Chapter objectives
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Explain the physical properties of water, including buoyancy, hydrostatic pressure, viscosity, and fluid dynamics.
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Identify the effect of immersion on weight bearing and appropriately apply weight-bearing guidelines in a clinical situation.
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Explain the physiologic responses of water immersion.
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Compare and contrast the physiologic response of exercise on land and in water.
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Identify and apply recommended guidelines for cardiovascular conditioning in water.
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Design a comprehensive aquatic-based rehabilitation program for an athlete that uses the principles of stretching, strengthening, balance, and aerobic conditioning.
Introduction
Since the earliest recording of history, water has offered a healing environment. Accordingly, it has been used throughout history to treat diseases and illnesses and for recreation. Although commonly used in Europe, aquatic-based rehabilitation in the United States did not begin until the early 1900s. Today, patients participating in aquatic rehabilitation benefit from a marriage of many different approaches, including the Bad Ragaz Ring method (Switzerland) and the Halliwick method (London). Although once viewed as a tool to be used solely in the acute stages of rehabilitation, where land-based rehabilitation was of limited use, aquatic-based rehabilitation has evolved considerably. It is now used throughout the rehabilitation process and serves as a component of prevention and wellness programs.
The purpose of this chapter is to discuss aquatic-based rehabilitation and training for the athlete. The physical properties of water are reviewed, and current research regarding aquatic-based cardiovascular conditioning programs is discussed. Extremity-, spine-, and core body–strengthening exercises are reviewed, as are balance and coordination exercises. Finally, recommendations regarding the transition from aquatic-based programs to resumption of competitive sports are made, and sample training programs are suggested.
Physical properties of water
Almost all of the biologic effects of immersion are related to the principles of water; a thorough understanding of these principles and their effect on the body is essential to support and provide a rationale for the use of aquatic-based rehabilitation. The key characteristics of water that affect the physiology of the human body include buoyancy and specific gravity, hydrostatic pressure, viscosity, and fluid dynamics.
Buoyancy and Specific Gravity
Archimedes’ principle states that when a body is immersed in a fluid, it will experience an upward thrust equal to the weight of the fluid that was displaced. Buoyancy is the upward thrust acting in the opposite direction of gravity; it is related to the specific gravity of the immersed object. By definition, the specific gravity of water is 1.0; the specific gravity of a human averages 0.974, which implies that on average, humans float when immersed. However, lean body mass (bone, muscle, and organs) has a specific gravity of 1.1, whereas fat has a specific gravity of 0.9. Therefore, lean athletes may have difficulty floating because of their body composition, which causes them to rest slightly below the water’s surface, or their lean extremities may sink while their trunk remains at the surface. Consequently, buoyant equipment may be necessary on the trunk or at various points along the limb to maintain buoyancy of a lean athlete in the pool.
Buoyancy has numerous clinical implications. Buoyancy-assisted exercises, which are movements toward the surface of the water, increase mobility and range of motion (ROM). Buoyancy-supported exercises are movements that are perpendicular to the upward thrust of buoyancy and parallel to the bottom of the pool. When performing buoyancy-supported exercises, buoyancy neither assists nor impedes the movement; it is similar to active ROM on land. Finally, buoyancy-resisted exercises directly oppose the upward thrust of buoyancy and are directed toward the bottom of the pool. They are similar to resisted exercises on land. Buoyancy-related movements and sample exercises are summarized in Table 11-1 .
Because athletes are often very lean, they may have difficulty maintaining buoyancy. To properly position the athlete in the water, the clinician may need to strategically place buoyant equipment along the athlete’s limbs, trunk, or both.
| Movement | Examples |
|---|---|
| Buoyancy assisted (PROM, AAROM) | Standing shoulder abduction from 0° to 90° In the prone position, shoulder flexion from 90° to 180° Standing knee extension from 90° to 0° (with the hip in 90° of flexion) Standing knee flexion from 0° to 90° (with the hip in neutral) |
| Buoyancy supported (AROM) | Trunk lateral flexion or hip abduction in the supine position (with flotations supporting the body) Standing shoulder horizontal adduction |
| Buoyancy resisted (RROM) | Standing elbow extension (with the shoulder in neutral) Standing knee extension (from 90° of knee flexion with the hip in neutral) Shoulder flexion from 0° to 90° in the prone position |
Hydrostatic Pressure
Pascal’s law of hydrostatic pressure states that at any given depth, the pressure from the liquid is exerted equally on all surfaces of the immersed object. As the depth of the liquid increases, so does hydrostatic pressure. Water exerts a pressure of 22.4 mm Hg/ft of water depth. At a water depth of 4 feet, the force from hydrostatic pressure is 89.6 mm Hg, which is slightly greater than diastolic pressure. Therefore, hydrostatic pressure may be used in rehabilitation to control effusion in an injured extremity while allowing the athlete to exercise. The effects of hydrostatic pressure occur immediately on immersion; blood is pushed proximally from the lower extremities, the chest wall is compressed, and the diaphragm is displaced upwardly. These changes have a significant impact on exercise training parameters.
