Number 36

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


Brent S. Rushall, Ph.D., R.Psy.
[February 15, 2007]

The buoyant force from water and the pull of gravity determines how an individual floats. It varies considerably between swimmers. It requires technique adjustments because it interacts with any movement principle. Unfortunately, it is rarely discussed and when it is it is almost universally misunderstood by swimming coaches. Rather than being ignored, the characteristics of a swimmer's flotation should always be considered when teaching a new technique element or altering an old one. The determination of technique instruction or changes should first involve an analysis of a swimmer's flotation. All aspects of "ideal" technique will be modified by flotation effects.

Basic Physics of Flotation

When an object floats in a dense fluid, it is acted upon by two forces. One is the ever-present force due to gravity and the other is the buoyant force, the force exerted by the fluid on the object. When the object is symmetrical, such as a rectangular block of Styrofoam, the two forces coincide. When the object floats, the buoyant force equals the gravitational force. Since the floating object displaces its weight in fluid (Archimedes' Principle), equilibrium is reached and the Styrofoam block rests at the surface in the fluid. The water "supports" the weight of the object. This simple event is illustrated in Figure 1. Confusion often reigns between the roles of the volume and the weight of the water displaced. Archimedes' Principle is appropriate to understand what occurs. It commonly is stated as follows:

When a body is immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced.

Figure 1. The forces acting on a floating object in a fluid.

Weight is the common measure of the gravitational force. When the volume and weight of water displaced are equal to the volume and weight of the object, the object does not float or sink. The state of "neutral buoyancy" is achieved. This is useful for activities such as scuba diving, where a buoyancy compensator is used to produce neutral buoyancy. If the effects of that device were not used, a diver would be fighting constantly against sinking or rising in the water resulting in the unnecessary expenditure of energy and air supply.

When the volume of an object is greater than the volume of the fluid displaced, some of the object will show above the fluid surface. Specific gravity (in water) is the useful measure of the capacity to float. It is the fraction: the weight of the object divided by the weight of an equal volume of water. For example, if a 99 kg person displaces 100 kg of water when fully immersed, the specific gravity of the individual would be .99 (99/100). The volume of the person above the surface would weigh 1 kg. Because of varying densities within the human body, it is not possible to determine the percent of volume of the swimmer that would float out of the water.

In humans, the predominant constituent matters are bone, muscle, fat, air in the lungs and other structures, and fluids (e.g., blood). The proportions of these substances in an individual's physical make-up determine the specific gravity, the ability to float, and the characteristics of floating. There is considerable variation in these factors among humans.

Fat has a specific gravity of less than 1.0 and floats in water, while both bone and muscle have a specific gravity of slightly more than 1.0. Thus, persons with a high proportion of fat will float while some individuals with very low fat levels, heavy bones, and high muscle mass will sink. Normal persons usually float to varying degrees and in varying ways.

Center of Buoyancy

The Center of Gravity is the point through which the gravitational force acts (usually in the region between the points of the hips when the body is in the anatomical position). Its magnitude is described as "weight". The position of the body segments determines the actual site of the Center of Gravity.

The Center of Buoyancy is the point through which the buoyant force acts (usually in the lower chest area but it too is determined by the position of body segments). Some body parts are more buoyant than others, and so the center of buoyancy usually does not coincide with the center of gravity. The center of buoyancy relates to the body's volume while the center of gravity relates to the body's mass. Since these two factors normally are different, they are usually sited in different areas of the body. The distance between the centers of gravity and buoyancy usually is greater for males than females.

The divergence between the locations of the centers of gravity and flotation presents a problem for humans. In most cases, rather than floating level, the body rotates until the centers of gravity and buoyancy are aligned vertically. The body then displays a motionless float at that angle. The water supports the weight of a motionless swimmer but only at that angle. This is illustrated in Figure 2. When the body commences in a horizontal (streamlined) position the relationship of the center of buoyancy to the center of gravity will produce a rotational force and subsequent movement until the angle of flotation is attained.

Figure 2. The roles of the centers of buoyancy and gravity and how they determine
the angle at which a swimmer floats.

Occasionally, there are individuals who can float horizontally with a considerable volume of the body above the surface. Such persons predominantly are female and their centers of gravity and buoyancy almost coincide. An example of the floating position of a "floater" is provided in Figure 3. Such individuals swim with ease

Figure 3. A "floater": A person with a lower than normal specific
gravity and the centers of gravity and buoyancy
very close together..

Factors that Affect the Capacity to Float

Various factors affect how a person floats. Some of the more salient variables that should be of interest to swimming coaches are listed below.

