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Swimming relies on the natural buoyancy of the human body, with the relative density of the average body, compared to water, of 0.98, creating a floating effect. This varies on the basis of body composition, with body fat lowering the density, and increasing floatation.

The relative density difference means that water supports the body during swimming, and therefore makes swimming low impact compared to surface activities such as running where weight is put on to the joints, and also creates resistance when moving through the water. The resistance is used by swimming strokes to create propulsion, but creates drag on the body.

This means that hydrodynamics are an important factor in stroke technique in terms of swimming faster, and swimmers wishing to swim faster, or wishing to tire less will try and reduce the drag caused by the body through the water. In order to be more hydrodynamic, people can increase the power of the strokes, or reduce water resistance, although increasing power to overcome resistance needs to increase by a factor of three to achieve the same effect as reducing resistance.

Efficient swimming by reducing water resistance involves having a horizontal water position, rolling the body in order to reduce the breadth of the body in the water and extending the arms as far as possible in order to reduce wave resistance.

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Technique

Swimming can be undertaken using a wide range of different styles, known as 'strokes', and these strokes are used for a different purposes, or to distinguish between classes in competitive swimming. It is not necessary to use a defined stroke for propulsion through the water, and untrained swimmers may use a 'doggy paddle' of arm and leg movements which mimics the strokes of quadruped animals such as dogs in the water.

There are four main strokes used in competition and recreation swimming, which are front crawl, breaststroke, backstroke and butterfly. Competitive swimming in Europe started around 1800, mostly using breaststroke and in 1873 John Arthur Trudgen introduced the trudgen to Western swimming competitions, after copying the front crawl used by Native Americans, but substituting a scissor kick for the traditional flutter kick in order to reduce splashing. Butterfly was developed in the 1930s and was at first a variant of breaststroke until it was accepted as a separate style in 1952.

Other strokes exist for specific purposes, such as training or rescue, and it is also possible to adapt strokes to not use parts of the body, either to isolate certain body parts, such as swimming with arms only or legs only to train them harder, or for use by amputees or those suffering paralysis.

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Historic record

Swimming has been recorded since prehistoric times, and the earliest records of swimming date back to Stone Age paintings from around 7,000 years ago. Written references date from 2000 BC. Some of the earliest references include the Epic of Gilgamesh, the Iliad, the Odyssey, the Bible (Ezekiel 47:5, Acts 27:42, Isaiah 25:11), Beowulf, and other sagas. In 1538, Nikolaus Wynmann, a German professor of languages, wrote the first swimming book, The Swimmer or A Dialogue on the Art of Swimming (Der Schwimmer oder ein Zweigespräch über die Schwimmkunst).

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Purpose

There are many reasons why people swim, from swimming as a recreational pursuit to swimming as a necessary part of a job or other activity. Some people may also be forced into swimming involuntarily as a result of falling into water.

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Recreation

The largest reason for people swimming is as a recreation activity, with swimming consistently ranking as one of the physical activities people are most likely to take part in. Recreational swimming can be used for people to exercise, to relax or to rehabilitate. The support of the water, and the reduction in impact, makes swimming accessible for people who are unable to undertake activities such as running.

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Health

Swimming is primarily a cardiovascular/aerobic exercise due to the long exercise time, requiring a constant oxygen supply to the muscles, except for short sprints where the muscles work anaerobically. As with most aerobic exercise, swimming is believed to reduce the harmful effects of stress. Swimming can also improve posture.

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Sport

Swimming (sport)

Swimming as a sport predominantly involves competition amongst participants to be the fastest over a given distance under self propulsion. Different distances are swum in different levels of competition. For example, swimming has been part of Olympic Swimming since 1896, and the current program contains events from 50m to 1500m in length, across all four main strokes and medley.

The sport is governed internationally by the Fédération Internationale de Natation (FINA), and competition pools for FINA events are 25 or 50 metres in length. In the United States of America, USA Swimming is the governing body and a pool 25 yards in length is commonly used for competition.

There are other swimming and water related sporting disciplines including diving, synchronised swimming and water polo, as well as sports which include a swimming element, such as triathlon and modern pentathlon.

