SWIMMING SCIENCE BULLETIN

Number 19

Produced, edited, and copyrighted by
Professor Brent S. Rushall, San Diego State University

FORCES IN SWIMMING -- A RE-EVALUATION OF CURRENT STATUS: PART IV

Brent S. Rushall -- San Diego State University
Eric J. Sprigings -- University of Saskatchewan
Larry E. Holt -- Dalhousie University
Jane M. Cappaert -- United States Swimming


PART IV

RESISTIVE FORCES

When swimmers are not creating propulsive forces of sufficient magnitude, they slow down. It is frequently observed that some individuals seem to slip through the water requiring less effort 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.

Karpovich (1933) described three forms of resistance in swimming:

  1. skin friction which is analogous to the "stickiness" of the swimmer for moving through the water;
  2. eddy resistance, the amount of water that is sucked along behind a swimmer as forward progression is achieved and is proportional to the cross-sectional area that is pushed against the water, and
  3. wave-making resistance which is caused by swimmers' movements that move large amounts of water (e.g., excessive diving of the shoulders and body at the butterfly entry).

That classification, as is explained later, is meaningful to coaches. However, for a period of time swimming researchers followed the tact of talking about passive and active drags (e.g., Chatard et al., 1990). Passive drag is the amount of resistance that exists when a swimmer does not move. Active drag is the resistance created by movements and is added to passive drag. Others directed attention to partial causes of resistance. For example, Counsilman (1977, pp. 142-143) drew attention to (a) head-on or frontal resistance, (b) tail suction or eddy resistance, and (c) skin friction. It should be noted that frontal and eddy resistance are features of the same category of resistance, namely form drag.

It is important to consider resistance in terms of swimming performance. If resistance can be minimized, then the propulsive effects of a swimmer's efforts will be maximized. The coaching of swimming technique should focus on actions that increase swimming speed and decrease impedances to progress.

An understanding of resistances 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. It appears that reduction in drag is a preferred approach to improving speed through the water than is performing some subtle adjustment to technique. An emerging coaching approach for technique appears to be to perform propelling actions but not at the expense of creating any unnecessary drag. If a choice is to be made, it should be to preserve the minimum drag position and action over attempting any "stronger" action that could cause the drag (resistance) to increase. That is a very different approach to coaching stroke technique than is entertained by most coaches.

Resistance should be as low as possible on all parts of the body except the forearms and hands. Since they should attempt to create the greatest amount of drag force possible, their surface and position should maximize drag resistance. The conclusion is that drag forces should be minimized in most of the swimming action except for those surfaces that contribute to propulsion.

Sheehan and Laughrin (1992) recently drew attention to the classifications of resistance that were highlighted by Karpovich. They described their qualitative and quantitative effects as well as suggesting how they should be measured. The benefit of classifying three types of resistance is that each has direct application to coaching techniques. When only categories of active and passive resistance are used, they are too general to cover meaningful concepts for coaching. Sheehan and Laughrin's classifications are described below.

Frictional Resistance (Drag)

Frictional or surface drag is developed when water passes over a rough surface. This is part of passive drag. Skin roughness, body contouring, hair, and swim suit fabrics are examples of the roughness that creates 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. Figure 13 illustrates the major features concerning frictional drag.

Frictional resistance

Figure 13. Features concerning frictional drag in swimmers. The major implication is that the greater the amount of water dragged along by a swimmers, the greater will be the frictional resistance.

The secret of reducing skin/suit-friction drag is to maintain laminar flow, a condition where the fluid glides smoothly over the surface. Water in laminar flow behaves as if it was a sandwich of many sheets, each one sliding smoothly against its neighbor. The layer closest to the skin is pulled along nearly at the speed of the surface itself while the layer furthest away is hardly in motion. The entire sandwich of layers influenced by the surface itself is quite thin, but if it is in laminar flow, it provides lubrication allowing the body to slide through the water. It is very difficult to establish laminar flow. The slightest irregularity, a bump, sharp edge, or natural roughness is enough to spoil laminar flow and cause turbulent flow. Turbulent flow, in which water in contact with the skin or fabric swirls violently in a tangle of microscopic eddies, causes friction that robs the swimmer of speed, power, and efficiency. It is unlikely that the human body can attain laminar flow in all but a few minor places. For swimming efficiency, it is probably better to attempt to reduce turbulent flow that will result in better "sliding" through the water.