In general, in water greater than 4 feet in depth, hydrostatic pressure will exceed the athlete’s diastolic pressure. Therefore, hydrostatic pressure may allow an athlete with an ankle injury to exercise acutely without increasing effusion.
Viscosity
Viscosity is defined as the friction occurring between individual molecules in a liquid, and it is responsible for resistance to flow. Resistance occurs as the molecules adhere to the surface of the moving object. Consequently, viscosity is noticeable only with motion through the liquid. Because of viscosity, all fast movements are subject to resistance in water, even in a buoyancy-assisted direction. In addition, when a patient who experiences pain with movement decreases the speed of movement, resistance also decreases, thereby allowing the patient to control the intensity of exercise.
Fluid Dynamics
Two different types of water flow exist: laminar flow and turbulent flow. Laminar flow is the smooth, streamlined flow of water molecules. It has the least amount of resistance because the water molecules are all traveling the same speed and direction. In contrast, turbulent flow is interrupted flow, as when laminar flow encounters an object, which causes the water molecules to rebound in all directions. As an object moves through water, resistance is created from pressure. Positive pressure exists in front of a moving object and impedes movement. The area immediately behind a moving object, the wake, is an area of low pressure and can hold the object back. This drag force created behind the object produces the majority of resistance to movement.
The shape of the object moving the water affects drag. A tapered, streamlined object will produce minimal disruption of flow, whereas an unstreamlined object will create rapid disruption of flow and cause turbulence. For example, forward walking is more resistive than walking sideways ( Fig. 11-1, A and B ).
Resistance is an important variable that clinicians can use when designing a rehabilitation program for an athlete. Resistance can be modified in three ways. First, resistance is affected by speed of movement. Drag force is proportional to the velocity of movement: the faster the movement, the greater the drag force and resistance to movement. For example, standing knee flexion and extension at 45°/sec will not be as challenging as performing the same movement at 120°/sec. Second, the frontal surface area of the object is directly proportional to the drag force produced and may be used to alter resistance. For instance, shoulder internal and external rotation with the forearms pronated creates less drag than performing the same exercise with the forearm neutral ( Fig. 11-2, A and B ). Finally, the flow of the water is another factor that affects resistance. Turbulent water flow increases the friction between molecules and therefore increases resistance to movement. This is the theory behind competitive swimmers’ preference to race through “still” water; resistance is less in nonturbulent water, which makes it easier for athletes to “pull” themselves through the water. All these factors and their effects on movement must be taken into consideration when designing a rehabilitation program for athletes. Table 11-2 summarizes methods to alter resistance in the pool.
| Methods to Alter Resistance | Effect | |
|---|---|---|
| To Make an Exercise More Challenging | To Make an Exercise Less Challenging | |
| Speed of movement | ↑ Speed | ↓ Speed |
| Object surface area | ↑ Surface area | ↓ Surface area |
| Water flow | Turbulent flow | Streamlined flow |
The type of muscle contraction is also an important consideration when designing a resistance program. Exercises performed against the water’s resistance almost always elicit concentric muscle contractions. Consider performance of shoulder internal and external rotation with the shoulder in neutral; the movements elicit concentric contractions of the rotator cuff muscles in a reciprocal fashion. However, it is possible to elicit eccentric muscle contractions in two ways: by using large pieces of buoyant equipment or with the use of a current. For example, an eccentric contraction of the hamstrings can be achieved when performing knee extension from 90° to 0° (with the hip in 90° of flexion) if a large flotation device is placed on the ankle. The athlete must eccentrically contract the hamstrings in a controlled fashion ( Fig. 11-3 ). In addition, eccentric contractions can be generated with the use of a current, as with the Swim Ex or Hydro Track AquaCiser III. For example, eccentric contractions of the left infraspinatus/teres minor can be accomplished by having athletes stand with the left side of their body against the current. They can achieve an eccentric contraction by holding a piece of equipment that increases surface area while allowing their shoulder to go from external rotation (in 0° shoulder abduction) to internal rotation in a controlled fashion. The faster the current, the greater the eccentric contraction generated.
