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Aquatic immersion has profound biological effects, extending across essentially all homeostatic

systems. These effects are both immediate and delayed and allow water to be used with

therapeutic efficacy for a great variety of rehabilitative problems. Aquatic therapies are

beneficial in the management of patients with musculoskeletal problems, neurologic

problems, cardiopulmonary pathology, and other conditions.


Nearly all the biological effects of immersion are related to the fundamental principles of hydrodynamics.The essential physical properties of water that effect physiologic change are density and specific gravity, hydrostatic pressure, buoyancy, viscosity, and thermodynamics.


Although the human body is mostly water, the body’s density is slightly less than that of water and averages a specific gravity of 0.974, with men averaging higher density than women. Lean

body mass, which includes bone, muscle, connective tissue, and organs, has a typical density near 1.1, whereas fat mass, which includes both essential body fat plus fat in excess of essential

needs, has a density of about 0.9 . Highly fit and muscular men tend toward specific gravities greater than one, whereas an unfit or obese man might be considerably less. Consequently,

the human body displaces a volume of water weighing slightly more than the body, forcing the body upward by a force equal to the volume of the water displaced .


Pressure is directly proportional to both the liquid density and to the immersion depth when the fluid is incompressible. Water exerts a pressure of 22.4 mm Hg/ft of water depth, which

translates to 1mmHg/1.36 cm (0.54 in.) of water depth. Thus a human body immersed to a depth of 48 inches is subjected to a force equal to 88.9mmHg, slightly greater than normal diastolic

blood pressure. Hydrostatic pressure is the force that aids resolution of edema in an injured body part.

Hydrostatic pressure effects begin immediately on immersion, causing plastic deformation of the body over a short period. Blood displaces cephalad, right atrial pressure begins to rise, pleural surface pressure rises, the chest wall compresses, and the diaphragm is displaced cephalad.


A human with specific gravity of 0.97 reaches floating equilibrium when 97% of his or her total body volume is submerged. As the body is gradually immersed, water is displaced, creating

the force of buoyancy, progressively offloading immersed joints.

With neck-depth immersion, only about 15 lb of compressive force (the approximate weight of the head) is exerted on the spine, hips, and knees. A person immersed to the symphysis pubis has effectively offloaded 40% of his or her body weight, and when further immersed to the umbilicus, approximately 50%. Xiphoid immersion offloads body weight by 60% or more, depending on whether the arms are overhead or beside the trunk. Buoyancy may be of great therapeutic utility. 


A fractured pelvis may not become mechanically stable under full body loading for a period of many weeks. With water immersion, gravitational forces may be partially or completely offset so that only muscle torque forces act on the fracture site, allowing active assisted range-of-motion activities, gentle strength building, and even gait training. Similarly, a lower extremity patient with weight-bearing restrictions may be placed in an aquatic depth where it is nearly impossible to

exceed those restrictions.


Viscosity refers to the magnitude of internal friction specific to a fluid during motion. A limb moving relative to water is subjected to the resistive effects of the fluid called drag force

and turbulence when present. Under turbulent flow conditions, this resistance increases as a log function of velocity.

Viscous resistance increases as more force is exerted against it, but that resistance drops to 0 almost immediately on cessation of force because there is only a small amount of inertial moment as viscosity effectively counteracts inertial momentum. Thus, when a person rehabilitating in water feels pain and stops movement, the force drops precipitously as water viscosity damps movement almost instantaneously. This allows enhanced control of strengthening activities

within the envelope of patient comfort


Water’s heat capacity is 1,000 times greater than an equivalent volume of air. The therapeutic utility of water depends greatly on both its ability to retain heat and its ability to transfer heat energy. Water is an efficient conductor, transferring heat 25 times faster than air. This thermal conductive property, in combination with the high specific heat of water, makes the use of water in rehabilitation very versatile because water retains heat or cold while delivering it easily to the

immersed body part. Water may be used therapeutically over a wide range of temperatures. 


Cold plunge tanks are often used in athletic training at temperatures of 10°–15°C to produce a decrease in muscle pain and speed recovery from overuse injury. 

Most public and competitive pools operate in the range of 27°–29°C, which is often too

cool for general rehabilitative populations, because these populations are usually less active in the water. Typical therapy pools operate in the range of 33.5°–35.5°C, temperatures that permit lengthy immersion durations and exercise activities sufficient to produce therapeutic effects without chilling or overheating.

Hot tubs are usually maintained at 37.5°– 41°C, although the latter temperature is rarely comfortable for more than a few minutes, and even the lower typical temperature does not allow for active exercise. Heat transfer begins immediately on immersion, and as the heat capacity of the human body is less than that of water (0.83 versus 1.00), the body equilibrates faster than water does.


Because an individual immersed in water is subjected to external water pressure in a gradient, which within a relatively small depth exceeds venous pressure, blood is displaced

upward through the venous and lymphatic systems, first into the thighs, then into the abdominal cavity vessels, and finally into the great vessels of the chest cavity and into the heart. 

