Produced, edited, and copyrighted by
Professor Brent S. Rushall, San Diego State University
Volume 3, Number 3: April 26, 1996


[This is a slightly modified version of an article of the same name published in the New South Wales Swimmer, 10(2), 5-7, 1993.]

Many readers may have wondered why swimmers slow down when they go through the water. Some individuals seem to slip through the water easier than others. Some swimmers look to be swimming well at slow speeds but when they attempt to speed up they do not improve as much as others. One of the explanations for such differences could be the amount of resistance, more commonly referred to as drag," that is created by the swimmer.

An understanding of drag is an important feature of modern swimming and coaching. It is a topic that is starting to have resurgent interest and is now considered to be more important than previously thought. In fact, it appears that reduction in drag is a better approach to improving speed through the water than is performing some subtle adjustment in technique. The emerging coaching approach for technique appears to be to perform propelling actions but to not create any unnecessary drag. If a choice is to be made, it should be preserve the minimum drag position and action over attempting any stronger action that could cause the drag (resistance) to increase. Analyses of gold medal crawl strokers at the Barcelona Olympic Games show that Keiren Perkins had the least drag and the weakest pull but was the most outstanding swimmer. This is a very different approach to swimming propulsion than is entertained by many Australian [and American] coaches.

There are several classifications of drag, a simple one being active and passive drag. However, that simplicity obscures a full understanding of what types of drag effect swimmers and the degree to which each is detrimental at various swimming speeds. A better classification for swimming contains three types of drag, each arising from different physical mechanisms and detracting from performance in different amounts.

1. Frictional Drag

This is developed when water passes over a surface, generally the rougher the surface, the more drag. This is part of passive drag. Skin roughness, body contouring, hair, and swim suit fabric are examples of roughness that create friction as a swimmer moves through water. The relationship of frictional drag to velocity is linear, causing a minor effect upon performance as speed is increased.

Frictional drag can be reduced by shaving hair off the body and legs, but not the forearms. The reduced resistance causes a reduction in the energy per stroke when compared to an unshaven condition. Tight swim suits of sheer fabrics with a structure that minimizes seams and edges is another way of reducing frictional resistance. Wearing a latex cap also provides a smoother surface than does a head of hair and thus, further reduces drag.

One should be wary of oils and topical applications that are advertised as being able to reduce frictional drag. Many have such a repellent value to water, that their surface tension exceeds that which exists on a clean oil-free shaved skin surface. The value of warm showers as part of a passive warm-up has been known for more than 50 years. When taking the final shower, a detergent soap should be used to remove oils from the skin. The effect will be marginal but positive.

2. Form Drag

The shape of the moving swimmer presented to the water is the second component of passive drag but may also be part of active drag if it is changed by the way the swimmer moves. To a minor extent, it is affected by the density of water and is part of the explanation of the difference between salt (denser water, faster times) and fresh water performances.

The largest factor in shape is the cross-sectional area (often termed frontal resistance) of the body. Form drag increases by the square of the velocity and so becomes increasingly important the faster a swimmer travels. However, form drag is not always detrimental. It contributes to hydrodynamic lift and is critical to propulsion in some strokes. Form drag is passive when a swimmer's pure size contributes to the resistance. It is active and disadvantageous when the swimmer's position in the water is not fully streamlined (e.g., swimming with a head-up position in backstroke which causes the hips to drop deeper than the cross-sectional area presented by the shoulders and chest area; looking directly ahead in breaststroke causing the hips to drop and the general body angle to be tilted rather than being flat as possible). If a swimmer's action or swimming "posture" deliberately creates an increased cross-sectional area then progress through the water will be slowed more than should be expected. In that case, the incorrect swimming alignment produces extra resistance which is actively created although it could be reduced. Form drag increases in the seriousness of its effects, the faster a swimmer goes. A possible method of finding a few extra seconds in a swimming race is to minimize form drag. That is a lot easier to do than trying to find more strength or endurance.