Shaving hair off the body and legs, but not the forearms can reduce frictional drag. The reduced resistance causes a reduction in the energy per stroke when compared to an unshaven condition (Sharp & Costill, 1990). Tight swimsuits of sheer fabrics with structures that minimize seams and edges are other ways of reducing frictional resistance. Wearing a latex cap also provides a smoother surface than does a head of hair and thus, further reduces drag.

It must be emphasized that the frictional surface must not be perfectly smooth, but rather, have a fine texture that holds a thin water film that becomes part of the swimmer and is carried along. That results in friction only being between water and water that is much less than between a very smooth skin and water (Imhoff & Pranger, 1975).

Form Resistance (Drag)

Form drag is caused by the shape (geometry) of the swimmer and is the second component of passive drag but may also be part of active drag. To a minor extent, it is affected by the density of water and is part of the explanation of the difference between salt and fresh water performances. Figure 14 illustrates the major features concerning form drag.

Form resistance

Figure 14. Features concerning form drag in swimmers. Four examples of increasing and decreasing streamline are shown. A: a head-up position (hyperextension of the neck) in crawl stroke tends to curve the spine and sink the hips lower than necessary. Straightening the neck (looking to the bottom) produces a more favorable streamline. B: crossing the entry behind the head in backstroke causes a hip movement from side to side, increasing form drag. If the entry is made a position where streamlining is not disrupted, form drag should be minimized. C: the head-up position of the breaststroker causes the hips to sink increasing form drag. That problem is accentuated if the arm action occurs under the body. When the arm action is well forward of the torso and head looks down, form drag is minimized. D: excessive head lifting and neck hyperextension during breathing can cause the legs and hips to drop lower in the water increasing form drag. When the head lift and neck movement are reduced, disruptions to streamlining and form drag are minimized.

The largest factor in shape is the cross-sectional area (frontal resistance) of the body. Form drag increases by the square of the velocity and so becomes increasingly important and influential 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. In fact, it is critical to propulsion where it is accentuated on the hands and forearms, and in some circumstances, the legs and feet. Form drag is passive when a swimmer's pure size contributes to the resistance. It is active and detrimental 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 in the water than the cross-sectional area presented by the shoulders and chest area alone; looking directly ahead in breaststroke which causes the hips to drop and the general body angle to be tilted rather than being as 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 it should be. In that case, the incorrect swimming alignment produces extra resistance, which is actively created although it could be reduced. Form drag increases in seriousness as a swimmer's speed increases.

Form drag can be lessened by accentuating streamlining at every opportunity (i.e., the swimmer has to create the thinnest and straightest form while going through the water). A general concept for most strokes is to have the shoulder/chest area create a gap in the water and the hips and legs follow through that space. That usually translates into swimming as flat as possible. Even the new breaststroke kick is designed to reduce the dropping of the knees that was a noted feature of the old action. When a breaststroker kicks and at the same time allows the hips to rise, that elevates the knees and reduces their contribution to form drag as well as producing a propelling force that is more horizontal and beneficial than the old, slightly downward kick. Most new advances in technique have aimed at maximizing streamlining, that is, reducing form drag. The streamlined position of Kieren Perkins, when compared to that of Joerg Hoffman, could account for some portion of his superiority (see Figure 15).

Perkins and Hoffman

Figure 15. A comparison of the streamline positions of Kieren Perkins (at the 1992 Olympic Games) and Joerg Hoffman (at the 1991 World Championships). Perkins' superior streamlining may account for much of his superiority over his arch rival Hoffman.