Central venous pressure rises with immersion to the xiphoid and increases until the body is completely immersed.

There is an increase in pulse pressure as a result of the increased cardiac filling and decreased heart rate during thermoneutral or cooler immersioN. Central blood volume increases by approximately 0.7 L during immersion to the neck, a 60% increase in central volume, with

one-third of this volume taken up by the heart and the remainder by the great vessels of the lungs. 

Cardiac volume increases 27%–30% with immersion to the neck.Stroke volume increases as a result of this increased stretch. Although normal resting stroke volume is about 71 mL/beat, the additional 25 mL resulting from immersion equals about 100 mL, which is close to the exercise maximum for a sedentary deconditioned individual on land and produces both an increase in end-diastolic volume and a decrease in end-systolic volume. 

Mean stroke volume thus increases 35% on average during neck depth immersion even at rest. As cardiac filling and stroke volume increase with progress in immersion depth from symphysis to xiphoid, the heart rate typically drops and typically at average pool temperatures the rate lowers by 12%–15%. This drop is variable, with the amount of decrease dependent on water temperature. In warm water, heart rate generally rises significantly, contributing to yet a further rise in cardiac output at high temperatures.

The relationship of heart rate to VO2 during water exercise parallels that of land-based exercise, though water heart rate averages 10 beats/min less.

During immersion to the neck, decreased sympathetic vasoconstriction reduces both peripheral venous tone and systemic vascular resistance by 30% at thermoneutral temperatures, dropping during the first hour of immersion and lasting for a period of hours thereafter. This decreases

end-diastolic pressures. Systolic blood pressure increases with increasing workload, but generally is approximately 20% less in water than on land.

Based on a substantial body of research, aquatic therapy in pool temperatures between 31°–

38°C appears to be a safe and potentially therapeutic environment for both normotensive and hypertensive patients. Exposure to a warm environment causes peripheral vasodilatation, a reduction in vascular resistance and cardiac after load might be therapeutic.

Researchers found that during a single 10-min immersion in a hot water bath (41°C), both pulmonary wedge pressure and right atrial pressure dropped by 25%, whereas cardiac output and stroke volume both increased.

Studies of elderly individuals with systolic congestive heart failure during warm water immersion found that most such individuals demonstrated an increase in cardiac output and ejection fractions during immersion.

The pulmonary system is profoundly affected by immersion of the body to the level of the thorax. Part of the effect is due to shifting of blood into the chest cavity, and part is due to compression of the chest wall itself by water. The combined effect is to alter pulmonary function, increase the work of breathing, and change respiratory dynamics. Vital capacity decreases by 6%–9% when comparing neck submersion to controls submerged to the xiphoid with about half of this

reduction due to increased thoracic blood volume, and half due to hydrostatic forces counteracting the inspiratory musculature.

The combined effect of all these changes is to increase the total work of breathing when submerged to the neck. The total work of breathing at rest for a tidal volume of 1 liter increases by 60% during submersion to the neck. Of this increased effort three-fourths is attributable to redistribution of blood from the thorax, and the rest to increased airway resistance and increased hydrostatic force on the thorax. Most of the increased work occurs during inspiration. Because fluid dynamics enter into both the elastic workload component as well as the dynamic component of breathing effort, as respiratory rate increases turbulence enters into the equation. 

Consequently there must be an exponential workload increase with more rapid breathing, as

during high level exercise with rapid respiratory rates.

Inspiratory muscle weakness is an important component of many chronic diseases, including congestive heart failure and chronic obstructive lung disease. Because the combination

of respiratory changes makes for a significantly challenging respiratory environment, especially because respiratory rates increase during exercise, immersion may be used for respiratory training and rehabilitation. 


For an athlete used to land-based conditioning exercises, a program of water-based exercise results in a significant workload demand on the respiratory apparatus, primarily in the muscles

of inspiration. Because inspiratory muscle fatigue seems to be a rate- and performance-limiting factor even in highly trained athletes, inspiratory muscle strengthening exercises have proven to be effective in improving athletic performance in elite cyclists and rowers. The challenge of inspiratory resistance posed during neck-depth immersion could theoretically raise the respiratory muscular strength and endurance if the time spent in aquatic conditioning is sufficient in intensity and duration to achieve respiratory apparatus strength gains.

The common response is a perception of easier breathing at peak exercise levels.


Respiratory strengthening may be a very important aspect of high level athletic performance.

When an athlete begins to experience respiratory fatigue, a cascade of physiologic changes follows. The production of metabolites, plus neurologic signaling through the sympathetic nervous system, sends a message to the peripheral arterial tree to shunt blood from the locomotor

musculature. With a decline in perfusion of the muscles of locomotion, the rate of fatigue increases quite dramatically.



Respiratory muscle weakness, especially in the musculature of inspiration, has been found in chronic heart failure patients and this weakness is correlated closely with cardiac function and may be a significant factor in the impaired exercise capacity seen in individuals with chronic heart failure.

Aquatic therapy may be very useful in the management of patients with neuromuscular impairment of the respiratory system, such as is seen in spinal cord injury and muscular dystrophy.

