Are you in Canada? Click here to proceed to the HK Canada website.

For all other locations, click here to continue to the HK US website.

Human Kinetics Logo

Purchase Courses or Access Digital Products

If you are looking to purchase online videos, online courses or to access previously purchased digital products please press continue.

Mare Nostrum Logo

Purchase Print Products or eBooks

Human Kinetics print books and eBooks are now distributed by Mare Nostrum, throughout the UK, Europe, Africa and Middle East, delivered to you from their warehouse. Please visit our new UK website to purchase Human Kinetics printed or eBooks.

Feedback IconFeedback

Physical properties and principles of water and aquatic exercise

This is an excerpt from Rehabilitation of Musculoskeletal Injuries 5th Edition With HKPropel Online Video by Peggy A. Houglum,Kristine L. Boyle-Walker & Daniel E. Houglum.

Before you can apply aquatic exercises, you must understand how water affects the body’s ability to move and exercise. Although some of water’s properties can be determined by formulas, we will not focus on the precise mathematical applications here. It is important only to appreciate that these formulas can help us to understand the impact of water properties on exercise.

Specific Gravity

Specific gravity is also called relative density. It refers to the density of an object relative to the density of water.1, 2 It is, then, a ratio of an object’s weight to the weight of an equal volume of water. The specific gravity of water is 1. If an object has a specific gravity greater than 1, it will sink in water since its relative weight per volume is more than that of water. If an object has a specific gravity of less than 1, it will float in water. If the object’s specific gravity is 1, it will float just below the water’s surface.

Specific gravity for the human body varies from one person to another and from one body segment to another.2 The person’s specific gravity depends on the body’s composition of lean and fat mass and the distribution of body fat. The specific gravity of fat is 0.8, bone is 1.5 to 2.0, and lean muscle is 1.0.3 The average range of specific gravity for the human body is 0.95 to 0.97.4 Since the specific gravity of the average human body is less than 1, people will usually float. Women usually have more body fat than men, so women float better than men. A lean, muscular person may have a specific gravity of 1.10; an obese person may have a specific gravity of 0.93.5 These wide variations in individual specific gravities lead to a wide range of abilities to float. Patients who are more muscular and have less fat mass may have a difficult time floating, so they may need flotation devices during aquatic exercises.


Archimedes’ principle of buoyancy states that a body partially or fully immersed in a fluid will experience an upward thrust of that fluid that is equal to the weight of the fluid the body displaces.6 Buoyancy and specific gravity are closely related in that a body with a specific gravity of less than 1 will float because the weight of the water it displaces is more than the weight of the full body. For example, if a person has a specific gravity of 0.95, 95% of the body is submerged and 5% of the body floats above the water’s surface. The amount of water displaced is 95% of the body weight. Specific-gravity values, in essence, indicate the amount of the body that floats and the amount that is submerged; the weight of the body or part of the body submerged is equal to the weight of the water it displaces.

Center of Buoyancy

Center of buoyancy is the center of gravity of the displaced fluid and the point at which the buoyant force acts on the body. In water, two opposing forces act on the body. Buoyancy is the upward force, and gravity is the downward force. Each has a center point of balance. When a floating body is in equilibrium, the center of buoyancy and the center of gravity are in vertical alignment with each other (figure 10.1). In this position, the body is balanced. If the center of buoyancy and the center of gravity are not in vertical alignment with each other, the body is out of equilibrium and will tend to roll or turn. For example, if you place a kickboard between your knees, the center of buoyancy will cause your lower extremities to move upward to float.

Figure 10.1 When the center of buoyancy and the center of gravity are not in vertical alignment, a person must actively work to keep from rolling in the water. (a) The body is in equilibrium; the centers of gravity and buoyancy are aligned vertically. (b) The body is not in equilibrium; the centers of gravity and buoyancy are not aligned vertically.
Figure 10.1 When the center of buoyancy and the center of gravity are not in vertical alignment, a person must actively work to keep from rolling in the water. (a) The body is in equilibrium; the centers of gravity and buoyancy are aligned vertically. (b) The body is not in equilibrium; the centers of gravity and buoyancy are not aligned vertically.


Hydrodynamics is the branch of physics that explores the motion of solid objects in fluids and the forces imparted on those objects by the fluid. The fluid’s resistance to movement, the size and shape of the object moving, and the speed of the object all govern movement through water. Some of the factors that affect a body’s movement through fluid are interrelated and are important for the clinician to understand when he or she makes decisions about the aquatic exercises to include in a patient’s rehabilitation program.

Viscosity is the resistance to movement within a fluid caused by the friction of the fluid’s molecules. Properties that influence viscosity include cohesion (the attraction of water molecules to adjacent water molecules), adhesion (the attraction of water molecules to the person’s body), and surface tension (the attraction of water molecules on the surface to each other).7 Movement within the water is resisted by the adhesion of water molecules to the person in the water and the cohesion of water molecules to each other. Surface tension provides resistance when a body or body segment tries to break the water’s surface.

Drag is the water’s resistance to a body that is moving through it. The three types of drag are form drag, wave drag, and frictional drag.7

Form Drag Form drag is the resistance that an object encounters in a fluid. The amount of form drag is determined by the object’s size and shape.8 A larger object has more drag than a smaller object. A broad object has more drag than a streamlined object. Form drag is directly related to turbulence.9 The greater the form drag, the greater the turbulence. Turbulence produces a low-pressure area behind the object that tends to pull the object backward, like what is seen behind a speedboat moving on a lake (figure 10.2).

