Why Is Swimming So Hard? Drag, Buoyancy and the Physics of Moving Through Water
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The fastest human on land covers 100 metres in under ten seconds; the fastest in water takes nearly 47, and the record has moved by a little over five seconds in a century of trying. The difference is the medium. Water is about 800 times denser than air and 50 times more viscous, and a body moving through it is pushed back by a drag that grows with the square of speed, so that going twice as fast costs four times the force and eight times the power. Swimming is the sport in which the environment does most of the resisting, and the science of it is mostly the science of getting out of the water's way.
Three kinds of drag
The water resists a swimmer in three ways, and technique is the art of reducing each:
- •Form drag: the resistance of the body's shape pushing water aside, which depends on the frontal area presented; a swimmer lying flat and high presents a small target, one whose hips sag presents a large one
- •Friction drag: the rubbing of water along the skin, which is why swimmers shave and why the textured suits of 2008 were banned
- •Wave drag: the energy spent making waves at the surface, which rises steeply with speed and becomes the largest term at racing pace; it is why swimmers go faster underwater, where there are no waves, and why the rules limit them to 15 metres of it after each turn
Floating and staying flat
A human body is very nearly the density of water, with lungs full of air making it float and muscle and bone making it sink, and most people float with the head up and the legs down because the legs are denser than the chest. That is the wrong shape. A swimmer's first task is to lie flat, which means pressing the chest and head down so that the hips and legs rise, and keeping the body long and narrow in the line of travel. The kick in front crawl contributes little propulsion, perhaps a tenth, and its main job is to hold the legs up and the body straight; a swimmer who kicks hard with sagging hips is spending energy to increase their own drag. Elite swimmers float better than novices not because they are lighter but because they hold the position that lets the water pass.
How the arms push
The arm stroke was assumed for a century to work like a paddle, pulling straight back against the water, and film of champions in the 1970s showed something else: the hand traces a curved path, sweeping outward, inward and back, and gets its force partly by acting as a hydrofoil, generating lift across the direction of motion as an aircraft wing does, and partly by pushing against still water rather than water already moving backward. The modern understanding is a mix of the two, with a high elbow keeping the forearm vertical so that the whole limb acts as a paddle and the pull driven by the rotation of the body about its long axis rather than by the shoulder alone. Stroke rate and stroke length trade off; sprinters take short fast strokes, distance swimmers long slow ones, and the product of the two is speed.
The suits
In 2008 and 2009 swimmers wearing full-body polyurethane suits broke 130 world records in eighteen months, because the suits compressed the body into a smaller cross-section, trapped air that raised the swimmer higher in the water, and were smoother than skin. The governing body banned them from 2010, restricting suits to textiles and to the waist-to-knee or shoulder-to-knee, and several of the records set in the suit era stood for a decade. The episode was the clearest demonstration that swimming speed is limited by drag rather than by strength, since the same athletes were suddenly several percent faster without becoming stronger, and it is why swimming remains the sport whose equipment rules are most tightly policed.
The exercise
The same resistance that makes swimming slow makes it a good workout. Moving against water works nearly every large muscle group at once, the horizontal position and the support of the water take the load off the joints and spine, which is why it is prescribed for arthritis, injury and pregnancy, and the heart and lungs work as hard as in running at a fraction of the impact. The breathing is the part beginners find hardest, since it must be timed to the stroke and done in a fraction of a second with the face turned into a bow wave, and it is the reason inexperienced swimmers tire so fast: they are not out of strength but out of breath. An hour of moderate swimming burns roughly what an hour of jogging does, and a swimmer who learns to lie flat and breathe in rhythm finds the water stops fighting them, or at least fights less.
The takeaway
Swimming is hard because water is 800 times denser than air and resists a moving body with form, friction and wave drag that rise with the square of speed, so that the sport is won by reducing drag rather than by strength: lying flat and high, keeping the body long, holding a high elbow so the forearm paddles, and going underwater where there are no waves. The banned suits of 2008 proved the point by making the same athletes several percent faster, and the same resistance makes swimming a full-body exercise that spares the joints.