  1. The distribution of body tissues on a unique physical structure causes each individual to float differently. When adipose tissue is concentrated more in one part of the body (e.g., around the thighs and hips of "pear-shaped" women) the center of buoyancy moves closer to the center of gravity reducing the rotational sinking effect in the lower part of the swimmer. Such individuals would need to devote less effort to force production to streamline the total shape. Conversely, swimmers with very little fat below the center of gravity, but with some above, will sink markedly. They will need to work harder with kicking to maintain streamline while swimming.
  2. The proportion of fat in an individual's overall physique will govern the tendency to float (among other things; see below). English Channel swimmers are benefited by being "fat" not only because the extra adipose tissue acts as an insulation factor but because it assists flotation. Less energy is used for internal body heating and for maintaining streamline.
  3. The volume of air in the lungs has a pronounced effect. After inhalation, a greater amount of water is displaced without any increase in weight. Thus, floating is easier when the lungs and chest are expanded but the angle of float will be increased.
  4. As the relative proportions of the major body tissues change with age, so does a person's specific gravity. Usually, specific gravity is lower in children and aged persons. It would be wrong to expect a young girl to float with the same proportion and parts of her body above the water as a champion woman swimmer (a common modeling technique of many coaches).
  5. Females tend to have a lower specific gravity than males because they are predisposed to having a higher percentage of body fat.
  6. The degree of tendency to float causes inter-individual variations in floating positions. Synchronized swimmers have low bone density, are fatter than competitive swimmers, and have good lung volumes. These features facilitate the repertoire of skills and stunts they need to perform. Competitive swimmers are benefited by the same physical tendencies.
  7. Racial differences. Asian Indians are lean "sinkers" because they have little fat and a high percentage of bone and muscle in their physical make-up. On the other hand, the Inuit are fat and round, adaptations that minimize heat loss. That combination also makes them float well. Unfortunately, political correctness will not make this disposition disappear. In some situations, it must be considered.

The density of water also determines how a person floats. Usually, one assumes water to be fresh and of a standard density. However, salt water is denser than fresh water. A swimmer would float slightly higher in salt water than fresh water1. Water can reach a saturation level of "saltiness". The Dead Sea is perhaps the most famous body of water that consists of salt-saturated water. People have no problem floating, usually horizontally, in such a natural phenomenon (see Figure 4).

Figure 4. People floating in the salt-saturated water of the Dead Sea.
The volume of water displaced is much less than would be
displaced in normal fresh water although the weight of the
displaced water would be the same in both fluids.

Implications for Competitive Swimming

The centers of gravity and buoyancy are determined by the physical make-up of the swimmer. The position a swimmer naturally attains in the water determines to a large part what a swimmer sees and feels. Changes to physical make-up produce changes to the flotation experience, which could affect the way a swimmer swims. Usually with changes of the swimming experience, there are changes in technique that compensate for them.

As swimmers grow, their physical attributes are altered dramatically. What an age-group swimmer experiences one day could be altered the next by a growth surge in one or more parts of the body. For example, if a pubescent boy's legs grow noticeably in the space of a few months, the distance between his centers of gravity and buoyancy will increase causing the legs to sink deeper in the water. If that swimmer continued swimming with the pre-growth technique, the efficiency of propulsion would decrease because of increased frontal resistance due to an increased flotation angle and possibly a minor increase in surface resistance. His specific gravity would also likely change and cause him overall to sink lower in the water. What he would see and feel as he swam would be altered. Those important factors would slow the swimmer unless compensatory technique changes were made to counteract the growth effects on flotation.

Most swimmers who participate in serious training from a late-infancy/pre-pubescent age to at least growth maturation undergo many physical alterations that require technique adjustments which in turn, demand correct adaptive coaching. For example, most young boys and girls in the under-10 years age-group float reasonably well and have little difficulty in adopting a horizontal streamline for crawl and backstroke2. As puberty and adolescence approach and proceed, gender differences in growth occur. Usually, when compared to girls, boys develop a higher specific gravity because of greater muscle mass and different limb to trunk ratios. Girls on the other hand, usually broaden at the hips and increase or maintain body fat changing their center of gravity and shape for moving through the water. Then as swimmers progress along the maturation path in adolescence, even further changes occur. All those variations require coaches to change the way they prime their teaching content for factors such as streamline for individual swimmers.