Form drag can be lessened by accentuating streamlining at every opportunity (i.e., the swimmer has to create the smallest hole while going through the water). A general concept for most strokes is to have the shoulder/chest area bore a hole in the water and the hips and legs follow through that hole. That usually translates into swimming as flat as possible. Even the new breaststroke kick is designed to reduce the dropping of the knees, a noted feature of older actions. When a breaststroker kicks and at the same time allows the hips to rise marginally, that elevates the knees and reduces the thigh contribution to form drag as well as producing a propelling force that is more horizontal and beneficial than the older, slightly downward kick.

The height of the waves that are created by a swimmer are a simple index of the streamline of a swimmer. It is easy to look at the bow wave that is created and determine its relationship to the height of lane markers. The smaller it becomes, while maintaining a constant velocity, the better is the streamline. This is the simplest way that a coach can evaluate this important factor. Most new advances in technique are aimed at maximizing streamlining, that is, reducing form drag.

3. Wave Drag

Unnecessary movements in a swimming technique move water creating waves, wakes, and turbulence and are a major part of active drag. Since waves and moving water carry energy, that energy is derived from the swimmer. Energy that could be applied to productive force is lost to unnecessary water movement. Although body position in the water has been described as contributing to form drag by increasing frontal resistance, if that body position is not constant but occurs cyclically in a stroke, energy will be lost to the water movements that result.

Examples of wave production are: accentuated vertical movements (e.g., "flying" out of the water in butterfly, lifting the head when breathing in crawl stroke), lateral movements (e.g., hip sway in backstroke that results from the hand entering behind the head; breathing backwards in crawl stroke that produces a reactionary side hip movement and wide kick), and any action that is not in a horizontal plane. Any bouncing or jerkiness in a swimmer's stroke also creates wave drag. Because of the limitations of the human anatomy, it is not possible to remove all movements outside of the direct horizontal plane, but when they are exaggerated, wave drag becomes the major resistance problem.

This is the worst category of drag creation because it scales as the cube of swimming velocity. The faster a swimmer goes, its contribution to resistance increases dramatically. At racing speeds it creates much more drag than either form or frictional drag. A swimmer can have a great degree of control over wave drag. If a correct and efficient form of propulsion is executed, it is minimal.

Wave drag can be minimized by reducing unnecessary vertical and lateral movements. Attempts to over-extend forward and backward reaching in strokes that produce even the slightest bending of the body up, down, or sideways, are not worthy of adoption because of the detrimental consequences of the resistance created. Similarly, attempts to swim "over the water" also generate large vertical movement components and those unnecessary movements accentuate wave drag.

There are some beneficial vertical movements that can contribute to forward propulsion. The wave action that travels down the body in correctly executed underwater double-legged kicking and butterfly, is helpful. However, if that action is exaggerated to the point where the undulation is too large, particularly of the torso, head, and outstretched arms, and the wave action is not as fast as the swimmer's velocity, it will actually slow the swimmer than if no wave action was attempted at all.

The effects of slowing are different for each category of drag. If a swimmer doubled the speed of swimming, frictional drag would be twice as much as at the original speed, form drag would be four times as much, and wave drag would be eight times as much. The detrimental contributions of these types of drag become increasingly different and magnified as the swimmer's velocity increases. There comes a time when, because of wave and form drag that are involved in a swimmer's technique, any attempt to swim faster would consume so much energy to overcome the increased drag functions, that the energy required could not be mustered. In races, rather than trying harder to swim faster with a "rough" style, speed would be generated better by holding the effort level constant and trying to swim smoother and more streamlined.

It is because of the increasing importance of wave drag as velocity increases that unnecessary movements should be eliminated from swimmers' techniques. Streamlining is a relatively simple act in technique modification that will also affect how easily a swimmer slips through the water. Streamlining reduces form and wave drag. Shaving and wearing a technically efficient suit also are easy actions that will reduce frictional drag and consequently, will assist in speeding up swimmers.

These components of resistance cannot be ignored by a caring coach. They slow swimmers. If a swimmer attempts to go faster by producing more effort, and that effort alters technique to produce greater amounts of unproductive movements, then the added resistance caused by those movements may offset any potential speed benefits generated by the extra effort.

The technique of swimming fast must be efficient and produce the least resistance possible. Attention to drag factors will contribute to propelling efficiency and will make swimming fast a lot easier and rewarding.

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