In general terms, swim resistance is the effect of water upon the motion of a swimmer. Swim resistance is a concept closely related to drag, the hydrodynamic principle of resistance created by a fluid to forward motion. The resistance met by a swimmer in their forward progress caused by the water is passive drag; the resistance against which the swimmer is exerting a force is active drag.
There are three types of resistance that affect swimming function, namely frontal resistance, skin friction, and eddy resistance.
Frontal resistance occurs when the swimmer adopts a body position that exposes a greater than necessary body surface to the water, thus increasing the effect of the water's resistance force. To limit the effect of frontal resistance, swimmers may seek to position themselves as high on the water surface as possible, to produce an effect similar to that of a hydrofoil. This technique is sometimes useful for swimmers with a smaller body mass. Alternatively, the swimmers will roll their body from side to side as the swim strokes are executed; the turning movement keep the body higher in the water.
Swimming research has established that for a freestyle swimmer, a body position of between 30% and 40% angle in the water allows the swimmer to generate optimum speed.
Swimmers keep their head as close to the surface of the water while breathing to counter act the natural tendency of the lower part of the body to sink, producing greater swim resistance, when the swimmer raises their head to breath.
Skin friction is the type of drag created when the swimmer's body and swim suit pass through water. The most time honored technique to counter skin friction is the shaving of the swimmer's body hair on any places that have contact with the water. The shaving of body hair may have a slight impact on the hydrodynamic characteristics of the swimmer's body. A similar physical result is achieved through the design of the racing suits that mimic the characteristics of shark skin. The suit surface is constructed with a series of ridges that tend to reduce the drag created when a suit passes through water, as the ridges act to deflect water away from the surface of the suit.
The full body racing suit is also designed to preserve the symmetry and sleekness of the body as it moves through the water, by limiting the amount of movement in the swimmers muscles that is not required for propulsion. The suits maintain a constant body silhouette without affecting muscle function.
Eddy resistance is caused when a swimmer creates eddies and water turbulence through poor stroke technique. Whenever a swimmer executes a front crawl stroke, an eddy, shaped as a vortex or whirlpool, forms at the water surface. If the stroke is executed inefficiently, the vortex will remain. If the vortexes accumulate around the body of the swimmer, the water resistance is increased.