Principles of Aquatic Therapy

It is necessary to understand the basic principles and properties of water, including relative density, buoyancy, viscosity, resistance, hydrostatic pressure, and surface tension to appreciate the benefits of aquatic therapy. These are important components to consider when planning an aquatic rehabilitation program.

Buoyancy

A body immersed in water is subjected to two forces, gravity and buoyancy. Buoyancy is defined as the upward thrust of water acting on a body that creates an apparent decrease in the weight of a body while immersed.² Buoyancy stems from Archimedes’ principle: When a body is fully or partially submerged in a fluid at rest, it experiences an upward thrust equal to the weight of the fluid displaced.^1,2,4,6 The amount of water displaced depends on the density of the body immersed relative to the density of water.² An immersed limb or body with a relative density less than that of water will be assisted to the surface by buoyant forces.⁷ An object with a relative density less than 1 floats because the weight of the object is less than the weight of the water displaced.^5,8 Buoyancy acts directly through the center of buoyancy.³ The center of buoyancy is the centroid of the displaced volume of water and depends on the distribution of the displaced volume of fluid relative to the body.⁹ Problems arise when the center of gravity and the center of buoyancy are not in the same vertical line. This can occur with the use of improperly placed floating devices, which can disrupt the vertical line, causing an animal to tilt or flip over so as to reach a state of equilibrium.³ The location of the center of buoyancy varies depending on the percent of body fat and distribution of adipose tissue. The more caudal the distribution of adipose tissue (which tends to occur more often in human females than males), the closer the center of buoyancy is located to the center of mass.

Buoyancy aids in the rehabilitation of weak muscles and painful joints. It allows the patient to exercise in an upright position and may decrease pain by minimizing the amount of weight bearing on joints. For example the percentage of weight borne by human females when immersed in water is approximately 5.9% to 8.7% at the seventh cervical vertebral level, 25% to 31% at the level of the xiphoid, and 40% to 51% at the level of the anterior superior iliac spine.¹⁰ Males bear 6.8% to 10%, 30% to 37%, and 50% to 56% of their body weight in the upright position at these same levels.¹⁰ Other studies of humans have also found similar percentages of weight bearing at these same levels.^11,12 In a study performed on dogs the amount of body weight borne when immersed in water (as a percentage of body weight on dry ground) was approximately 91% when the water was at the level of the lateral malleolus of the tibia, 85% at the level of the lateral condyle of the femur, and 38% at the level of the greater trochanter of the femur (Figure 31-1).¹³ This information may be particularly useful when treating patients with arthritis because joints may be unloaded as a result of the buoyant properties of water.

Aquatic Exercise Research

There are many benefits of aquatic therapy. Exercising in water is effective for improving strength, muscular endurance, cardiorespiratory endurance, range of motion (ROM), agility, and psychological well-being, while minimizing pain.17–19 The purpose of this section is to provide the reader with information regarding human studies of the benefits of aquatic therapy. Table 31-1 provides a summary of these studies. It is likely that similar principles apply to animals.

Gravity is the primary resistant force to exercise on land, whereas viscosity, friction, and turbulence are the primary resistant forces to exercise in water; these properties have a direct effect on heart rate and oxygen uptake.²⁰ Arm exercises on land produced an increase in heart rate from a resting rate of 77 beats per minute to 92 beats per minute in men (age 21 to 27) and from 74 to 96 beats per minute in women (age 20 to 29). Exactly the same arm exercises in water produced an increase in heart rate from 60 to 99 beats per minute in men and from 60 to 98 beats per minute in women. Compared with the subjects exercising on land, the subjects exercising in water exercised at slightly higher heart rates and had a greater overall increase in heart rate because of the initial lower resting heart rate. Subjects in this study had lower resting heart rates in water than on land because of two factors: the reflex response of the cardiovascular system to the cold receptors of the skin, and the effect of hydrostatic water pressure exerted on the submerged body.²⁰ The water temperature ranged from 26° to 26.5° C (78.8° to 79.7° F) during aquatic exercise. As discussed later, water temperature directly affects heart rate. Leg exercises further increased heart rate during water exercises: 76 (resting) to 124 (exercise) beats per minute for men on land versus 60 (resting) to 139 (exercise) beats per minute in water, and 73 (resting) to 126 (exercise) beats per minute for women on land versus 60 (resting) to 126 (exercise) beats per minute in water. The increased weight of legs as compared with arms may increase the necessary energy expenditure for exercises performed on land because of the increased mass and pull of gravity. This study demonstrated that heart rate and oxygen uptake were greater when performing exercises in water compared with performing the same exercises on land.²⁰