Consider the hypothetical example of the boy swimmer depicted above. When a young swimmer can adopt a streamlined crawl stroke position, he is able to carry his head and shoulders relative to his hips in a particular manner. The coach likely would only have to instruct him to swim flat and keep his head down [Heaven forbid that he is instructed to look ahead at 45.] With the change in leg structure and length due to accelerated growth in that body region, he would need to be told to compensate for the new natural angle of flotation (the angle would be increased). Instruction would have to change possibly by telling him to bury his head and shoulders deeper in the water and even have some water go over them (particularly when swimming slow). Another prime might be to have the top of his buttocks level with the back of his head. The experience would be to swim with the leading portion of the swimmer deeper/lower in the water than before. The compensatory movement to this new orientation should be for the legs to rise back to a streamlined position. Most coaches forget that when the position of one end of a swimmer is changed, the other end acts in a mirrored/opposite manner (e.g., raise the head and the feet sink; lower the head and the feet rise). The focus of coaching in this situation changes from a whole body perspective when young to one of accentuating what is done with a part of the body in the later years. The swimmer's manner of thinking about streamline would have to be altered to bring about consistent performance of the adjustment to present an habitual maintenance of streamline until the next growth change occurred. The versatility of a coach's repertoire of instructional primes for technique instruction needs to be substantial to accommodate inter- and intra-individual variations in physical structure and its consequential effects on flotation.

Coaching mature swimmers also needs consideration of the effects of physical changes on flotation. Swimmers who "bulk-up", assumedly with more muscle mass, will increase in specific gravity (they will sink more), as well as alter what the water will feel like flowing over newly positioned portions of the skin. Further, if the increased mass is in one part of the body, for example the shoulders and upper torso, the position of the centers of gravity and buoyancy will also change. Such alterations will present demands for technique changes. If swimmers are not instructed correctly, those changes could be serendipitous and wrong, dooming the swimmer to performance leveling or even worsening. Land-based training has been known for some time to be unrelated to swimming performance improvements in mature athletes. Before swimmers embark on the "obvious value of strength gains" (a swimming myth), the potential detrimental effects from such auxiliary training, which could outweigh any real or perceived benefits, should be contemplated.

Somewhat akin to musculature changes are the effects of noticeable increases or decreases in weight and what part of the body is altered the greatest with those changes. Unfortunately, most coaches think there is an ideal weight or percent body fat for all swimmers of both genders. That is wrong. The flotation characteristics of every swimmer are particularly individual and need to be considered accordingly.

Performing periodic flotation tests on all swimmers is one way of monitoring the effects of physical structure and instructed technique changes on the sensations produced by being supported by the water. Any structural change in a swimmer alters how a swimmer floats, which then requires the coach to make coaching adjustments to maintain a swimmer in a fluid-supported streamlined posture. Flotation characteristics and capacities need to be considered when instructing technique and developing performance expectations.

The Inherent Qualities of the Human Body Moving Through Water

While the human body is articulated complexly, its progress through water can be understood by comparing its non-force producing segments to that of boat hulls. At most stages of techniques, the head and trunk are the non-propulsive segments of the swimmer and constitute the primary load that is being propelled. Since no appreciable contributory forces are developed by these sectors of the body, any resistances that occur should be minimized.

To all intents and purposes, a swimmer's body (head and trunk) functions as a "displacement hull". It moves through the water, pushing it to the sides, under, or over the swimmer, that is, it displaces water. That contrasts with the other general classification of boat hulls, the "planning hull". With a planning hull, after gaining sufficient speed the vessel rides nearly on top of the water. All boats at rest or moving slowly are displacement hulls.

A floating displacement hull displaces a volume of water equal to its weight. At slow velocities, water is diverted around the hull while forming a bow wave. As velocity increases, the bow wave increases and the hull attempts to climb over it. However, a displacement-hull boat is not designed to do this. Consequently, a hull can move through the water only up to a velocity limit. Usually, the longer and narrower the displacement-hull, the greater is its potential velocity through water. [This implies that taller swimmers have the potential to move through the water faster than shorter swimmers; but there are other factors that need to be considered for this principle to be in effect.]

The concept of a displacement hull can be applied to swimmers to gain some knowledge of performance dynamics. The analogy is imperfect, but it does give information that is useful for understanding changes in performance in one swimmer and performance differences between two or more swimmers. Some of the important implications are discussed below.

A human swimmer is not designed to progress through water efficiently. One could argue that humans were not designed for movement in water at all. This poor design factor dictates that despite all efforts and designed movements, maximum progression velocity is quite low, at best being slightly more than 2 m/s. The ill-design of a human is demonstrated starkly by the prone-float push-and-glide maneuver. Despite attempts at streamlining, a swimmer stops progress within several seconds. Initial progress negatively accelerates very rapidly and then as the swimmer slows progress continues in incrementally less amounts.

