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Making a Splash: Aquatic therapy can offer more bang for the (therapy) buck

Updated: 17 hours ago

Byline: Andrea Salzman, MS, PT

Water works, right? I mean, we keep hearing that water offers a unique therapeutic environment which can be harnessed to permit activities unachievable on land. If this is true, there ought to be something special, something tangibly different about the environment in a therapy pool. Something so special that it is both difficult and clinically impractical to achieve a similar environment on land. Let's take a look at what makes water-based therapy so different.


Archimedes' principle states: "when a body is wholly or partially immersed in a fluid, it

experiences an upthrust equal to the weight of fluid displaced." This upthrust, or buoyancy,

counterbalances gravity and supports the body, resulting in an apparent reduction in weight

bearing through the spine and lower extremities. Buoyancy can provide either assistance and

support or resistance to movement of the body in the water, depending on the position of the


Basically, buoyancy is an upward thrust which acts in the opposite direction to the force of

gravity. Therefore, a body in water is subject to two opposing forces: gravity acting through a

center of gravity (COG) and buoyancy acting through the center of buoyancy (COB). These

forces, when not perfectly aligned, create a moment around a pivot point and the body rotates.

There are three ways of looking at Archimedes' principle:

1. An object immersed in water loses weight (it becomes buoyant). The weight loss is equal to the weight of the water that it displaced.

2. When a body is immersed in a fluid at rest, it experiences an upward thrust equal to the weight of the water it displaced.

3. A body will float if it displaces water equal to its own weight.

When discussing whether a body or body part immersed in water will sink or float, it is

important to understand the concept of relative density (alternatively known as specific gravity). Relative density is "the ratio of the mass of an object to an equal volume of water." Water has a specific gravity equal to 1. It serves as the reference point for all objects. Objects with relative density less than water float, and those with relative density greater than water sink. Objects with relative density near the value of water hover just below the surface. The human body has elements which tend to sink (dense muscle) and elements which tend to float (fatty tissue and air-filled lungs). This tendency to float counterbalances gravity and supports the body, resulting in an apparent reduction in weight. This reduction in weight can provide relief from compressive forces on painful joints. It is therefore possible for a person to stand, even walk, with reduced pain without external support or abnormal protective mechanisms in the water. Thus, the patient can initiate "normal" weight bearing tasks such as gait, transfers, and balance drills in the water and offset any deconditioning effects of immobility or reduced movement.

As already mentioned, the relative density of water has been arbitrarily set at 1 (RD = 1). It

follows, then, that if an object has a relative density greater than 1, the object will sink. There are many factors which will increase the relative density of an object: spastic limbs; bulky muscular body; a tense, fearful patient; kyphotic trunk alignment; disproportionate higher and lower trunk size (hydrocephalus); disproportionate limb/trunk ratio (short legs, long trunk, lower center of gravity), and deflated lungs. If relative density is less than 1, an object will float on the surface. There are many factors which decrease the relative density of an object: flaccid limbs, high adipose body, a relaxed patient and inflated lungs.

A Therapeutic Environment

So, how can "buoyancy" create a therapeutic environment? For one, exercise in water produces less spinal and lower extremity joint compression than the identical exercise performed on land.

This reduction in compression creates an environment in which weight bearing and joint

compression (of the lower extremities and spine) can be applied in a graded or progressive

manner by the therapist. Weight bearing may be systematically reduced by increasing the amount of the body submerged. Static (standing) immersion to C-7 levels reduces weight to less than 10 percent of normal weight. Immersion to the xiphosternum reduces weight to 25 percent to 37 percent of normal. Immersion to the level of the anterior superior iliac spine (ASIS) reduces weight to 40 percent to 56 percent of normal.

During slow walking, patients must be immersed to the ASIS before weight bearing can be

reduced to 75 percent of normal. They must be immersed to the clavicle to reduce weight bearing to 50 percent, and they must be immersed above the clavicle to obtain a weight reduction of more than 75 percent.

During fast walking, patients must be immersed deeper than the xiphosternum in order for

weight bearing to be less than 50 percent and they must be immersed deeper than C-7 for weight bearing to be less than 25 percent of normal.

Muscle activity is also systematically reduced during immersion. One study examined the effects of immersion in warm water on muscle sympathetic activity (MSA) and electromyography (EMG) of the soleus muscle, and skin sympathetic activity (SSA) of the sole of the foot. In this study, as the level of immersion increased, both MSA and EMG activity decreased proportionally as weight bearing diminished. With immersion to the cervical spine, both MSA and EMG became almost absent. In other words, the subjects' calf muscles became less active in a buoyant environment. In effect, the calf muscle responsible for maintaining upright posture in a gravity-based environment had diminished responsibilities.

Also, as the level of immersion was increased, the sympathetic activity of the skin on the sole of the foot decreased. With immersion to the cervical spine, the SSA showed a marked and proportional decrease in activity. Translated, this means that although immersion results in less motor activity for postural muscles such as the calf, it also results in less sensory input to the skin (and probably joint) receptors which record weight bearing.

Buoyancy can be used to decrease the fight against gravity's downward thrust by producing:

• A decrease in weight bearing through joints;

• A decrease in joint stress;

• A decrease in splinting or guarding of antigravity muscles;

• An increase in freedom of movement.