The metabolic requirements are also greater for exercises performed in water than for exercises performed on land. In a study by Johnson et al.,²⁰ men and women performed the same arm and leg exercises in water and on land. Oxygen uptake during exercise in water was 34% greater in men and 27% greater in women as compared with the same exercises on land.²⁰ The differences between men and women are probably owing to different body compositions (male body composition tends to be more muscular, which requires more oxygen) and heights (men tend to be taller and have longer extremities, thereby having a longer lever arm, which requires more exertion to move). Another study found that subjects could walk or jog at slower speeds in water than on land while expending the same amount of energy as measured by oxygen consumption. In this study subjects walked at 2.6 to 3.5 km/hour (1.6 to 2.2 mph) in water, whereas subjects on land walked or jogged at speeds of 5.5 to 13.4 km/hour (3.4 to 8.3 mph). Both expended the same amount of energy, despite the greater velocity of people walking on land.²¹ A study by Whitley and Schoene¹⁹ supports these data with the observation that various types of exercises performed in water (aquatic calisthenics, shoulder and leg exercises, bicycle ergometry exercises, walking, and jogging) require a significantly greater amount of energy expenditure compared to the same exercises performed on land. Subjects in a study by Hall et al.²² also expended more energy walking in water than on land at speeds greater than 4 km/hour (2.5 mph).

When aerobic exercise is used in the rehabilitation process, it may aid in improving cardiovascular fitness and weight reduction. Melton-Rogers et al.²³ compared exercise during land bicycle ergometry with running in water with a flotation device in individuals with rheumatoid arthritis. Higher ratings of perceived exertion (RPE) by the subjects and respiratory exchange ratios were seen during water running.²³ These studies demonstrate that walking in water at specific speeds may be sufficient to meet heart rate requirements for developing and maintaining cardiorespiratory fitness.

Aquatic exercises are also beneficial for muscle strengthening. Bravo et al.¹⁷ performed a study on postmenopausal women who exercised in a pool with waist-deep water for 60 minutes, three times a week for 12 months. The average strength gain as a component of functional fitness was approximately 18%. Because of buoyancy water walking can provide an excellent alternative method of achieving and maintaining muscle strength and ROM in individuals with lower limb joint problems such as osteoarthritis (OA). Although exercise in water may not be as effective in achieving maximum muscle performance as land exercises, rehabilitation in water may minimize the amount of joint effusion and lead to greater functional improvement.²⁴ Closed-chain exercises such as walking in water are more closely associated to movements in everyday activities than open-chain exercises because they recruit muscles in a more functional manner.²⁵ Performing closed-chain exercises in an environment in which weight-bearing forces are decreased may minimize or eliminate damage and inflammation to the soft tissues, while maximizing functional training. The aqueous environment may also aid in reducing knee pain and joint effusion,²⁴ which may facilitate the recovery of lower-extremity function after cranial cruciate ligament (CCL) stabilization in dogs.

Water’s buoyancy can ease the performance of exercise activities while also providing proprioceptive feedback to aid in the rehabilitation process. The effect of buoyancy allows for gentler active exercises by decreasing the loads placed on the injured tissues compared with exercises performed on land.¹⁸ Aquatic exercises may be used as a transition to land-based exercises in postsurgery or postinjury rehabilitation. Water exercises are generally less painful than land exercises because of the support that buoyancy provides. Therefore water exercises may result in less discomfort maintain ROM and functional movement, and provide a better sense of security when initiating active movements.¹⁸

There are many physiologic effects resulting from exercise in heated water. Among them are increased circulation to muscles, increased joint flexibility, and decreased joint pain.²⁶ Templeton et al. performed a study to determine whether subjects with rheumatic diseases experienced changes in joint flexibility and functional ability following an aquatic therapy program.²⁶ Subjects exercised in water for 8 weeks. Exercises consisted of therapeutic games emphasizing both isotonic and isometric exercises of the trunk, neck, and upper and lower extremities. Water temperature was maintained at 32.9° C (91.3° F), and the air temperature was maintained at 31.7° C (89° F). Active ROM and a Functional Status Index test that recorded the degree of assistance, pain, and difficulty experienced in the performance of 18 tasks were measured before and after the 8-week aquatic therapy session. The results of the study indicated that pain decreased with therapy, and active ROM and functional ability increased. The decreased pain and improvement in performing daily activities had a significant impact on the overall increased functional ability of the subjects.²⁶ These findings support aquatic therapy as an effective rehabilitation technique for increasing functional ability and joint flexibility in people with severe joint disease. Osteoarthritic and rheumatoid arthritic patients participated in another study of the effects of aquatic exercise on ROM and strength.²⁷ Control subjects were asked to refrain from participating in any new physical activity throughout the duration of the study, whereas the exercise group participated in an aquatic exercise program. After 6 weeks, hip strength and ROM were measured. Hip strength in the aquatic exercise group increased 10.9%, whereas ROM increased 11.8% as compared with the control group.²⁷

