming that is incomprehensible to most of the rest of us mortals," says Laughlin.

Kim McDonald is a science writer at the Chronicle of Higher Education.

CAPTION: How the Body Cuts Through Water (This graphic was not available)