Programs typically used include chest-depth aerobic activity for general rehabilitation populations usually at therapy pool temperatures. For chronic obstructive pulmonary disease

patients, depth should start at waist level, and progress into deeper water as strength and respiratory tolerance improves.


Water immersion causes significant effects on the musculoskeletal system. 

The effects are caused by the compressive effects of immersion as well as reflex regulation of blood vessel tone. During immersion, it is likely that most of the increased cardiac output is redistributed to skin and muscle. Resting muscle blood flow has been found to increase from a dry baseline of 1.8 mL/min/100 g tissue to 4.1 mL/min/100 g tissue with neck immersion. With muscle blood flow increased 225% above dry land flow, even higher than the rise in cardiac output during immersion, it is therefore reasonable to conclude that oxygen availability to muscles is significantly increased during immersion at rest.


The hydrostatic effects of immersion, possibly combined with temperature effects, have been shown to significantly improve dependent edema and subjective pain symptoms in patients

with venous varicosities.

An aquatic exercise program may be designed to vary the amount of gravity loading by using buoyancy as a counterforce.

For acute injury, such as tibial stress fracture, programs typically should start at non–weight-bearing depths, limiting activity below pain onset, and progressing in weight bearing and exercise levels as symptoms permit.


Rehabilitative programs for specific joints may be more effective as either closed or open kinetic chain programs. Shallow-water vertical exercises generally approximate closed chain exercise,

albeit with reduced joint loading because of the counterforce produced by buoyancy. 

Deep water exercises more generally approximate an open chain system, as do horizontal exercises, such as swimming. Paddles and other resistive equipment tend to close the kinetic chain.

Offloading of body weight occurs as a function of immersion, but the water depth chosen may be adjusted for the amount of loading desired.

The spine is especially well protected during aquatic exercise programs, which facilitates

early rehabilitation from back injuries.

Acute joint symptoms may respond to warm water immersion and gentle active or active assisted

range of motion, whereas subacute or chronic arthritis often responds to more active exercise regimens.

Arthritis Exercise program has been found effective in reducing disability and improving functional fitness and strength in older adults with arthritis.

Fibromyalgic patients have demonstrated reduction in pain, improvement in sleep patterns, fibromyalgia impact, mood state disorders, and when compared with land-based exercise programs, the aquatic groups typically showed faster and larger gains.


Aquatic cross training can present a very significant aerobic challenge to the athlete, there are differences in motor activity, muscle recruitment and cardiovascular performance.

For many athletic activities, aquatic cross-training can sustain or even build aerobic fitness, with the side benefits of reduced joint loading, decreased muscle soreness and improved performance,

and a significant potential for improved respiratory function.

Programs typically used for vertical water exercise include buoyancy-assisted deep water running and cross-country skiing, aquatic treadmill running, waist-depth aqua-running, and upper extremity work using resistive devices in cool pool environments.

When aquatic exercise is compared with land-based equivalent exercise in effect on maximum VO2 gains in unfit individuals, aquatic exercise is seen to achieve equivalent results, and when water temperature is below thermoneutral (37°C), the gains achieved are usually accompanied by a lower heart rate. Thus, water-based exercise programs may be used effectively to sustain or increase aerobic conditioning in athletes who need to keep weight off a joint, such as when in

injury recovery or during an intensive training program in which joint or bone micro trauma is likely with exclusively land-based training.

The 2 major compensatory mechanisms that assist cooling in warm air temperatures are peripheral vasodilatation combined with increased cardiac output. These mechanisms work to counterpurposes in warm water (greater than 37°C), because they facilitate heat gain when the surrounding environment does not allow evaporative and radiant cooling.


Aquatic exercise at conventional pool temperatures has been shown to be safe during all

trimesters of pregnancy, and facilitate aerobic conditioning,

while reducing joint loading. Aquatic exercise at conventional temperatures has also been shown to improve amniotic fluid production, which may be a useful side effect.

Typical prenatal programs should include cool to neutral temperature pool aerobic exercise at chest or deeper depth, along with spinal stabilization drills.


Aquatic exercise would seem to offer the safest and most protective environment for obese individuals because of the buoyancy effects of immersion, which minimizes the risk of

joint injury. With body weight reduced to essentially negligible levels, the immersed individual can exercise vigorously and is capable of producing increases in VO2 max over relatively short periods.

Aquatic exercise programs may be highly beneficial in the restoration of fitness in obese patients

because of the protective effects against heavy joint loading in the aquatic environment. On dry land, the ability to achieve an aerobic exercise level for sufficient time to produce a conditioning effect may be difficult in this population, and a program that begins in water and moves to land as strength, endurance, and tolerance builds may be a more effective method of achieving both conditioning and weight loss.

The advantages of aquatic exercise also include the heat conductive effects of water, which greatly reduces risk of heat stress when done in cooler pools. 

Aquatic therapy programs for this population should include chest depth or deeper sustained aerobic exercise, alternated with balance and coordination drills.

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