Buoyancy can also promote ease of handling of the large or heavy patient, allow access to body parts which would be inaccessible if the patient was positioned on a plinth or chair, and allow progression of resistance in a logical, graded fashion, from buoyancy-assisted (easiest); buoyancy-eliminated (harder), or buoyancy-resisted (hardest).

Hydrostatic Pressure

Pascal's Law states that "Fluid pressure is exerted equally on all surfaces of an immersed body at rest at a given depth." Thus we know that pressure increases as depth increases. Since the density of the fluid in most therapeutic pools is fixed and unalterable, this pressure gradient can be used therapeutically.

In essence, hydrostatic pressure increases the pressure on the outside of an immersed standing body, resulting in:

• A reduction in edema in the lower extremities (by providing graduated pressure at greater


• An offsetting of blood pooling in lower extremities;

• A desensitization effect (by constantly stimulating phasic receptors);

• A slowing of the heart rate during exercise in water (especially in cooler water) by increasing the shift of blood to thorax, increasing pre-load of the heart, and thus increasing stroke volume;

• A resistance to chest wall expansion.


Viscosity is nothing more than the inherent friction that exists between molecules of a liquid

which cause a resistance to flow. Molecules of a liquid adhere to the surface of a body moving through that liquid resulting in resistance. When examining the qualities of viscosity, it is important to remember that:

• Resistance increases as viscosity increases. (When an object moves through a fluid of a higher viscosity, it creates greater turbulence at a given speed which creates more resistance).

• Resistance to movement at a given velocity is greater in water than in air, as fluid is more

viscous than air.

• Viscosity decreases as temperature increases (the molecules separate).

It is possible to use viscosity therapeutically. How? Movement of a body part through water

results in greater somatosensory input to receptors than movement of that body part through air. Water is more viscous than air, and resistance to flow through water is greater than resistance to flow through air. Thus, it takes more force to push through water molecules than to push through air molecules. Additionally, the faster an object is pushed through the water, the more turbulence is created and this creates additional resistance to movement. It seems likely that somatosensory input is increased more by moving an object through a viscous liquid than by moving through a less viscous gas (air). Movement may cause distention or stretch of the skin resulting in stimulation of rapidly adapting mechanoreceptors, perhaps contributing to better proprioception.


In streamlined flow of a liquid, a thin layer of fluid molecules slide over one another. Resistance is directly proportional to the velocity of movement and no eddy currents are created. In unstreamlined (turbulent) flow of a liquid, there is an irregular, rapid, random movement of fluid molecules. Resistance is directly proportional to the velocity of movement squared and eddy currents are created. When an object moves through a fluid, there is an increase in the pressure in the front of an object combined with a reduction in pressure in the back. This results in the water wanting to move from an area of high pressure to an area of lower pressure. The area of "low pressure" is known as the wake. Eddy currents form in this wake and "pull" the object back. The negative pressure (or drag) behind a moving object (the wake) is responsible for 90 percent of the impedance of movement. Surprisingly, the bow wave (the positive pressure in front of the object) is only responsible for 10 percent of the impedance.

The principle of flow can be used therapeutically to increase resistance by creating positive and negative drag. Resistance can be altered by varying velocity of movement; opposing inertia; altering streamlining; making quick reversals of flow (reversals in direction resulting in turbulence); using rebound off the side of the pool. It is also possible to use the concepts of flow to decrease resistance by taking advantage of the "pull" of wake or by performing movements in a more streamlined position.

Surface Tension

Surface tension is the force exerted between the surface "skin" molecules of a fluid. In other words, refraction is the inherent attraction between neighboring molecules of the same type of matter.

When examining the qualities of surface tension, it is important to remember that:

• Resistance increases as surface tension increases.

• Resistance increases if an exercise or activity requires the body to "break" through the surface of the water.

• Since the surface "skin" must be broken to allow movement from water to air, it is possible for a therapeutic activity to be made more difficult by requiring a patient to repeatedly move a limb from air to water.

Thermal Shifts

At temperatures above "thermoneutral" (approximately 92 degrees to 95 degrees Fahrenheit at rest), body temperature increases due to the reduced ability of the body to dissipate heat through the skin. Thermal energy (heat) is exchanged between water and the body and between air and the body. Energy exchange between a submerged body and the water occurs through both convection and conduction. Thermal energy is also exchanged between the body and the air through radiation and evaporation—methods which become more critical if the total body is immersed and the water temperature prevents heat dissipation from occurring during aquatic exercise.

Conductivity of heat through water is much greater than that through air. Thus the rate of heat loss (or gain) is much greater in water than on land. Heating a thin layer of water next to the body does not result in the formation of an insular zone (as it does in air) and thus cooler water temperatures can elevate the energy expended on executing a task by requiring muscle shivering to maintain temperature during exercise.

Although dependent on the population using the facility, therapeutic pools are generally heated to between 92 degrees to 97 degrees Fahrenheit. Immersion in water warmer than the skin will result in a rise in superficial tissue temperature which creates a palliative effect like that experienced during the therapeutic use of paraffin, Fluidotherapy ®  and moist heat.


Therapists who work in the water can offer their patients both sides of the coin: gravity and

buoyancy; ease and resistance. In the pool, patients find a place where they can work quickly

toward their therapeutic goals.

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