Water temperature may have a significant effect on the cardiovascular response to exercise. Subjects in cool water exercise at lower heart rates than those exercising on land at the same workload for several reasons.²⁸ If the water temperature is low, peripheral vasoconstriction occurs, blood moves centrally, venous return is enhanced, and stroke volume increases. Therefore to maintain the same cardiac output, heart rate decreases. In one study, subjects training in cool water (20° C [68° F]) had enhanced stroke volume and decreased heart rate, thereby increasing exercise efficiency.²⁸ It is also our experience that dogs exercise at lower heart rates walking on underwater versus land treadmills at the same velocity and length of time (unpublished data, Millis DL, 2000).

Blood pressure is also affected by water temperature. In a study of subjects at rest while submerged in 32° C (89.6° F) water, systolic blood pressure decreased 14 ± 14 mm Hg and diastolic pressure decreased 6 ± 10 mm Hg because of peripheral arterial dilation. On the other hand, exercising in water at temperatures that exceed body temperature may increase cardiovascular demands above those of exercise alone.¹² In one study, cardiac output was measured at rest while people were immersed in water temperatures of 25°, 34°, and 40° C (77°, 93.2°, and 104° F).³⁰ Cardiac output of those immersed in 40° C water increased significantly. The authors of this study recommend that elite athletes train in water temperatures between 26° C and 28° C (78.8° F and 82.4° F) to prevent any heat-related complications.³⁰ This temperature range is also recommended by the authors for pools in which healthy dogs are training. Lower temperatures are also apparently tolerated quite well by dogs, especially those with thicker hair coats.

All of the benefits of aquatic therapy in humans (improved strength, muscle endurance, cardiorespiratory endurance, increased ROM, decreased stress on healing tissues, and minimized pain) may also apply to dogs. The type of aquatic therapy used by therapists, veterinarians, and owners depends on the specific rehabilitation needs for each individual. An understanding of the benefits of aquatic therapy aids the prescription of specific protocols.

Aquatic Therapy Equipment

This section provides the reader with information concerning the types of aquatic therapy and equipment currently available for dogs. The anticipated use, size of the dogs receiving therapy, and funds available help determine the type of equipment best suited for rehabilitation. Some of the available forms of aquatic therapy include underwater treadmills, pools of various sizes, and whirlpools (Figure 31-2).

Underwater treadmills are very useful for aquatic therapy. In addition to the buoyancy and resistance effects of water, the treadmill encourages walking or jogging with a more normal gait pattern than is achieved with swimming. The height of the water can be adjusted, which alters the buoyancy and alters joint motion. Underwater treadmills are very useful in neurologic patients to help with gait patterning with the support of the water. Units typically have entrance and exit doors, a storage tank for water, heater, filters, and water treatment with chlorine or bromine. Dogs typically walk into the unit, the doors are sealed shut, and the water is pumped into the chamber until the desired height is reached. The treadmill is then turned on at the appropriate speed. On occasion, a therapist may accompany a patient to assist with gait training. After treatment is completed, the water is pumped out, the doors are opened and the patient is rinsed off and dried. Units are available in some countries that have the water in the treadmill compartment, with an adjustable treadmill that lowers into the water. This type of unit helps to save space because a storage tank is not necessary, but the dog must be elevated on a lift to step on the treadmill; therefore, caution must be used to avoid the danger of the dog jumping from the lift or treadmill.

Swimming pools may also be used for rehabilitation. Several options must be considered when installing a pool. Some of the considerations include whether jets or propellers used to circulate the water and add resistance to exercises will be useful; the location of the jets; the possibility of adding an underwater treadmill into the pool; hooks on the inner walls to which safety straps may be attached; manual versus computer-controlled water treatment and heating systems; above-ground versus in-ground pools; and stairs, lifts, or ladders to enter the pool. In addition the physical aspects of the enclosure and surrounding area must be considered. For example the humidity and condensation associated with a heated indoor pool must be considered. The floor surface around the pool must also be carefully designed to avoid slipping and falling by both patients and handlers.

Other forms of aquatic therapy include hot tubs or whirlpools used in hospitals (Figure 31-3). Cost-effective forms of aquatic therapy for smaller dogs include bathtubs, large basins, and plastic pools. Taking a dog to a lake or river is also a cost-effective means of providing aquatic therapy, but caution must be exercised when swimming dogs with recent incisions in these environments. Strong water currents and natural hazards must also be avoided.