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 based on body composition, lung inflation, muscle and fat content, centre of gravity 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. Because they tend to have a lower centre of gravity and higher muscle content, human males find it more difficult to float or be buoyant. See also: Hydrostatic weighing.
Since the human body is less dense than water, water can support the body's weight 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 hydrodynamically effective, swimmers can either increase the power of their strokes or reduce water resistance. However, 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.
Swimmers may opt to do some pre-swim exercises such as "arm swings", in which they rotate their arms like a windmill, to help loosen their joints and warm their muscles immediately before swimming. Outside the pool, swimmers may incorporate strength training exercises such as squats.
A 2022 systematic review, published in Science & Sports journal, found that adding certain strength training exercises to traditional swimming training may improve both short and medium distance swimming performance. The authors concluded it would be "appropriate" to have strength sessions separated from swimming sessions within a training programme but cautioned against increasing the training volume too much, to avoid "greater fatigue" among swimmers.
