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 hydrodynamically effective, 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. 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 (see mammalian diving reflex). 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 (fingers and toes). 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. Technique 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, breaststroke, backstroke and butterfly. Competitive swimming in Europe started around 1800, mostly using the breaststroke. In 1873, John Arthur Trudgen introduced the trudgen to Western swimming competitions. Butterfly was 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