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Occupation

​Professional swimmers performing a water ballet in Guardalavaca, Cuba
Some occupations require the workers to swim. For example, abalone- or pearl-divers swim and dive to obtain an economic benefit, as do spear fishermen.

Swimming is used to rescue people in the water who may be in distress, including tired swimmers, non-swimmers who have accidentally entered the water, or water users who have come to harm. Lifeguards or volunteer lifesavers are deployed at many pools and beaches worldwide to fulfill this purpose, and they, as well as rescue swimmers, may use specific swimming styles for rescue purposes.

Swimming is also used in marine biology to observe plants and animals in their natural habitat. Other sciences use swimming, for example Konrad Lorenz swam with geese as part of his studies of animal behavior.

Swimming also has military purposes. Military swimming is usually done by special forces, such as Navy SEALs. Swimming is used to approach a location, gather intelligence, sabotage or combat, and to depart a location. This may also include airborne insertion into water or exiting a submarine while it is submerged. Due to regular exposure to large bodies of water, all recruits in the United States Navy, Marine Corps, and Coast Guard are required to complete basic swimming or water survival training.

Swimming is also a professional sport. Companies sponsor swimmers who are at the international level. Many swimmers compete competitively in order to represent their home country in the olympics. Cash awards are also given at many of the major competitions for breaking records.

Professional swimmers may also earn a living as entertainers, performing in water ballets.

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Locomotion

Locomation by swimming over brief distances is frequent when alternatives are precluded. There have been cases of political refugees swimming in the Baltic Sea and of people jumping in the water and swimming ashore from vessels not intended to reach land where they planned to go. Swimming travel is central to the plot of the motion picture "Welcome". US president John F. Kennedy led his sailors swimming island to island after his torpedo boat was sunk in World War II. His senator brother Ted Kennedy claimed to have left Chappaquiddick Island by swimming.

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Risks

A sign warns hikers on the trail to Hanakapiai Beach.
There are many risks associated with voluntary or involuntary human presence in water, which may result in death directly or through drowning asphyxiation. Swimming is both the goal of much voluntary presence, and the prime means of regaining land in accidental situations.

Most recorded water deaths fall into these categories:
Panic where the inexperienced swimmer or non swimmer becomes mentally overwhelmed by the circumstances of their immersion, leading to sinking and drowning. Occasionally panic can kill through hyperventilation even in very shallow water.
Exhaustion where the person is unable to sustain effort to swim or tread water, often leading to death through drowning.

An adult with fully developed and extended lungs has generally positive or at least neutral buoyancy, and can float with modest effort when calm and in still water. A small child has negative buoyancy and will either sink rapidly or have to make a sustained effort to stay near the surface.
Hypothermia where the person loses critical core temperature, leading to unconsciousness or heart failure.
Dehydration from prolonged exposure to hypertonic salt water, less frequently salt water aspiration syndrome where inhaled salt water creates foam in the lungs that restricts breathing.

Hypothermia and dehydration also kill directly, without causing drowning, even when the person wears a life vest.
Blunt trauma in fast moving flood or river water.

Less common are
Other adverse effects: Exostosis is an abnormal bony overgrowth narrowing the ear canal due to frequent, long-term splashing or filling of cold water into the ear canal, also known as surfer's ear.
Infection due to water-borne bacteria, viruses, or parasites.
Chlorine inhalation (in swimming pools).

Adverse encounters with aquatic life: Stings from sea lice, jellyfish, fish, sea shells, and some species of coral.
Puncture wounds caused by crabs, lobsters, sea urchins, zebra mussels, stingrays, flying fish, sea birds, and rubbish.
Hemorrhaging bites from fish, marine mammals, and marine reptiles, occasionally resulting from predation.
Venomous bites from sea snakes and certain species of octopus.
Electrocution or mild shock from electric eels and electric rays.

Around any pool area, safety equipment is often considered important and is a zoning requirement for most residential pools in the United States. Supervision by personnel trained in rescue techniques is required at most competitive swimming meets and public pools.