Wave Resistance (Drag)

Wave drag occurs when a swimmer creates waves, wakes, and turbulence and is a large component of active drag. Since waves carry energy, the source of that energy comes from the swimmer. Energy that could be applied to productive force is lost by unnecessary wave production. Although body position in the water has been described as contributing to form drag by increasing frontal resistance, it also contributes to wave drag by increasing following turbulence (usually termed "eddy resistance"). Figure 16 illustrates the major features concerning wave drag.

Wave resistance

Figure 16. Features concerning wave drag in swimmers. Actions that shift large amounts of water transfer much energy to that water instead of using it for propulsion. Examples of three common movements that increase wave drag in swimmers are illustrated. A: excessive diving at the butterfly entry. B: excessive reaching across behind the head at the backstroke entry which causes the hips to move laterally. C1: raising the head excessively to breathe in crawl stroke produces an exaggerated kick. C2: lowering the excessively raised head back into the water moves a large volume of water. These actions also contribute to increased form drag that makes them doubly troublesome.

Examples of wave production are;

  1. accentuated vertical movements (e.g., "flying" out of the water in butterfly, lifting the head when breathing in crawl stroke),
  2. lateral movements (e.g., hip sway in backstroke that results from the hand entry being placed behind the head; breathing backwards in crawl stroke that produces a hip movement and sideways kick), and
  3. any action that is not in a longitudinal horizontal direction.

Any bouncing or jerkiness in a swimmer's style also creates wave drag. Because of the human anatomy, it is not possible to remove all movements outside of the direct horizontal-longitudinal plane, but when they are exaggerated, wave drag becomes a major problem.

This is the worst form of drag because it increases as the cube of swimming velocity. The faster a swimmer goes, its contribution to resistance increases dramatically. This type of drag is one over which a swimmer has a great degree of control. Usually, increases in wave drag are also accompanied by increases in form drag, which makes their effects on propulsion particularly noticeable.

Wave drag, can be minimized by reducing unnecessary vertical and lateral movements. Attempts to over-extend forward and backward that produce even the slightest bending of the body up or down are not worthy of adoption because of the detrimental consequences of the wave drag that is created. Similarly, attempts to swim over the water in crawl stroke and butterfly also generate large vertical forces and unnecessary movements that create accentuated wave drag.

The effects of slowing are different for each form 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. It can be seen that these features of swimming efficiency become increasingly more important as a swimmer attempts to increase speed. There comes a time when, because of the 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.

Although the three forms of drag have been explained separately, in the dynamic actions of swimming one unnecessary action could cause detrimental increases in each form of resistance. For example, if a swimmer exaggerated the head and shoulder movement in butterfly by diving unnecessarily at the entry, the following would likely occur:

  1. frictional drag would increase because of the complete covering of the upper body and back of the head;
  2. form drag would increase because of the increased departure from a streamlined alignment caused by excessive hip flexion; and
  3. wave drag would be increased because of the excessive displacement of water.

Thus, when considering resistances that result from swimming actions it is prudent to consider if the three forms of drag have been affected rather than just one or two.

It is because of the increasing importance of drag as speeds increase that unnecessary movements should be eliminated from swimmers' techniques. Streamlining (reducing all forms of drag on non-propulsive body segments) is a relatively simple act in technique modification that will also affect how easily a swimmer slips through the water. Streamlining reduces both 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.

Practical Implications

For every technique change that is attempted in swimmers, its effect on drag has to be considered. If drag is increased, then it most likely will not be of advantage to change a swimmer's technique.

A caring coach cannot ignore these components of resistance. 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.

The relationship of increases in resistance to increases in speed creates a hierarchy of coaching preferences.

  1. Streamlining and the minimization of drag are paramount.
  2. Actions which cause vertical movements, particularly of the head, shoulders, and hips, should be minimized and, where possible, eliminated.
  3. Actions that cause lateral movements, particularly of the head, shoulders, and hips, should be eliminated.
  4. Technique alterations and actions should only be those which do not upset the position and actions which minimize drag.
  5. Technique changes should not be made or modified if they produce a negative alteration in a swimmer's position and postural control in the water.

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