Resistance can be increased in three ways: by increasing the speed of movement, by increasing the frontal surface area of the object, and by performing exercises in turbulent (versus “still”) water. The clinician must choose the most appropriate progression based on the athlete’s functional needs.
Effect of Depth of Immersion on Weight Bearing
The depth of water affects weight bearing because of the buoyancy of water. This is an important issue when designing a rehabilitation program, specifically for the lower extremities, for which weight bearing may be a concern. General weight-bearing percentages for males and females are listed in Table 11-3 . These numbers reflect static weight bearing; increasing the activity level to a fast walk can increase weight bearing by as much as 76%. Consequently, two options for progression of lower extremity weight bearing are available: decreasing the depth of the water and increasing the amount of impact at any given depth.
| Anatomic Landmark | Weight-Bearing Percentages | |
|---|---|---|
| Males | Females | |
| Seventh cervical vertebrae | 8 | 8 |
| Xiphoid process | 35 | 28 |
| Anterior superior iliac spine | 54 | 47 |
Physiologic responses to aquatic exercise
Responses to Water Immersion
Physiologic changes occur when immersed in water, both at rest and during exercise. It is imperative that the clinician be aware of these changes to modify rehabilitation programs appropriately. Changes that occur at rest during water immersion are the result of hydrostatic pressure. Most importantly, a cephalad redistribution of blood flow of approximately 0.7 L occurs. Of the 0.7 L redistributed proximally, one third of the blood is redistributed to the heart and two thirds to the pulmonary arterial circulation. Because of the increase in blood volume, the heart distends and myocardial wall tension increases, which results in a Frank-Starling reflex and a subsequent increase in stroke volume (SV) by approximately 35%. Diuresis and hormonal changes have also been observed with sustained periods of water immersion ( Table 11-4 ).
| Measure | Response |
|---|---|
| Right atrial venous pressure | Increases (8-12 mm Hg) |
| Heart blood volume | Increases (180-250 mL) |
| Cardiac output | Increases (25% +) |
| Stroke volume | Increases (25% +) |
| Central venous pressure | Increases |
| Heart rate | Remains the same or decreases slightly |
| Systemic blood pressure | Remains the same or increases slightly |
Immersion and Response to Exercise
Several studies have been conducted to determine whether cardiovascular training can be performed effectively in the water. Davidson performed a study to determine the effects of deep water running (DWR) and road running (RR) on the maximal oxygen consumption ( 
o 2 max) of untrained females in a crossover design. They determined that V o 2 max values did not differ between groups and concluded that both DWR and RR can improve cardiovascular fitness in young, sedentary women.
Byrne et al compared heart rate (HR), SV, and cardiac output ( 
) when 20 subjects ambulated at 2 and 3 mph on a dry land treadmill and an underwater treadmill. Underwater treadmill walking resulted in greater 
and SV at both speeds. When oxygen consumption ( 
o 2 ) levels for land and water were matched, the underwater treadmill session demonstrated a significantly lower HR and higher SV. The authors concluded that underwater treadmill walking may be beneficial and that HR may be a poor indicator of aquatic exercise intensity.
Robertson and Factora compared heart rates during DWR and shallow water running (SWR) at the same rate of perceived exertion. The authors found a significant difference of 10 beats/min between mean DWR (145 beats/min) and mean SWR (155 beats/min). The authors concluded that clinicians should not prescribe SWR at HR values obtained from DWR exercise.
To determine whether a water running program could maintain aerobic performance, Wilber et al studied 16 trained male runners. The runners were assigned to either a treadmill running or a water running group and trained five times per week for 6 weeks. The results showed no group differences. The authors concluded that DWR may serve as an effective alternative to land running to maintain aerobic performance.
Based on these studies and others, it has been determined that in general, 
o 2 is approximately three times greater at a given speed in water than on land. Therefore, a training effect can be achieved at a lower intensity in water than on land. In addition, HR during cardiovascular training will be approximately 10 beats/min lower than values attained during comparable exercise on land.
Effects of Water Temperature
Water temperature can have a profound effect on the cardiovascular response to exercise. Water is an effective conductor, with heat transferred 25 times faster than in air. Avellini et al studied three groups of males: one group exercising on land and one group each exercising in 32°C (90°F) or 20°C (68°F) water, respectively. Based on their findings, the authors concluded that training in cold water enhances SV and decreases HR, thereby increasing exercise efficiency.
