Different Strokes Why Do Human Beings Find It So Incredibly Difficult to Swim? By Kim McDonald Special to The Washington Post Wednesday, August 11, 1999; Page H01 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 more dense 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. 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. For the rest of the physics of swimming, see The Chris Lewis Swimming Homepage