Swimming. Swimming is the self-propulsion of a person through water, usually for recreation, sport, exercise, or survival. Locomotion is achieved through coordinated movement of the limbs, the body, or both. Humans can hold their breath underwater and undertake rudimentary locomotive swimming within weeks of birth, as a survival response. Swimming is consistently among the top public recreational activities, and in some countries, swimming lessons are a compulsory part of the educational curriculum. As a formalized sport, swimming features in a range of local, national, and international competitions, including every modern Summer Olympics. Swimming relies on the nearly neutral buoyancy of the human body. On average, the body has a relative density of 0.98 compared to water, which causes the body to float. However, buoyancy varies on the basis of body composition, lung inflation, and the salinity of the water. Higher levels of body fat and saltier water both lower the relative density of the body and increase its buoyancy. Since the human body is only slightly less dense than water, water supports the weight of the body during swimming. As a result, swimming is low-impact compared to land activities such as running. The density and viscosity of water also create resistance for objects moving through the water. Swimming strokes use this resistance to create propulsion, but this same resistance also generates drag on the body. Hydrodynamics is important to stroke technique for swimming faster, and swimmers who want to swim faster or exhaust less try to reduce the drag of the body's motion through the water. To be more hydrodynamic, swimmers can either increase the power of their strokes or reduce water resistance, though power must increase by a factor of three to achieve the same effect as reducing resistance. Efficient swimming by reducing water resistance involves a horizontal water position, rolling the body to reduce the breadth of the body in the water, and extending the arms as far as possible to reduce wave resistance. Just before plunging into the pool, swimmers may perform exercises such as squatting. Squatting helps in enhancing a swimmer's start by warming up the thigh muscles. Main article: Infant swimming Human babies demonstrate an innate swimming or diving reflex from newborn until the age of approximately 6 months. Other mammals also demonstrate this phenomenon. The diving response involves apnea, reflex bradycardia, and peripheral vasoconstriction; in other words, babies immersed in water spontaneously hold their breath, slow their heart rate, and reduce blood circulation to the extremities.Because infants are innately able to swim, classes for babies of about 6 months old are offered in many locations. This helps build muscle memory and makes strong swimmers from a young age. Swimming can be undertaken using a wide range of styles, known as 'strokes,' and these strokes are used for 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, similar to the way four-legged animals swim. There are four main strokes used in competition and recreation swimming: the front crawl, also known as freestyle, the breaststroke, the backstroke and the butterfly. Competitive swimming in Europe started around 1800, mostly using the breaststroke. In 1873, John Arthur Trudgen introduced the trudgen to Western swimming competitions. The butterfly stroke developed in the 1930s, and was considered a variant of the breaststroke until accepted as a separate style in 1953. Butterfly is considered the hardest stroke by many people, but it is the most effective for all-around toning and the building of muscles. It also burns the most calories. Other strokes exist for specific purposes, such as training or rescue, and it is also possible to adapt strokes to avoid using 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 affected by paralysis. Main article: History of swimming 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, Beowulf, and other sagas. The coastal tribes living in the volatile Low Countries were known as excellent swimmers by the Romans.