In contrast, exercising in warm water can increase cardiovascular demands above those of exercise alone. Choukroun and Varene studied the effects of water temperature on the cardiovascular system of resting subjects immersed to the neck in 25°C (77°F), 34°C (93°F), and 40°C (104°F) water while measuring cardiovascular demands. 
increased significantly at 40°C (104°F). Because of the potential for heat-associated illness, temperature recommendations for intense training of athletes should be between 26°C and 28°C (79°F and 82°F) to prevent any heat-related complications.
Rehabilitation
As a result of the unique properties of water, aquatic rehabilitation offers advantages over land-based rehabilitation. It is imperative that the clinician apply aquatic principles appropriately when designing a rehabilitation program for an injured athlete. Buoyancy decreases weight bearing and joint compressive forces, which may be an important consideration with lower extremity rehabilitation. Buoyancy also allows exercises to be progressed from assisted, to supported, to resisted by simply changing the position of the athlete in the water. Because of viscosity, resistance is encountered in all directions of movement, and it can be modified by adjusting the lever arm, the speed of movement, or the turbulence of the water. Because resistance increases as the speed of movement increases, water provides an accommodating resistance to exercise and makes rehabilitation programs safe for an injured athlete.
Water also makes an ideal environment for cardiovascular conditioning. Training can be performed in a non–weight-bearing or partial–weight-bearing environment to allow the athlete to begin conditioning sooner. For example, an athlete with a lower extremity injury (e.g., an inversion ankle sprain) may perform DWR without the deleterious effects of increased edema because of the hydrostatic pressure of water. As healing continues, the athlete may be progressed to SWR, which minimizes the impact in comparison to land-based running.
Similar to land-based programs, the principles of tissue healing and exercise progression must be respected when designing an aquatic-based rehabilitation program for an athlete to prevent delayed healing and further injury. Cardiovascular conditioning should be addressed as soon as possible to prevent deconditioning. Stretching and ROM may be started when indicated, and strengthening, gait training, and increased weight bearing are initiated and progressed as tolerated with respect to tissue-healing constraints. The later phases of aquatic rehabilitation involve specific cardiovascular-conditioning drills combined with functional movement patterns, balance, impact loading, plyometrics, and sport-specific drills. An aquatic-based program can also be used to add variety to a maintenance program.
Cardiovascular Conditioning
Cardiovascular conditioning is a key component of a comprehensive rehabilitation program. With proper use of a pool, athletes can maintain or improve their cardiovascular function while resting an injury. This is particularly useful for athletes with a lower extremity injury, who may have weight-bearing or impact restrictions.
Appropriate warm-up and cool-down periods are fundamental components of the rehabilitation program. These activities should be performed in the water and may include walking, jogging, bicycling, or calisthenics; stretching should complement the warm-up and cool-down sessions. Many different cardiovascular activities may be performed. Depending on the stage of rehabilitation, the clinician may have the athlete rest the affected area or may choose to challenge muscles specific to the sport or activity. For example, a basketball player with a subacute knee injury may perform deep water cross-country skiing (possibly with a knee immobilizer to minimize knee motion). In the later phases of rehabilitation, SWR with impact may be appropriate. Both running and cross-country skiing may be performed in shallow or deep water, and the athlete may be tethered to the side of the pool in either depth. When teaching these techniques, the clinician should encourage proper form. The athlete is encouraged to maintain an upright posture and avoid excessive leaning, which mimics a swimming stroke. Running involves a small amount of ROM of the upper extremity but requires hip and knee flexion in the lower extremity. In contrast, cross-country skiing requires a large amount of hip and shoulder flexion and extension. Techniques may be modified to protect an injured area. When performing drills or cardiovascular conditioning in shallow water, the athlete may want to use aqua shoes or wear an old pair of running shoes in the pool. This will minimize foot irritation while offering support to the foot.
When basic cardiovascular-conditioning techniques have been mastered, advanced techniques such as vertical kicking may be incorporated, if indicated. Vertical kicking in deep water may be performed with or without fins and should be initiated with a small flutter kick. The athlete may initially require a flotation vest as well ( Fig. 11-4 ). The athlete may be progressed to a dolphin kick, which is used with the butterfly stroke but performed in an upright position, especially if dynamic lumbar stabilization is indicated. Ideally, kicking should be performed without upper extremity assistance, with the arms held behind the back or out of the water.