A swimmer's shape limits progress because it incurs form and wave resistances of considerable magnitude and surface resistance to a lesser extent. The influence of those resistances increases markedly as velocity increases. There is a velocity level that produces sufficient resistance which negates any further velocity improvement. Thus, swimmers have a maximum velocity limit.

Flotation affects all competitive swimming strokes while characteristics of those strokes impose more restrictions on the maximum velocity that can be achieved. However, it behooves a swimmer to concentrate on minimizing resistance so that progression can approach its maximum. Since flotation usually produces a rotational force in the swimmer, if that rotation is not counteracted, resistance will be increased.

The displacement-hull analogy of the human body implies that it is erroneous to attempt to rise up in the manner of a planning hull. Swimmers do not and cannot achieve a planning-hull position. Throughout time, there have been directions given to swimmers and coaches to move out of a displacement-hull mode. The fabled Tarzan and Olympic Champion, Johnny Weissmuller, advocated swimming with the head up to "climb over the top of the water". Similarly, butterfly swimmers have been encouraged to fly over the water, an impossible and detrimental strategy. A characteristic of the majority of top-class swimmers is that the head position is kept low and the body is streamlined during forward progression. This has been referred to as swimming through the water. This feature is characteristic of displacement-hull designed nuclear submarines. It is illustrated in Figure 5.

Figure 5. The nuclear submarine USS Houston displaying
displacement-hull characteristics. This illustrates the general
principle of progressing through the water which should be
followed by competitive swimmers.

While a swimmer proceeds forward, most of the swimmer's energy is used to push water out of the way; water is displaced under and to both sides of the swimmer. Wave resistance will be reduced proportionally by any amount of water that can go over the top of the swimmer. This is the reason why swimmers progress better underwater (at approximately a depth of one meter or more to achieve maximum benefits). Any action or directive that leads to a part of the body being out of the water more than is supported by natural flotation will slow a swimmer.

As a swimmer increases velocity, there will be a minor tendency for the swimmer to rise in the water. This should make more of the dorsal aspect of the swimmer visible. However, increased visibility comes mostly from the increased bow wave that accompanies higher velocities. That higher bow wave produces a greater trailing trough that also exposes more of the swimmer. Some coaches take the increased visibility of some swimmers to be "because of the swimmer swimming over the surface". Swimmers are coached to swim higher in the water with the belief that resistance will be reduced but physics does not accommodate that belief. For every effort to rise up, a counter-reactive action on the other side of the center of buoyancy will occur making another part of the body sink. However, if a counter-reactive force can be produced on the same side of the center of buoyancy (usually an arm action) the rise will be counter-balanced but at the cost of the potential propulsive force developed by the arms being reduced.

Implications for Coaching

There are number of coaching implications (principles) that should be considered any time that a swimmer's technique is changed. The more important ones are listed below.

  1. When a movement element is altered in a swimmer, there usually will be a reciprocal change in a part of the body on the other side of the swimmer's center of buoyancy. Two technique alterations occur despite the intention of the coach to change only one.
  2. It is possible to alter a movement element on one side of the center of buoyancy without causing a reaction on the other side. To achieve this outcome it is necessary to perform a counter-balancing action (usually with the arms) on the same side of the center.
  3. Because of the rotational flotation characteristics of most swimmers, there usually will need to be some movements continually performed that produce force components that will counteract the rotary flotation force.
  4. The continual alteration of the positions of the body, head, and limbs while performing cycles of the various competitive swimming strokes will persistently alter the rotational flotation characteristics of the swimmer. Those characteristics will be altered further by the velocity of progression through the water, the stroke being performed, the deliberate technique features of the stroke, and the stroke rate. Thus, there will be phases in each stroke cycle that will require greater counter-balancing actions than at other phases.
  5. Attempts to raise a part of the body above that which is accommodated by floating will increase resistances (mostly form and wave resistances) and will require greater forces to be produced to maintain a semblance of streamline.
  6. To achieve a streamlined posture in the water many swimmers will have to dedicate some force/energy production to achieve a desirable position.
  7. A coach who attempts to change one feature of a swimmer's technique without considering the counter-balancing or counteracting movements will be ineffective and possibly a hindrance to the swimmer's development.


1In the 1950's and 60's there was a belief that swimmers were advantaged unfairly by swimming in salt-water meets as opposed to those in fresh water. This matter was considered by the world-governing body of swimming and found to be inconsequential. Thus, an intended stipulation that world-records could only be made in fresh water was shelved.

2The flotation of young people is aided by their normally shorter legs which shifts the center of gravity closer to the center of buoyancy. Their overall greater specific gravity will cause them to sink more but streamline will be easier to achieve because of the relative positions of the centers of gravity and buoyancy.

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