Figure 10.2 Form drag: (a) laminar flow (which produces minimal form drag) and (b) turbulent flow. Form drag is caused by turbulence behind an object moving through a fluid.
Figure 10.2 Form drag: (a) laminar flow (which produces minimal form drag) and (b) turbulent flow. Form drag is caused by turbulence behind an object moving through a fluid.

A streamlined object moving through water produces a laminar flow—a smooth movement of water that causes a minimal amount of resistance. There is less form drag because there is less turbulence. The water molecules all travel at the same speed past the moving body. Friction of the fluid is minimal because the water molecules separate easily, moving smoothly behind the object.

On the other hand, a broad object produces a turbulent flow as it moves through the water. The object has more form drag because of the greater turbulence created behind it. The layers of the water move irregularly as they run into the object and rush to move past and behind it. This causes a circular movement of the water layers as they rejoin behind the object. This circular motion of water layers pulling against the moving object is called an eddy. In essence, the turbulence creates a backward pull on the forward-moving object, adding to the effort the object must make to move through the water. Because of the disturbance caused by the eddy, a wake, or trail, is left in the water (seen as either bubbles behind the body or white water, depending on the amount of turbulence created).

Form drag can be used in an aquatic therex program as a means of altering resistance to exercises. A change in the position of the body or body segment can increase or decrease form drag. For example, moving the arm horizontally in the water with the palm down causes less form drag than with the hand in a vertical position. Shortening or lengthening the body’s extremity decreases or increases the form drag, respectively, since a longer lever arm pushes more water than a shorter one. Adding equipment such as hand paddles increases the surface area of the hand, and adding long paddles increases the lever-arm length; both provide additional form drag to increase the resistance of an exercise.

Wave Drag Wave drag is the water’s resistance because of turbulence caused primarily by the speed of the object in the water.10 The greater the speed of the object, the greater the wave drag. Wave drag is reduced if movement remains underwater because less wake is produced.11 The amount of water wake is an indication of wave drag. Swimming pools often have a splash gutter around the periphery to reduce wave drag for swimmers.

Exercises performed in calm water produce less resistance than those performed in turbulent water. The person can create wave drag during an exercise by changing positions often and rapidly. Increasing the speed of an exercise also increases the wave drag, thereby increasing the exercise’s resistance. For example, walking in water provides the body with 5 to 6 times the resistance that walking in air does. Running in water, however, increases the resistance to more than 40 times that of air.12

Frictional Drag Frictional drag is the result of water’s surface tension. This is not a factor in rehabilitation, but it becomes an important element for competitive swimmers. Frictional drag can add crucial milliseconds to a race time; swimmers reduce frictional drag by shaving body hair before competition.13 Recently, custom-made bodysuits constructed from unique new fibers have reduced frictional drag.14

Hydrostatic Pressure

Pascal’s law states that pressure from a fluid is exerted equally on all surfaces of an immersed object at any given depth (figure 10.3).15 The more deeply the object is immersed, the greater the pressure it encounters. Atmospheric pressure at the surface is 14.7 psi (pounds per square inch). For every foot of submersion, water pressure increases by 0.43 psi.5 Hydrostatic pressure can positively affect edema both by reducing postinjury edema and by allowing exercise without the risk of increasing it.

Figure 10.3 Pascal’s law.
Figure 10.3 Pascal’s law.

Clinical Tips

Several major physical properties of water, including specific gravity, buoyancy, center of buoyancy, and hydrodynamics, affect the way people exercise in water. These properties contribute to an optimal environment for exercise when body segments are very weak or weight bearing is restricted. Several aquatic exercises can be initiated early in a rehabilitation program, and others may continue well past the time when the patient has achieved enough strength and weight-bearing ability to perform land-based exercises.

Weight Bearing in Water

Since buoyancy and gravity are opposing forces acting on a body in water, the more deeply the body submerges in water, the less weight is borne by the lower extremities (figure 10.4). Because a male’s center of gravity is higher than a female’s, the specific percentage of body weight borne at different depths varies slightly from female to male. For example, with the body immersed to the xiphoid process, females bear 28% of their weight, whereas males bear 35% of their weight.16 Note that individual percentages will vary depending on body structure and weight distribution. In most cases, the percentage differences between men and women are not enough to require a distinction between the sexes when it comes to determining the most appropriate water depth for exercises. Figure 10.4 may be used as a general rule for males and females.

Figure 10.4 Weight bearing in water at different depths.
Figure 10.4 Weight bearing in water at different depths.

These percentages provide useful information, especially in the early stages of rehabilitation. For example, an injured basketball player who is partial weight bearing to 50% on the left lower extremity can perform left leg exercises in water that is at anterior superior iliac spine (ASIS) level. As the patient is permitted to bear more weight on the limb, he or she can perform the exercises in shallower water.

Changing walking speed in the water changes the weight-bearing forces in the water.17 Generally, the faster a person walks in the water, the higher the weight-bearing percentages. For example, if you walk at a slow pace, you can be in water that is at a level below the waist for you to be 50% weight bearing. If you walk at a fast rate, however, 50% weight bearing occurs in water above the axilla level.